/* $OpenBSD: subr_blist.c,v 1.4 2023/05/30 08:30:01 jsg Exp $ */
/* DragonFlyBSD:7b80531f545c7d3c51c1660130c71d01f6bccbe0:/sys/kern/subr_blist.c */
/*
 * BLIST.C -    Bitmap allocator/deallocator, using a radix tree with hinting
 * 
 * Copyright (c) 1998,2004 The DragonFly Project.  All rights reserved.
 * 
 * This code is derived from software contributed to The DragonFly Project
 * by Matthew Dillon <dillon@backplane.com>
 * 
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 * 3. Neither the name of The DragonFly Project nor the names of its
 *    contributors may be used to endorse or promote products derived
 *    from this software without specific, prior written permission.
 * 
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
 * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 * 
 *
 *      This module implements a general bitmap allocator/deallocator.  The
 *      allocator eats around 2 bits per 'block'.  The module does not
 *      try to interpret the meaning of a 'block' other than to return
 *      SWAPBLK_NONE on an allocation failure.
 *
 *      A radix tree is used to maintain the bitmap.  Two radix constants are
 *      involved:  One for the bitmaps contained in the leaf nodes (typically
 *      32), and one for the meta nodes (typically 16).  Both meta and leaf
 *      nodes have a hint field.  This field gives us a hint as to the largest
 *      free contiguous range of blocks under the node.  It may contain a
 *      value that is too high, but will never contain a value that is too
 *      low.  When the radix tree is searched, allocation failures in subtrees
 *      update the hint.
 *
 *      The radix tree also implements two collapsed states for meta nodes:
 *      the ALL-ALLOCATED state and the ALL-FREE state.  If a meta node is
 *      in either of these two states, all information contained underneath
 *      the node is considered stale.  These states are used to optimize
 *      allocation and freeing operations.
 *
 *      The hinting greatly increases code efficiency for allocations while
 *      the general radix structure optimizes both allocations and frees.  The
 *      radix tree should be able to operate well no matter how much
 *      fragmentation there is and no matter how large a bitmap is used.
 *
 *      Unlike the rlist code, the blist code wires all necessary memory at
 *      creation time.  Neither allocations nor frees require interaction with
 *      the memory subsystem.  In contrast, the rlist code may allocate memory
 *      on an blist_free() call.  The non-blocking features of the blist code
 *      are used to great advantage in the swap code (uvm/uvm_swap.c).  The
 *      rlist code uses a little less overall memory than the blist code (but
 *      due to swap interleaving not all that much less), but the blist code
 *      scales much, much better.
 *
 *      LAYOUT: The radix tree is laid out recursively using a
 *      linear array.  Each meta node is immediately followed (laid out
 *      sequentially in memory) by BLIST_META_RADIX lower level nodes.  This
 *      is a recursive structure but one that can be easily scanned through
 *      a very simple 'skip' calculation.  In order to support large radixes,
 *      portions of the tree may reside outside our memory allocation.  We
 *      handle this with an early-termination optimization (when bighint is
 *      set to -1) on the scan.  The memory allocation is only large enough
 *      to cover the number of blocks requested at creation time even if it
 *      must be encompassed in larger root-node radix.
 *
 *      NOTE: The allocator cannot currently allocate more than
 *      BLIST_BMAP_RADIX blocks per call.  It will panic with 'allocation too
 *      large' if you try.  This is an area that could use improvement.  The
 *      radix is large enough that this restriction does not effect the swap
 *      system, though.  Currently only the allocation code is effected by
 *      this algorithmic unfeature.  The freeing code can handle arbitrary
 *      ranges.
 *
 *      NOTE: The radix may exceed BLIST_BMAP_RADIX bits in order to support
 *            up to 2^(BLIST_BMAP_RADIX-1) blocks.  The first division will
 *            drop the radix down and fit it within a signed BLIST_BMAP_RADIX
 *            bit integer.
 *
 *      This code can be compiled stand-alone for debugging.
 */

#ifdef _KERNEL

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/blist.h>
#include <sys/malloc.h>

#else

#ifndef BLIST_NO_DEBUG
#define BLIST_DEBUG
#endif

#include <sys/types.h>
#include <assert.h>
#include <err.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stdarg.h>
#include <limits.h>

#define malloc(s,t,f)   calloc(1, s)
#define mallocarray(n,s,t,f)    reallocarray(NULL, n, s)
#define free(p,t,s)     free(p)
#define KASSERT(exp)    assert(exp)
#define KDASSERT(exp)   assert(exp)

#include "../sys/blist.h"

#define panic(...)      do { errx(1, __VA_ARGS__); } while (0)

