/*-
 * SPDX-License-Identifier: BSD-3-Clause
 *
 * Copyright (c) 2007-2009 Google Inc.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
 * met:
 *
 * * Redistributions of source code must retain the above copyright
 *   notice, this list of conditions and the following disclaimer.
 * * 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.
 * * Neither the name of Google Inc. 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
 * OWNER 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.
 *
 * Copyright (C) 2005 Csaba Henk.
 * All rights reserved.
 *
 * Copyright (c) 2019 The FreeBSD Foundation
 *
 * Portions of this software were developed by BFF Storage Systems, LLC under
 * sponsorship from the FreeBSD Foundation.
 *
 * 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.
 *
 * THIS SOFTWARE IS PROVIDED BY AUTHOR 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 AUTHOR 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.
 */

#include <sys/types.h>
#include <sys/param.h>
#include <sys/module.h>
#include <sys/systm.h>
#include <sys/errno.h>
#include <sys/param.h>
#include <sys/kernel.h>
#include <sys/conf.h>
#include <sys/uio.h>
#include <sys/malloc.h>
#include <sys/queue.h>
#include <sys/lock.h>
#include <sys/sx.h>
#include <sys/mutex.h>
#include <sys/rwlock.h>
#include <sys/priv.h>
#include <sys/proc.h>
#include <sys/mount.h>
#include <sys/vnode.h>
#include <sys/stat.h>
#include <sys/unistd.h>
#include <sys/filedesc.h>
#include <sys/file.h>
#include <sys/fcntl.h>
#include <sys/bio.h>
#include <sys/buf.h>
#include <sys/sysctl.h>
#include <sys/vmmeter.h>

#include <vm/vm.h>
#include <vm/vm_extern.h>
#include <vm/pmap.h>
#include <vm/vm_map.h>
#include <vm/vm_page.h>
#include <vm/vm_object.h>
#include <vm/vnode_pager.h>

#include "fuse.h"
#include "fuse_file.h"
#include "fuse_node.h"
#include "fuse_internal.h"
#include "fuse_ipc.h"
#include "fuse_io.h"

/* 
 * Set in a struct buf to indicate that the write came from the buffer cache
 * and the originating cred and pid are no longer known.
 */
#define B_FUSEFS_WRITE_CACHE B_FS_FLAG1

SDT_PROVIDER_DECLARE(fusefs);
/* 
 * Fuse trace probe:
 * arg0: verbosity.  Higher numbers give more verbose messages
 * arg1: Textual message
 */
SDT_PROBE_DEFINE2(fusefs, , io, trace, "int", "char*");

SDT_PROBE_DEFINE4(fusefs, , io, read_bio_backend_start, "int", "int", "int", "int");
SDT_PROBE_DEFINE2(fusefs, , io, read_bio_backend_feed, "int", "struct buf*");
SDT_PROBE_DEFINE4(fusefs, , io, read_bio_backend_end, "int", "ssize_t", "int",
                "struct buf*");
int
fuse_read_biobackend(struct vnode *vp, struct uio *uio, int ioflag,
    struct ucred *cred, struct fuse_filehandle *fufh, pid_t pid)
{
        struct buf *bp;
        struct mount *mp;
        struct fuse_data *data;
        daddr_t lbn, nextlbn;
        int bcount, nextsize;
        int err, n = 0, on = 0, seqcount;
        off_t filesize;

        const int biosize = fuse_iosize(vp);
        mp = vnode_mount(vp);
        data = fuse_get_mpdata(mp);

        if (uio->uio_offset < 0)
                return (EINVAL);

        seqcount = ioflag >> IO_SEQSHIFT;

        err = fuse_vnode_size(vp, &filesize, cred, curthread);
        if (err)
                return err;

        for (err = 0, bp = NULL; uio->uio_resid > 0; bp = NULL) {
                if (fuse_isdeadfs(vp)) {
                        err = ENXIO;
                        break;
                }
                if (filesize - uio->uio_offset <= 0)
                        break;
                lbn = uio->uio_offset / biosize;
                on = uio->uio_offset & (biosize - 1);

                if ((off_t)lbn * biosize >= filesize) {
                        bcount = 0;
                } else if ((off_t)(lbn + 1) * biosize > filesize) {
                        bcount = filesize - (off_t)lbn *biosize;
                } else {
                        bcount = biosize;
                }
                nextlbn = lbn + 1;
                nextsize = MIN(biosize, filesize - nextlbn * biosize);