#endif

/*
 * static support functions
 */

static swblk_t blst_leaf_alloc(blmeta_t *scan, swblk_t blkat,
                                swblk_t blk, swblk_t count);
static swblk_t blst_meta_alloc(blmeta_t *scan, swblk_t blkat,
                                swblk_t blk, swblk_t count,
                                swblk_t radix, swblk_t skip);
static void blst_leaf_free(blmeta_t *scan, swblk_t relblk, swblk_t count);
static void blst_meta_free(blmeta_t *scan, swblk_t freeBlk, swblk_t count, 
                                        swblk_t radix, swblk_t skip,
                                        swblk_t blk);
static swblk_t blst_leaf_fill(blmeta_t *scan, swblk_t blk, swblk_t count);
static swblk_t blst_meta_fill(blmeta_t *scan, swblk_t fillBlk, swblk_t count,
                                        swblk_t radix, swblk_t skip,
                                        swblk_t blk);
static void blst_copy(blmeta_t *scan, swblk_t blk, swblk_t radix,
                                swblk_t skip, blist_t dest, swblk_t count);
static swblk_t  blst_radix_init(blmeta_t *scan, swblk_t radix,
                                                swblk_t skip, swblk_t count);
static int blst_radix_gapfind(blmeta_t *scan, swblk_t blk, swblk_t radix, swblk_t skip,
    int state, swblk_t *maxbp, swblk_t *maxep, swblk_t *bp, swblk_t *ep);

#if defined(BLIST_DEBUG) || defined(DDB)
static void     blst_radix_print(blmeta_t *scan, swblk_t blk,
                                        swblk_t radix, swblk_t skip, int tab);
#endif

/*
 * blist_create() - create a blist capable of handling up to the specified
 *                  number of blocks
 *
 *      blocks must be greater than 0
 *
 *      The smallest blist consists of a single leaf node capable of
 *      managing BLIST_BMAP_RADIX blocks.
 *
 *      The pages are addressable in range [0, nblocks[
 */

blist_t
blist_create(swblk_t blocks)
{
        blist_t bl;
        swblk_t radix;
        swblk_t skip = 0;

        KASSERT(blocks > 0);

        /*
         * Calculate radix and skip field used for scanning.
         *
         * Radix can exceed BLIST_BMAP_RADIX bits even if swblk_t is limited
         * to BLIST_BMAP_RADIX bits.
         *
         * XXX check overflow
         */
        radix = BLIST_BMAP_RADIX;

        while (radix < blocks) {
                radix *= BLIST_META_RADIX;
                skip = (skip + 1) * BLIST_META_RADIX;
                KASSERT(skip > 0);
        }

        bl = malloc(sizeof(struct blist), M_VMSWAP, M_WAITOK | M_ZERO);

        bl->bl_blocks = blocks;
        bl->bl_radix = radix;
        bl->bl_skip = skip;
        bl->bl_rootblks = 1 +
            blst_radix_init(NULL, bl->bl_radix, bl->bl_skip, blocks);
        bl->bl_root = mallocarray(bl->bl_rootblks, sizeof(blmeta_t),
                              M_VMSWAP, M_WAITOK);

#if defined(BLIST_DEBUG)
        printf(
                "BLIST representing %lu blocks (%lu MB of swap)"
                ", requiring %6.2fM of ram\n",
                bl->bl_blocks,
                bl->bl_blocks * 4 / 1024,
                (bl->bl_rootblks * sizeof(blmeta_t) + 1023) / (1024.0 * 1024.0)
        );
        printf("BLIST raw radix tree: %lu records, top-radix %lu\n",
                bl->bl_rootblks, bl->bl_radix);
#endif
        blst_radix_init(bl->bl_root, bl->bl_radix, bl->bl_skip, blocks);

        return(bl);
}

void
blist_destroy(blist_t bl)
{
        KASSERT(bl != NULL);

        free(bl->bl_root, M_VMSWAP, sizeof(blmeta_t) * bl->bl_rootblks);
        free(bl, M_VMSWAP, sizeof(struct blist));
}

/*
 * blist_alloc() - reserve space in the block bitmap.  Return the base
 *                   of a contiguous region or SWAPBLK_NONE if space could
 *                   not be allocated.
 */

swblk_t
blist_alloc(blist_t bl, swblk_t count)
{
        swblk_t blk = SWAPBLK_NONE;

        if (bl) {
                if (bl->bl_radix == BLIST_BMAP_RADIX)
                        blk = blst_leaf_alloc(bl->bl_root, 0, 0, count);
                else
                        blk = blst_meta_alloc(bl->bl_root, 0, 0, count,
                                              bl->bl_radix, bl->bl_skip);
                if (blk != SWAPBLK_NONE) {
                        bl->bl_free -= count;

                        KDASSERT(blk < bl->bl_blocks);
                        KDASSERT(bl->bl_free <= bl->bl_blocks);
                }
        }
        return(blk);
}

swblk_t
blist_allocat(blist_t bl, swblk_t count, swblk_t blkat)
{
        swblk_t blk = SWAPBLK_NONE;

        if (bl) {
                KDASSERT(blkat < bl->bl_blocks);
                KDASSERT(blkat + count <= bl->bl_blocks);

                if (bl->bl_radix == BLIST_BMAP_RADIX)
                        blk = blst_leaf_alloc(bl->bl_root, blkat, 0, count);
                else
                        blk = blst_meta_alloc(bl->bl_root, blkat, 0, count,
                                              bl->bl_radix, bl->bl_skip);
                if (blk != SWAPBLK_NONE) {
                        bl->bl_free -= count;

                        KDASSERT(blk < bl->bl_blocks);
                        KDASSERT(bl->bl_free <= bl->bl_blocks);
                }
        }
        return(blk);
}

/*
 * blist_free() -       free up space in the block bitmap.  Return the base
 *                      of a contiguous region.  Panic if an inconsistency is
 *                      found.
 */

void
blist_free(blist_t bl, swblk_t blkno, swblk_t count)
{
        if (bl) {
                KDASSERT(blkno < bl->bl_blocks);
                KDASSERT(blkno + count <= bl->bl_blocks);

                if (bl->bl_radix == BLIST_BMAP_RADIX)
                        blst_leaf_free(bl->bl_root, blkno, count);
                else
                        blst_meta_free(bl->bl_root, blkno, count, bl->bl_radix, bl->bl_skip, 0);
                bl->bl_free += count;

                KDASSERT(bl->bl_free <= bl->bl_blocks);
        }
}

/*
 * blist_fill() -       mark a region in the block bitmap as off-limits
 *                      to the allocator (i.e. allocate it), ignoring any
 *                      existing allocations.  Return the number of blocks
 *                      actually filled that were free before the call.
 */

swblk_t
blist_fill(blist_t bl, swblk_t blkno, swblk_t count)
{
        swblk_t filled;

        if (bl) {
                KDASSERT(blkno < bl->bl_blocks);
                KDASSERT(blkno + count <= bl->bl_blocks);

                if (bl->bl_radix == BLIST_BMAP_RADIX) {
                        filled = blst_leaf_fill(bl->bl_root, blkno, count);
                } else {
                        filled = blst_meta_fill(bl->bl_root, blkno, count,
                            bl->bl_radix, bl->bl_skip, 0);
                }
                bl->bl_free -= filled;
                KDASSERT(bl->bl_free <= bl->bl_blocks);
                return (filled);
        } else {
                return 0;
        }
}

/*
 * blist_resize() -     resize an existing radix tree to handle the
 *                      specified number of blocks.  This will reallocate
 *                      the tree and transfer the previous bitmap to the new
 *                      one.  When extending the tree you can specify whether
 *                      the new blocks are to left allocated or freed.
 */

void
blist_resize(blist_t *pbl, swblk_t count, int freenew)
{
    blist_t newbl = blist_create(count);
    blist_t save = *pbl;

    *pbl = newbl;
    if (count > save->bl_blocks)
            count = save->bl_blocks;
    blst_copy(save->bl_root, 0, save->bl_radix, save->bl_skip, newbl, count);

    /*
     * If resizing upwards, should we free the new space or not?
     */
    if (freenew && count < newbl->bl_blocks) {
            blist_free(newbl, count, newbl->bl_blocks - count);
    }
    blist_destroy(save);
}

#define GAPFIND_FIRSTFREE 0
#define GAPFIND_FIRSTUSED 1

/*
 * blist_gapfind() -    return the largest gap (free pages) in blist.
 *                      the blist isn't modified. the returned range
 *                      is [maxbp, maxep[ . The size of the gap is
 *                      maxep - maxbp. If not found, the size is 0.
 */

void
blist_gapfind(blist_t bl, swblk_t *maxbp, swblk_t *maxep)
{
        int state;
        swblk_t b, e;

        /* initialize gaps (max and current) */
        *maxbp = *maxep = 0;
        b = e = 0;

        /* search the larger gap from block 0 */
        state = blst_radix_gapfind(bl->bl_root, 0, bl->bl_radix, bl->bl_skip,
            GAPFIND_FIRSTFREE, maxbp, maxep, &b, &e);

        if (state == GAPFIND_FIRSTUSED) {
                e = bl->bl_blocks;
                if (*maxep - *maxbp < e - b) {
                        *maxbp = b;
                        *maxep = e;
                }
        }

        KDASSERT(*maxbp <= *maxep);
        KDASSERT(*maxbp <  bl->bl_blocks);
        KDASSERT(*maxep <= bl->bl_blocks);
}

/*
 * blst_radix_gapfind - search the larger gap in one pass
 *
 * - search first free block, from X -> set B
 * - search first used block, from B -> set E
 * - if the size (E - B) is larger than max, update it
 * - loop (with X=E) until end of blist
 * - max is the larger free gap
 */
static int
blst_radix_gapfind(blmeta_t *scan, swblk_t blk, swblk_t radix, swblk_t skip,
    int state, swblk_t *maxbp, swblk_t *maxep, swblk_t *bp, swblk_t *ep)
{
        swblk_t i;
        swblk_t next_skip;

        if (radix == BLIST_BMAP_RADIX) {
                /* leaf node: we consider only completely free bitmaps as free */
                if (state == GAPFIND_FIRSTFREE) {
                        if (scan->u.bmu_bitmap == (u_swblk_t)-1) {
                                /* node is fully free */
                                *bp = blk;
                                return GAPFIND_FIRSTUSED;
                        }

                        /* it isn't fully free, not found, keep state */
                        return state;