                SDT_PROBE4(fusefs, , io, read_bio_backend_start,
                        biosize, (int)lbn, on, bcount);

                if (bcount < biosize) {
                        /* If near EOF, don't do readahead */
                        err = bread(vp, lbn, bcount, NOCRED, &bp);
                } else if ((vp->v_mount->mnt_flag & MNT_NOCLUSTERR) == 0) {
                        /* Try clustered read */
                        long totread = uio->uio_resid + on;
                        seqcount = MIN(seqcount,
                                data->max_readahead_blocks + 1);
                        err = cluster_read(vp, filesize, lbn, bcount, NOCRED,
                                totread, seqcount, 0, &bp);
                } else if (seqcount > 1 && data->max_readahead_blocks >= 1) {
                        /* Try non-clustered readahead */
                        err = breadn(vp, lbn, bcount, &nextlbn, &nextsize, 1,
                                NOCRED, &bp);
                } else {
                        /* Just read what was requested */
                        err = bread(vp, lbn, bcount, NOCRED, &bp);
                }

                if (err) {
                        brelse(bp);
                        bp = NULL;
                        break;
                }

                /*
                 * on is the offset into the current bp.  Figure out how many
                 * bytes we can copy out of the bp.  Note that bcount is
                 * NOT DEV_BSIZE aligned.
                 *
                 * Then figure out how many bytes we can copy into the uio.
                 */

                n = 0;
                if (on < bcount - bp->b_resid)
                        n = MIN((unsigned)(bcount - bp->b_resid - on),
                            uio->uio_resid);
                if (n > 0) {
                        SDT_PROBE2(fusefs, , io, read_bio_backend_feed, n, bp);
                        err = uiomove(bp->b_data + on, n, uio);
                }
                vfs_bio_brelse(bp, ioflag);
                SDT_PROBE4(fusefs, , io, read_bio_backend_end, err,
                        uio->uio_resid, n, bp);
                if (bp->b_resid > 0) {
                        /* Short read indicates EOF */
                        break;
                }
        }

        return (err);
}

SDT_PROBE_DEFINE1(fusefs, , io, read_directbackend_start,
        "struct fuse_read_in*");
SDT_PROBE_DEFINE3(fusefs, , io, read_directbackend_complete,
        "struct fuse_dispatcher*", "struct fuse_read_in*", "struct uio*");

int
fuse_read_directbackend(struct vnode *vp, struct uio *uio,
    struct ucred *cred, struct fuse_filehandle *fufh)
{
        struct fuse_data *data;
        struct fuse_dispatcher fdi;
        struct fuse_read_in *fri;
        int err = 0;

        data = fuse_get_mpdata(vp->v_mount);

        if (uio->uio_resid == 0)
                return (0);

        fdisp_init(&fdi, 0);

        /*
         * XXX In "normal" case we use an intermediate kernel buffer for
         * transmitting data from daemon's context to ours. Eventually, we should
         * get rid of this. Anyway, if the target uio lives in sysspace (we are
         * called from pageops), and the input data doesn't need kernel-side
         * processing (we are not called from readdir) we can already invoke
         * an optimized, "peer-to-peer" I/O routine.
         */
        while (uio->uio_resid > 0) {
                fdi.iosize = sizeof(*fri);
                fdisp_make_vp(&fdi, FUSE_READ, vp, uio->uio_td, cred);
                fri = fdi.indata;
                fri->fh = fufh->fh_id;
                fri->offset = uio->uio_offset;
                fri->size = MIN(uio->uio_resid,
                    fuse_get_mpdata(vp->v_mount)->max_read);
                if (fuse_libabi_geq(data, 7, 9)) {
                        /* See comment regarding FUSE_WRITE_LOCKOWNER */
                        fri->read_flags = 0;
                        fri->flags = fufh_type_2_fflags(fufh->fufh_type);
                }