                } else if (state == GAPFIND_FIRSTUSED) {
                        if (scan->u.bmu_bitmap == (u_swblk_t)-1) {
                                /* it is free, not found, keep state */
                                return state;
                        }

                        /* it is (at least partially) used */
                        *ep = blk;
                        if (*maxep - *maxbp < *ep - *bp) {
                                *maxbp = *bp;
                                *maxep = *ep;
                        }
                        return GAPFIND_FIRSTFREE;
                }
        }

        if (scan->u.bmu_avail == 0) {
                /* ALL-ALLOCATED */
                if (state == GAPFIND_FIRSTFREE) {
                        /* searching free block, not found, keep state */
                        return state;

                } else if (state == GAPFIND_FIRSTUSED) {
                        /* searching used block, found */
                        *ep = blk;
                        if (*maxep - *maxbp < *ep - *bp) {
                                *maxbp = *bp;
                                *maxep = *ep;
                        }
                        return GAPFIND_FIRSTFREE;
                }
        }

        if (scan->u.bmu_avail == radix) {
                /* ALL-FREE */
                if (state == GAPFIND_FIRSTFREE) {
                        /* searching free block, found */
                        *bp = blk;
                        return GAPFIND_FIRSTUSED;

                } else if (state == GAPFIND_FIRSTUSED) {
                        /* searching used block, not found, keep state */
                        return state;
                }
        }

        radix /= BLIST_META_RADIX;
        next_skip = (skip / BLIST_META_RADIX);

        for (i = 1; i <= skip; i += next_skip) {
                if (scan[i].bm_bighint == (swblk_t)-1)
                        /* Terminator */
                        break;

                state = blst_radix_gapfind(&scan[i], blk, radix, next_skip - 1,
                    state, maxbp, maxep, bp, ep);

                blk += radix;
        }

        return state;
}

#if defined(BLIST_DEBUG) || defined(DDB)

/*
 * blist_print()    - dump radix tree
 */

void
blist_print(blist_t bl)
{
        printf("BLIST {\n");
        blst_radix_print(bl->bl_root, 0, bl->bl_radix, bl->bl_skip, 4);
        printf("}\n");
}

#endif

/************************************************************************
 *                        ALLOCATION SUPPORT FUNCTIONS                  *
 ************************************************************************
 *
 *      These support functions do all the actual work.  They may seem
 *      rather longish, but that's because I've commented them up.  The
 *      actual code is straight forward.
 *
 */

/*
 * blist_leaf_alloc() - allocate at a leaf in the radix tree (a bitmap).
 *
 *      This is the core of the allocator and is optimized for the 1 block
 *      and the BLIST_BMAP_RADIX block allocation cases.  Other cases are
 *      somewhat slower.  The 1 block allocation case is log2 and extremely
 *      quick.
 */

static swblk_t
blst_leaf_alloc(blmeta_t *scan, swblk_t blkat __unused, swblk_t blk,
                swblk_t count)
{
        u_swblk_t orig = scan->u.bmu_bitmap;

        if (orig == 0) {
                /*
                 * Optimize bitmap all-allocated case.  Also, count = 1
                 * case assumes at least 1 bit is free in the bitmap, so
                 * we have to take care of this case here.
                 */
                scan->bm_bighint = 0;
                return(SWAPBLK_NONE);
        }
        if (count == 1) {
                /*
                 * Optimized code to allocate one bit out of the bitmap
                 */
                u_swblk_t mask;
                int j = BLIST_BMAP_RADIX/2;
                int r = 0;

                mask = (u_swblk_t)-1 >> (BLIST_BMAP_RADIX/2);

                while (j) {
                        if ((orig & mask) == 0) {
                            r += j;
                            orig >>= j;
                        }
                        j >>= 1;
                        mask >>= j;
                }
                scan->u.bmu_bitmap &= ~((u_swblk_t)1 << r);
                return(blk + r);
        }
        if (count <= BLIST_BMAP_RADIX) {
                /*
                 * non-optimized code to allocate N bits out of the bitmap.
                 * The more bits, the faster the code runs.  It will run
                 * the slowest allocating 2 bits, but since there aren't any
                 * memory ops in the core loop (or shouldn't be, anyway),
                 * you probably won't notice the difference.
                 */
                int j;
                int n = (int)(BLIST_BMAP_RADIX - count);
                u_swblk_t mask;

                mask = (u_swblk_t)-1 >> n;

                for (j = 0; j <= n; ++j) {
                        if ((orig & mask) == mask) {
                                scan->u.bmu_bitmap &= ~mask;
                                return(blk + j);
                        }
                        mask = (mask << 1);
                }
        }

        /*
         * We couldn't allocate count in this subtree, update bighint.
         */
        scan->bm_bighint = count - 1;

        return(SWAPBLK_NONE);
}

/*
 * blist_meta_alloc() - allocate at a meta in the radix tree.
 *
 *      Attempt to allocate at a meta node.  If we can't, we update
 *      bighint and return a failure.  Updating bighint optimize future
 *      calls that hit this node.  We have to check for our collapse cases
 *      and we have a few optimizations strewn in as well.
 */
static swblk_t
blst_meta_alloc(blmeta_t *scan, swblk_t blkat,
                swblk_t blk, swblk_t count,
                swblk_t radix, swblk_t skip)
{
        int hintok = (blk >= blkat);
        swblk_t next_skip = ((swblk_t)skip / BLIST_META_RADIX);
        swblk_t i;

#ifndef _KERNEL
        printf("blist_meta_alloc blkat %lu blk %lu count %lu radix %lu\n",
                blkat, blk, count, radix);
#endif

        /*
         * ALL-ALLOCATED special case
         */
        if (scan->u.bmu_avail == 0)  {
                scan->bm_bighint = 0;
                return(SWAPBLK_NONE);
        }

        /*
         * ALL-FREE special case, initialize uninitialized
         * sublevel.
         *
         * NOTE: radix may exceed 32 bits until first division.
         */
        if (scan->u.bmu_avail == radix) {
                scan->bm_bighint = radix;

                radix /= BLIST_META_RADIX;
                for (i = 1; i <= skip; i += next_skip) {
                        if (scan[i].bm_bighint == (swblk_t)-1)
                                break;
                        if (next_skip == 1) {
                                scan[i].u.bmu_bitmap = (u_swblk_t)-1;
                                scan[i].bm_bighint = BLIST_BMAP_RADIX;
                        } else {
                                scan[i].bm_bighint = (swblk_t)radix;
                                scan[i].u.bmu_avail = (swblk_t)radix;
                        }
                }
        } else {
                radix /= BLIST_META_RADIX;
        }

        for (i = 1; i <= skip; i += next_skip) {
                if (scan[i].bm_bighint == (swblk_t)-1) {
                        /*
                         * Terminator
                         *
                         * note: check it first, as swblk_t may be unsigned.
                         *   otherwise, the second if() might match and the
                         *   Terminator will be ignored.
                         */
                        break;
                }

                if (count <= scan[i].bm_bighint &&
                    blk + (swblk_t)radix > blkat) {
                        /*
                         * count fits in object
                         */
                        swblk_t r;
                        if (next_skip == 1) {
                                r = blst_leaf_alloc(&scan[i], blkat,
                                                    blk, count);
                        } else {
                                r = blst_meta_alloc(&scan[i], blkat,
                                                    blk, count,
                                                    radix, next_skip - 1);
                        }
                        if (r != SWAPBLK_NONE) {
                                scan->u.bmu_avail -= count;
                                if (scan->bm_bighint > scan->u.bmu_avail)
                                        scan->bm_bighint = scan->u.bmu_avail;
                                return(r);
                        }
                        /* bighint was updated by recursion */
                } else if (count > (swblk_t)radix) {
                        /*
                         * count does not fit in object even if it were
                         * complete free.
                         */
                        panic("%s: allocation too large %lu/%lu",
                              __func__, count, radix);
                }
                blk += (swblk_t)radix;
        }

        /*
         * We couldn't allocate count in this subtree, update bighint.
         */
        if (hintok && scan->bm_bighint >= count)
                scan->bm_bighint = count - 1;
        return(SWAPBLK_NONE);
}

/*
 * BLST_LEAF_FREE() -   free allocated block from leaf bitmap
 */
static void
blst_leaf_free(blmeta_t *scan, swblk_t blk, swblk_t count)
{
        /*
         * free some data in this bitmap
         *
         * e.g.
         *      0000111111111110000
         *          \_________/\__/
         *              v        n
         */
        int n = blk & (BLIST_BMAP_RADIX - 1);
        u_swblk_t mask;

        mask = ((u_swblk_t)-1 << n) &
            ((u_swblk_t)-1 >> (BLIST_BMAP_RADIX - count - n));

        if (scan->u.bmu_bitmap & mask)
                panic("%s: freeing free block", __func__);
        scan->u.bmu_bitmap |= mask;

        /*
         * We could probably do a better job here.  We are required to make
         * bighint at least as large as the biggest contiguous block of
         * data.  If we just shoehorn it, a little extra overhead will
         * be incurred on the next allocation (but only that one typically).
         */
        scan->bm_bighint = BLIST_BMAP_RADIX;
}

/*
 * BLST_META_FREE() - free allocated blocks from radix tree meta info
 *
 *      This support routine frees a range of blocks from the bitmap.
 *      The range must be entirely enclosed by this radix node.  If a
 *      meta node, we break the range down recursively to free blocks
 *      in subnodes (which means that this code can free an arbitrary
 *      range whereas the allocation code cannot allocate an arbitrary
 *      range).
 */

static void
blst_meta_free(blmeta_t *scan, swblk_t freeBlk, swblk_t count,
               swblk_t radix, swblk_t skip, swblk_t blk)
{
        swblk_t i;
        swblk_t next_skip = ((swblk_t)skip / BLIST_META_RADIX);

#if 0
        printf("FREE (%04lx,%lu) FROM (%04lx,%lu)\n",
            freeBlk, count,
            blk, radix
        );
#endif