                SDT_PROBE1(fusefs, , io, read_directbackend_start, fri);

                if ((err = fdisp_wait_answ(&fdi)))
                        goto out;

                SDT_PROBE3(fusefs, , io, read_directbackend_complete,
                        &fdi, fri, uio);

                if ((err = uiomove(fdi.answ, MIN(fri->size, fdi.iosize), uio)))
                        break;
                if (fdi.iosize < fri->size) {
                        /* 
                         * Short read.  Should only happen at EOF or with
                         * direct io.
                         */
                        break;
                }
        }

out:
        fdisp_destroy(&fdi);
        return (err);
}

int
fuse_write_directbackend(struct vnode *vp, struct uio *uio,
    struct ucred *cred, struct fuse_filehandle *fufh, off_t filesize,
    int ioflag, bool pages)
{
        struct fuse_vnode_data *fvdat = VTOFUD(vp);
        struct fuse_data *data;
        struct fuse_write_in *fwi;
        struct fuse_write_out *fwo;
        struct fuse_dispatcher fdi;
        size_t chunksize;
        ssize_t r;
        void *fwi_data;
        off_t as_written_offset;
        int diff;
        int err = 0;
        bool direct_io = fufh->fuse_open_flags & FOPEN_DIRECT_IO;
        bool wrote_anything = false;
        uint32_t write_flags;

        data = fuse_get_mpdata(vp->v_mount);

        /* 
         * Don't set FUSE_WRITE_LOCKOWNER in write_flags.  It can't be set
         * accurately when using POSIX AIO, libfuse doesn't use it, and I'm not
         * aware of any file systems that do.  It was an attempt to add
         * Linux-style mandatory locking to the FUSE protocol, but mandatory
         * locking is deprecated even on Linux.  See Linux commit
         * f33321141b273d60cbb3a8f56a5489baad82ba5e .
         */
        /*
         * Set FUSE_WRITE_CACHE whenever we don't know the uid, gid, and/or pid
         * that originated a write.  For example when writing from the
         * writeback cache.  I don't know of a single file system that cares,
         * but the protocol says we're supposed to do this.
         */
        write_flags = !pages && (
                (ioflag & IO_DIRECT) ||
                !fsess_opt_datacache(vnode_mount(vp)) ||
                !fsess_opt_writeback(vnode_mount(vp))) ? 0 : FUSE_WRITE_CACHE;

        if (uio->uio_resid == 0)
                return (0);

        if (ioflag & IO_APPEND)
                uio_setoffset(uio, filesize);

        err = vn_rlimit_fsizex(vp, uio, 0, &r, uio->uio_td);
        if (err != 0) {
                vn_rlimit_fsizex_res(uio, r);
                return (err);
        }

        fdisp_init(&fdi, 0);

        while (uio->uio_resid > 0) {
                size_t sizeof_fwi;

                if (fuse_libabi_geq(data, 7, 9)) {
                        sizeof_fwi = sizeof(*fwi);
                } else {
                        sizeof_fwi = FUSE_COMPAT_WRITE_IN_SIZE;
                }

                chunksize = MIN(uio->uio_resid, data->max_write);

                fdi.iosize = sizeof_fwi + chunksize;
                fdisp_make_vp(&fdi, FUSE_WRITE, vp, uio->uio_td, cred);

                fwi = fdi.indata;
                fwi->fh = fufh->fh_id;
                fwi->offset = uio->uio_offset;
                fwi->size = chunksize;
                fwi->write_flags = write_flags;
                if (fuse_libabi_geq(data, 7, 9)) {
                        fwi->flags = fufh_type_2_fflags(fufh->fufh_type);
                }
                fwi_data = (char *)fdi.indata + sizeof_fwi;

                if ((err = uiomove(fwi_data, chunksize, uio)))
                        break;

retry:
                err = fdisp_wait_answ(&fdi);
                if (err == ERESTART || err == EINTR || err == EWOULDBLOCK) {
                        /*
                         * Rewind the uio so dofilewrite will know it's
                         * incomplete
                         */
                        uio->uio_resid += fwi->size;
                        uio->uio_offset -= fwi->size;
                        /* 
                         * Change ERESTART into EINTR because we can't rewind
                         * uio->uio_iov.  Basically, once uiomove(9) has been
                         * called, it's impossible to restart a syscall.
                         */
                        if (err == ERESTART)
                                err = EINTR;
                        break;
                } else if (err) {
                        break;
                } else {
                        wrote_anything = true;
                }

                fwo = ((struct fuse_write_out *)fdi.answ);

                if (fwo->size > fwi->size) {
                        fuse_warn(data, FSESS_WARN_WROTE_LONG,
                                "wrote more data than we provided it.");
                        /* This is bonkers.  Clear attr cache. */
                        ASSERT_CACHED_ATTRS_LOCKED(vp);
                        fvdat->flag &= ~FN_SIZECHANGE;
                        fuse_vnode_clear_attr_cache(vp);
                        err = EINVAL;
                        break;
                }

                /* Adjust the uio in the case of short writes */
                diff = fwi->size - fwo->size;

                as_written_offset = uio->uio_offset - diff;

                if (as_written_offset - diff > filesize) {
                        fuse_vnode_setsize(vp, as_written_offset, false);
                        getnanouptime(&fvdat->last_local_modify);
                }
                if (as_written_offset - diff >= filesize) {
                        ASSERT_CACHED_ATTRS_LOCKED(vp);
                        fvdat->flag &= ~FN_SIZECHANGE;
                }

                if (diff > 0) {
                        /* Short write */
                        if (!direct_io) {
                                fuse_warn(data, FSESS_WARN_SHORT_WRITE,
                                        "short writes are only allowed with "
                                        "direct_io.");
                        }
                        if (ioflag & IO_DIRECT) {
                                /* Return early */
                                uio->uio_resid += diff;
                                uio->uio_offset -= diff;
                                break;
                        } else {
                                /* Resend the unwritten portion of data */
                                fdi.iosize = sizeof_fwi + diff;
                                /* Refresh fdi without clearing data buffer */
                                fdisp_refresh_vp(&fdi, FUSE_WRITE, vp,
                                        uio->uio_td, cred);
                                fwi = fdi.indata;
                                MPASS2(fwi == fdi.indata, "FUSE dispatcher "
                                        "reallocated despite no increase in "
                                        "size?");
                                void *src = (char*)fwi_data + fwo->size;
                                memmove(fwi_data, src, diff);
                                fwi->fh = fufh->fh_id;
                                fwi->offset = as_written_offset;
                                fwi->size = diff;
                                fwi->write_flags = write_flags;
                                goto retry;
                        }
                }
        }

        fdisp_destroy(&fdi);

        if (wrote_anything) {
                CACHED_ATTR_LOCK(vp);
                fuse_vnode_undirty_cached_timestamps(vp, false);
                CACHED_ATTR_UNLOCK(vp);
        }

        vn_rlimit_fsizex_res(uio, r);
        return (err);
}

SDT_PROBE_DEFINE6(fusefs, , io, write_biobackend_start, "int64_t", "int", "int",
                "struct uio*", "int", "bool");
SDT_PROBE_DEFINE2(fusefs, , io, write_biobackend_append_race, "long", "int");
SDT_PROBE_DEFINE2(fusefs, , io, write_biobackend_issue, "int", "struct buf*");

int
fuse_write_biobackend(struct vnode *vp, struct uio *uio,
    struct ucred *cred, struct fuse_filehandle *fufh, int ioflag, pid_t pid)
{
        struct fuse_vnode_data *fvdat = VTOFUD(vp);
        struct buf *bp;
        daddr_t lbn;
        off_t filesize;
        ssize_t r;
        int bcount;
        int n, on, seqcount, err = 0;

        const int biosize = fuse_iosize(vp);

        seqcount = ioflag >> IO_SEQSHIFT;