        /*
         * ALL-ALLOCATED special case, initialize for recursion.
         *
         * We will short-cut the ALL-ALLOCATED -> ALL-FREE case.
         */
        if (scan->u.bmu_avail == 0) {
                scan->u.bmu_avail = count;
                scan->bm_bighint = count;

                if (count != radix)  {
                        for (i = 1; i <= skip; i += next_skip) {
                                if (scan[i].bm_bighint == (swblk_t)-1)
                                        break;
                                scan[i].bm_bighint = 0;
                                if (next_skip == 1) {
                                        scan[i].u.bmu_bitmap = 0;
                                } else {
                                        scan[i].u.bmu_avail = 0;
                                }
                        }
                        /* fall through */
                }
        } else {
                scan->u.bmu_avail += count;
                /* scan->bm_bighint = radix; */
        }

        /*
         * ALL-FREE special case.
         *
         * Set bighint for higher levels to snoop.
         */
        if (scan->u.bmu_avail == radix) {
                scan->bm_bighint = radix;
                return;
        }

        /*
         * Break the free down into its components
         */
        if (scan->u.bmu_avail > radix) {
                panic("%s: freeing already "
                      "free blocks (%lu) %lu/%lu",
                      __func__, count, (long)scan->u.bmu_avail, radix);
        }

        radix /= BLIST_META_RADIX;

        i = (freeBlk - blk) / (swblk_t)radix;
        blk += i * (swblk_t)radix;
        i = i * next_skip + 1;

        while (i <= skip && blk < freeBlk + count) {
                swblk_t v;

                v = blk + (swblk_t)radix - freeBlk;
                if (v > count)
                        v = count;

                if (scan->bm_bighint == (swblk_t)-1)
                        panic("%s: freeing unexpected range", __func__);

                if (next_skip == 1) {
                        blst_leaf_free(&scan[i], freeBlk, v);
                } else {
                        blst_meta_free(&scan[i], freeBlk, v,
                                       radix, next_skip - 1, blk);
                }

                /*
                 * After having dealt with the becomes-all-free case any
                 * partial free will not be able to bring us to the
                 * becomes-all-free state.
                 *
                 * We can raise bighint to at least the sub-segment's
                 * bighint.
                 */
                if (scan->bm_bighint < scan[i].bm_bighint) {
                    scan->bm_bighint = scan[i].bm_bighint;
                }
                count -= v;
                freeBlk += v;
                blk += (swblk_t)radix;
                i += next_skip;
        }
}

/*
 * BLST_LEAF_FILL() -   allocate specific blocks in leaf bitmap
 *
 *      Allocates all blocks in the specified range regardless of
 *      any existing allocations in that range.  Returns the number
 *      of blocks allocated by the call.
 */
static swblk_t
blst_leaf_fill(blmeta_t *scan, swblk_t blk, swblk_t count)
{
        int n = blk & (BLIST_BMAP_RADIX - 1);
        swblk_t nblks;
        u_swblk_t mask, bitmap;

        mask = ((u_swblk_t)-1 << n) &
            ((u_swblk_t)-1 >> (BLIST_BMAP_RADIX - count - n));

        /* Count the number of blocks we're about to allocate */
        bitmap = scan->u.bmu_bitmap & mask;
        for (nblks = 0; bitmap != 0; nblks++)
                bitmap &= bitmap - 1;

        scan->u.bmu_bitmap &= ~mask;
        return (nblks);
}

/*
 * BLST_META_FILL() -   allocate specific blocks at a meta node
 *
 *      Allocates the specified range of blocks, regardless of
 *      any existing allocations in the range.  The range must
 *      be within the extent of this node.  Returns the number
 *      of blocks allocated by the call.
 */
static swblk_t
blst_meta_fill(blmeta_t *scan, swblk_t fillBlk, swblk_t count,
               swblk_t radix, swblk_t skip, swblk_t blk)
{
        swblk_t i;
        swblk_t next_skip = ((swblk_t)skip / BLIST_META_RADIX);
        swblk_t nblks = 0;

        if (count == radix || scan->u.bmu_avail == 0) {
                /*
                 * ALL-ALLOCATED special case
                 */
                nblks = scan->u.bmu_avail;
                scan->u.bmu_avail = 0;
                scan->bm_bighint = count;
                return (nblks);
        }

        if (scan->u.bmu_avail == radix) {
                radix /= BLIST_META_RADIX;