        KASSERT(uio->uio_rw == UIO_WRITE, ("fuse_write_biobackend mode"));
        if (vp->v_type != VREG)
                return (EIO);
        if (uio->uio_offset < 0)
                return (EINVAL);
        if (uio->uio_resid == 0)
                return (0);

        err = fuse_vnode_size(vp, &filesize, cred, curthread);
        if (err)
                return err;

        if (ioflag & IO_APPEND)
                uio_setoffset(uio, filesize);

        err = vn_rlimit_fsizex(vp, uio, 0, &r, uio->uio_td);
        if (err != 0) {
                vn_rlimit_fsizex_res(uio, r);
                return (err);
        }

        do {
                bool direct_append, extending;

                if (fuse_isdeadfs(vp)) {
                        err = ENXIO;
                        break;
                }
                lbn = uio->uio_offset / biosize;
                on = uio->uio_offset & (biosize - 1);
                n = MIN((unsigned)(biosize - on), uio->uio_resid);

again:
                /* Get or create a buffer for the write */
                direct_append = uio->uio_offset == filesize && n;
                if (uio->uio_offset + n < filesize) {
                        extending = false;
                        if ((off_t)(lbn + 1) * biosize < filesize) {
                                /* Not the file's last block */
                                bcount = biosize;
                        } else {
                                /* The file's last block */
                                bcount = filesize - (off_t)lbn * biosize;
                        }
                } else {
                        extending = true;
                        bcount = on + n;
                }
                if (direct_append) {
                        /* 
                         * Take care to preserve the buffer's B_CACHE state so
                         * as not to cause an unnecessary read.
                         */
                        bp = getblk(vp, lbn, on, PCATCH, 0, 0);
                        if (bp != NULL) {
                                uint32_t save = bp->b_flags & B_CACHE;
                                allocbuf(bp, bcount);
                                bp->b_flags |= save;
                        }
                } else {
                        bp = getblk(vp, lbn, bcount, PCATCH, 0, 0);
                }
                if (!bp) {
                        err = EINTR;
                        break;
                }
                if (extending) {
                        /* 
                         * Extend file _after_ locking buffer so we won't race
                         * with other readers
                         */
                        err = fuse_vnode_setsize(vp, uio->uio_offset + n, false);
                        filesize = uio->uio_offset + n;
                        getnanouptime(&fvdat->last_local_modify);
                        ASSERT_CACHED_ATTRS_LOCKED(vp);
                        fvdat->flag |= FN_SIZECHANGE;
                        if (err) {
                                brelse(bp);
                                break;
                        } 
                }

                SDT_PROBE6(fusefs, , io, write_biobackend_start,
                        lbn, on, n, uio, bcount, direct_append);
                /*
                 * Issue a READ if B_CACHE is not set.  In special-append
                 * mode, B_CACHE is based on the buffer prior to the write
                 * op and is typically set, avoiding the read.  If a read
                 * is required in special append mode, the server will
                 * probably send us a short-read since we extended the file
                 * on our end, resulting in b_resid == 0 and, thusly,
                 * B_CACHE getting set.
                 *
                 * We can also avoid issuing the read if the write covers
                 * the entire buffer.  We have to make sure the buffer state
                 * is reasonable in this case since we will not be initiating
                 * I/O.  See the comments in kern/vfs_bio.c's getblk() for
                 * more information.
                 *
                 * B_CACHE may also be set due to the buffer being cached
                 * normally.
                 */

                if (on == 0 && n == bcount) {
                        bp->b_flags |= B_CACHE;
                        bp->b_flags &= ~B_INVAL;
                        bp->b_ioflags &= ~BIO_ERROR;
                }
                if ((bp->b_flags & B_CACHE) == 0) {
                        bp->b_iocmd = BIO_READ;
                        vfs_busy_pages(bp, 0);
                        fuse_io_strategy(vp, bp);
                        if ((err = bp->b_error)) {
                                brelse(bp);
                                break;
                        }
                        if (bp->b_resid > 0) {
                                /* 
                                 * Short read indicates EOF.  Update file size
                                 * from the server and try again.
                                 */
                                SDT_PROBE2(fusefs, , io, trace, 1,
                                        "Short read during a RMW");
                                brelse(bp);
                                err = fuse_vnode_size(vp, &filesize, cred,
                                    curthread);
                                if (err)
                                        break;
                                else
                                        goto again;
                        }
                }
                if (bp->b_wcred == NOCRED)
                        bp->b_wcred = crhold(cred);

                /*
                 * If dirtyend exceeds file size, chop it down.  This should
                 * not normally occur but there is an append race where it
                 * might occur XXX, so we log it.
                 *
                 * If the chopping creates a reverse-indexed or degenerate
                 * situation with dirtyoff/end, we 0 both of them.
                 */
                if (bp->b_dirtyend > bcount) {
                        SDT_PROBE2(fusefs, , io, write_biobackend_append_race,
                            (long)bp->b_blkno * biosize,
                            bp->b_dirtyend - bcount);
                        bp->b_dirtyend = bcount;
                }
                if (bp->b_dirtyoff >= bp->b_dirtyend)
                        bp->b_dirtyoff = bp->b_dirtyend = 0;

                /*
                 * If the new write will leave a contiguous dirty
                 * area, just update the b_dirtyoff and b_dirtyend,
                 * otherwise force a write rpc of the old dirty area.
                 *
                 * While it is possible to merge discontiguous writes due to
                 * our having a B_CACHE buffer ( and thus valid read data
                 * for the hole), we don't because it could lead to
                 * significant cache coherency problems with multiple clients,
                 * especially if locking is implemented later on.
                 *
                 * as an optimization we could theoretically maintain
                 * a linked list of discontinuous areas, but we would still
                 * have to commit them separately so there isn't much
                 * advantage to it except perhaps a bit of asynchronization.
                 */

                if (bp->b_dirtyend > 0 &&
                    (on > bp->b_dirtyend || (on + n) < bp->b_dirtyoff)) {
                        /*
                         * Yes, we mean it. Write out everything to "storage"
                         * immediately, without hesitation. (Apart from other
                         * reasons: the only way to know if a write is valid
                         * if its actually written out.)
                         */
                        SDT_PROBE2(fusefs, , io, write_biobackend_issue, 0, bp);
                        bwrite(bp);
                        if (bp->b_error == EINTR) {
                                err = EINTR;
                                break;
                        }
                        goto again;
                }
                err = uiomove((char *)bp->b_data + on, n, uio);

                if (err) {
                        bp->b_ioflags |= BIO_ERROR;
                        bp->b_error = err;
                        brelse(bp);
                        break;
                        /* TODO: vfs_bio_clrbuf like ffs_write does? */
                }
                /*
                 * Only update dirtyoff/dirtyend if not a degenerate
                 * condition.
                 */
                if (n) {
                        if (bp->b_dirtyend > 0) {
                                bp->b_dirtyoff = MIN(on, bp->b_dirtyoff);
                                bp->b_dirtyend = MAX((on + n), bp->b_dirtyend);
                        } else {
                                bp->b_dirtyoff = on;
                                bp->b_dirtyend = on + n;
                        }
                        vfs_bio_set_valid(bp, on, n);
                }

                vfs_bio_set_flags(bp, ioflag);

                bp->b_flags |= B_FUSEFS_WRITE_CACHE;
                if (ioflag & IO_SYNC) {
                        SDT_PROBE2(fusefs, , io, write_biobackend_issue, 2, bp);
                        if (!(ioflag & IO_VMIO))
                                bp->b_flags &= ~B_FUSEFS_WRITE_CACHE;
                        err = bwrite(bp);
                } else if (vm_page_count_severe() ||
                            buf_dirty_count_severe() ||
                            (ioflag & IO_ASYNC)) {
                        bp->b_flags |= B_CLUSTEROK;
                        SDT_PROBE2(fusefs, , io, write_biobackend_issue, 3, bp);
                        bawrite(bp);
                } else if (on == 0 && n == bcount) {
                        if ((vp->v_mount->mnt_flag & MNT_NOCLUSTERW) == 0) {
                                bp->b_flags |= B_CLUSTEROK;
                                SDT_PROBE2(fusefs, , io, write_biobackend_issue,
                                        4, bp);
                                cluster_write(vp, &fvdat->clusterw, bp,
                                    filesize, seqcount, 0);
                        } else {
                                SDT_PROBE2(fusefs, , io, write_biobackend_issue,
                                        5, bp);
                                bawrite(bp);
                        }
                } else if (ioflag & IO_DIRECT) {
                        bp->b_flags |= B_CLUSTEROK;
                        SDT_PROBE2(fusefs, , io, write_biobackend_issue, 6, bp);
                        bawrite(bp);
                } else {
                        bp->b_flags &= ~B_CLUSTEROK;
                        SDT_PROBE2(fusefs, , io, write_biobackend_issue, 7, bp);
                        bdwrite(bp);
                }
                if (err)
                        break;
        } while (uio->uio_resid > 0 && n > 0);