                /*
                 * ALL-FREE special case, initialize sublevel
                 */
                for (i = 1; i <= skip; i += next_skip) {
                        if (scan[i].bm_bighint == (swblk_t)-1)
                                break;
                        if (next_skip == 1) {
                                scan[i].u.bmu_bitmap = (u_swblk_t)-1;
                                scan[i].bm_bighint = BLIST_BMAP_RADIX;
                        } else {
                                scan[i].bm_bighint = (swblk_t)radix;
                                scan[i].u.bmu_avail = (swblk_t)radix;
                        }
                }
        } else {
                radix /= BLIST_META_RADIX;
        }

        if (count > (swblk_t)radix)
                panic("%s: allocation too large", __func__);

        i = (fillBlk - blk) / (swblk_t)radix;
        blk += i * (swblk_t)radix;
        i = i * next_skip + 1;

        while (i <= skip && blk < fillBlk + count) {
                swblk_t v;

                v = blk + (swblk_t)radix - fillBlk;
                if (v > count)
                        v = count;

                if (scan->bm_bighint == (swblk_t)-1)
                        panic("%s: filling unexpected range", __func__);

                if (next_skip == 1) {
                        nblks += blst_leaf_fill(&scan[i], fillBlk, v);
                } else {
                        nblks += blst_meta_fill(&scan[i], fillBlk, v,
                            radix, next_skip - 1, blk);
                }
                count -= v;
                fillBlk += v;
                blk += (swblk_t)radix;
                i += next_skip;
        }
        scan->u.bmu_avail -= nblks;
        return (nblks);
}

/*
 * BLIST_RADIX_COPY() - copy one radix tree to another
 *
 *      Locates free space in the source tree and frees it in the destination
 *      tree.  The space may not already be free in the destination.
 */

static void
blst_copy(blmeta_t *scan, swblk_t blk, swblk_t radix,
          swblk_t skip, blist_t dest, swblk_t count)
{
        swblk_t next_skip;
        swblk_t i;

        /*
         * Leaf node
         */

        if (radix == BLIST_BMAP_RADIX) {
                u_swblk_t v = scan->u.bmu_bitmap;

                if (v == (u_swblk_t)-1) {
                        blist_free(dest, blk, count);
                } else if (v != 0) {
                        for (i = 0; i < BLIST_BMAP_RADIX && i < count; ++i) {
                                if (v & ((swblk_t)1 << i))
                                        blist_free(dest, blk + i, 1);
                        }
                }
                return;
        }

        /*
         * Meta node
         */

        if (scan->u.bmu_avail == 0) {
                /*
                 * Source all allocated, leave dest allocated
                 */
                return;
        }
        if (scan->u.bmu_avail == radix) {
                /*
                 * Source all free, free entire dest
                 */
                if (count < radix)
                        blist_free(dest, blk, count);
                else
                        blist_free(dest, blk, (swblk_t)radix);
                return;
        }


        radix /= BLIST_META_RADIX;
        next_skip = ((u_swblk_t)skip / BLIST_META_RADIX);

        for (i = 1; count && i <= skip; i += next_skip) {
                if (scan[i].bm_bighint == (swblk_t)-1)
                        break;

                if (count >= (swblk_t)radix) {
                        blst_copy(
                            &scan[i],
                            blk,
                            radix,
                            next_skip - 1,
                            dest,
                            (swblk_t)radix
                        );
                        count -= (swblk_t)radix;
                } else {
                        if (count) {
                                blst_copy(
                                    &scan[i],
                                    blk,
                                    radix,
                                    next_skip - 1,
                                    dest,
                                    count
                                );
                        }
                        count = 0;
                }
                blk += (swblk_t)radix;
        }
}

/*
 * BLST_RADIX_INIT() - initialize radix tree
 *
 *      Initialize our meta structures and bitmaps and calculate the exact
 *      amount of space required to manage 'count' blocks - this space may
 *      be considerably less than the calculated radix due to the large
 *      RADIX values we use.
 */

static swblk_t
blst_radix_init(blmeta_t *scan, swblk_t radix, swblk_t skip, swblk_t count)
{
        swblk_t i;
        swblk_t next_skip;
        swblk_t memindex = 0;

        /*
         * Leaf node
         */

        if (radix == BLIST_BMAP_RADIX) {
                if (scan) {
                        scan->bm_bighint = 0;
                        scan->u.bmu_bitmap = 0;
                }
                return(memindex);
        }

        /*
         * Meta node.  If allocating the entire object we can special
         * case it.  However, we need to figure out how much memory
         * is required to manage 'count' blocks, so we continue on anyway.
         */

        if (scan) {
                scan->bm_bighint = 0;
                scan->u.bmu_avail = 0;
        }

        radix /= BLIST_META_RADIX;
        next_skip = ((u_swblk_t)skip / BLIST_META_RADIX);

        for (i = 1; i <= skip; i += next_skip) {
                if (count >= (swblk_t)radix) {
                        /*
                         * Allocate the entire object
                         */
                        memindex = i + blst_radix_init(
                            ((scan) ? &scan[i] : NULL),
                            radix,
                            next_skip - 1,
                            (swblk_t)radix
                        );
                        count -= (swblk_t)radix;
                } else if (count > 0) {
                        /*
                         * Allocate a partial object
                         */
                        memindex = i + blst_radix_init(
                            ((scan) ? &scan[i] : NULL),
                            radix,
                            next_skip - 1,
                            count
                        );
                        count = 0;
                } else {
                        /*
                         * Add terminator and break out
                         */
                        if (scan)
                                scan[i].bm_bighint = (swblk_t)-1;
                        break;
                }
        }
        if (memindex < i)
                memindex = i;
        return(memindex);
}

#if defined(BLIST_DEBUG) || defined(DDB)

static void
blst_radix_print(blmeta_t *scan, swblk_t blk, swblk_t radix, swblk_t skip, int tab)
{
        swblk_t i;
        swblk_t next_skip;

        if (radix == BLIST_BMAP_RADIX) {
                printf(
                    "%*.*s(%04lx,%lu): bitmap %0*llx big=%lu\n",
                    tab, tab, "",
                    blk, radix,
                    (int)(1 + (BLIST_BMAP_RADIX - 1) / 4),
                    scan->u.bmu_bitmap,
                    scan->bm_bighint
                );
                return;
        }

        if (scan->u.bmu_avail == 0) {
                printf(
                    "%*.*s(%04lx,%lu) ALL ALLOCATED\n",
                    tab, tab, "",
                    blk,
                    radix
                );
                return;
        }
        if (scan->u.bmu_avail == radix) {
                printf(
                    "%*.*s(%04lx,%lu) ALL FREE\n",
                    tab, tab, "",
                    blk,
                    radix
                );
                return;
        }

        printf(
            "%*.*s(%04lx,%lu): subtree (%lu/%lu) big=%lu {\n",
            tab, tab, "",
            blk, radix,
            scan->u.bmu_avail,
            radix,
            scan->bm_bighint
        );

        radix /= BLIST_META_RADIX;
        next_skip = ((u_swblk_t)skip / BLIST_META_RADIX);
        tab += 4;

        for (i = 1; i <= skip; i += next_skip) {
                if (scan[i].bm_bighint == (swblk_t)-1) {
                        printf(
                            "%*.*s(%04lx,%lu): Terminator\n",
                            tab, tab, "",
                            blk, radix
                        );
                        break;
                }
                blst_radix_print(
                    &scan[i],
                    blk,
                    radix,
                    next_skip - 1,
                    tab
                );
                blk += (swblk_t)radix;
        }
        tab -= 4;

        printf(
            "%*.*s}\n",
            tab, tab, ""
        );
}

#endif

#if !defined(_KERNEL) && defined(BLIST_DEBUG)

int
main(int ac, char **av)
{
        swblk_t size = 1024;
        swblk_t i;
        blist_t bl;

        for (i = 1; i < (swblk_t)ac; ++i) {
                const char *ptr = av[i];
                if (*ptr != '-') {
                        size = strtol(ptr, NULL, 0);
                        continue;
                }
                ptr += 2;
                fprintf(stderr, "Bad option: %s\n", ptr - 2);
                exit(1);
        }
        bl = blist_create(size);
        blist_free(bl, 0, size);

        for (;;) {
                char buf[1024];
                swblk_t da = 0;
                swblk_t count = 0;
                swblk_t blkat;


                printf("%lu/%lu/%lu> ",
                        bl->bl_free, size, bl->bl_radix);
                fflush(stdout);
                if (fgets(buf, sizeof(buf), stdin) == NULL)
                        break;
                switch(buf[0]) {
                case '#':
                        continue;
                case 'r':
                        if (sscanf(buf + 1, "%li", &count) == 1) {
                                blist_resize(&bl, count, 1);
                                size = count;
                        } else {
                                printf("?\n");
                        }
                case 'p':
                        blist_print(bl);
                        break;
                case 'a':
                        if (sscanf(buf + 1, "%li %li", &count, &blkat) == 1) {
                                printf("count %lu\n", count);
                                swblk_t blk = blist_alloc(bl, count);
                                if (blk == SWAPBLK_NONE)
                                        printf("    R=SWAPBLK_NONE\n");
                                else
                                        printf("    R=%04lx\n", blk);
                        } else if (sscanf(buf + 1, "%li %li", &count, &blkat) == 2) {
                                swblk_t blk = blist_allocat(bl, count, blkat);
                                if (blk == SWAPBLK_NONE)
                                        printf("    R=SWAPBLK_NONE\n");
                                else
                                        printf("    R=%04lx\n", blk);
                        } else {
                                printf("?\n");
                        }
                        break;
                case 'f':
                        if (sscanf(buf + 1, "%li %li", &da, &count) == 2) {
                                blist_free(bl, da, count);
                        } else {
                                printf("?\n");
                        }
                        break;
                case 'g': {
                                swblk_t b, e;
                                blist_gapfind(bl, &b, &e);
                                printf("gapfind: begin=%04lx end=%04lx size=%lu\n",
                                    b, e, e-b);
                                break;
                        }
                case 'l':
                        if (sscanf(buf + 1, "%li %li", &da, &count) == 2) {
                                printf("    n=%lu\n",
                                    blist_fill(bl, da, count));
                        } else {
                                printf("?\n");
                        }
                        break;
                case '?':
                case 'h':
                        puts(
                            "p          -print\n"
                            "a %li      -allocate\n"
                            "f %li %li  -free\n"
                            "l %li %li  -fill\n"
                            "g          -gapfind\n"
                            "r %li      -resize\n"
                            "h/?        -help\n"
                            "    hex may be specified with 0x prefix\n"
                        );
                        break;
                default:
                        printf("?\n");
                        break;
                }
        }
        return(0);
}

#endif