        vn_rlimit_fsizex_res(uio, r);
        return (err);
}

int
fuse_io_strategy(struct vnode *vp, struct buf *bp)
{
        struct fuse_vnode_data *fvdat = VTOFUD(vp);
        struct fuse_filehandle *fufh;
        struct ucred *cred;
        struct uio *uiop;
        struct uio uio;
        struct iovec io;
        off_t filesize;
        int error = 0;
        int fflag;
        /* We don't know the true pid when we're dealing with the cache */
        pid_t pid = 0;

        const int biosize = fuse_iosize(vp);

        MPASS(vp->v_type == VREG || vp->v_type == VDIR);
        MPASS(bp->b_iocmd == BIO_READ || bp->b_iocmd == BIO_WRITE);

        fflag = bp->b_iocmd == BIO_READ ? FREAD : FWRITE;
        cred = bp->b_iocmd == BIO_READ ? bp->b_rcred : bp->b_wcred;
        error = fuse_filehandle_getrw(vp, fflag, &fufh, cred, pid);
        if (bp->b_iocmd == BIO_READ && error == EBADF) {
                /* 
                 * This may be a read-modify-write operation on a cached file
                 * opened O_WRONLY.  The FUSE protocol allows this.
                 */
                error = fuse_filehandle_get(vp, FWRITE, &fufh, cred, pid);
        }
        if (error) {
                printf("FUSE: strategy: filehandles are closed\n");
                bp->b_ioflags |= BIO_ERROR;
                bp->b_error = error;
                bufdone(bp);
                return (error);
        }

        uiop = &uio;
        uiop->uio_iov = &io;
        uiop->uio_iovcnt = 1;
        uiop->uio_segflg = UIO_SYSSPACE;
        uiop->uio_td = curthread;

        /*
         * clear BIO_ERROR and B_INVAL state prior to initiating the I/O.  We
         * do this here so we do not have to do it in all the code that
         * calls us.
         */
        bp->b_flags &= ~B_INVAL;
        bp->b_ioflags &= ~BIO_ERROR;

        KASSERT(!(bp->b_flags & B_DONE),
            ("fuse_io_strategy: bp %p already marked done", bp));
        if (bp->b_iocmd == BIO_READ) {
                ssize_t left;

                io.iov_len = uiop->uio_resid = bp->b_bcount;
                io.iov_base = bp->b_data;
                uiop->uio_rw = UIO_READ;

                uiop->uio_offset = ((off_t)bp->b_lblkno) * biosize;
                error = fuse_read_directbackend(vp, uiop, cred, fufh);
                /* 
                 * Store the amount we failed to read in the buffer's private
                 * field, so callers can truncate the file if necessary'
                 */

                if (!error && uiop->uio_resid) {
                        int nread = bp->b_bcount - uiop->uio_resid;
                        left = uiop->uio_resid;
                        bzero((char *)bp->b_data + nread, left);

                        CACHED_ATTR_LOCK(vp);
                        if ((fvdat->flag & FN_SIZECHANGE) == 0) {
                                /*
                                 * A short read with no error, when not using
                                 * direct io, and when no writes are cached,
                                 * indicates EOF caused by a server-side
                                 * truncation.  Clear the attr cache so we'll
                                 * pick up the new file size and timestamps.
                                 *
                                 * We must still bzero the remaining buffer so
                                 * uninitialized data doesn't get exposed by a
                                 * future truncate that extends the file.
                                 * 
                                 * To prevent lock order problems, we must
                                 * truncate the file upstack, not here.
                                 */
                                SDT_PROBE2(fusefs, , io, trace, 1,
                                        "Short read of a clean file");
                                fuse_vnode_clear_attr_cache(vp);
                        } else {
                                /*
                                 * If dirty writes _are_ cached beyond EOF,
                                 * that indicates a newly created hole that the
                                 * server doesn't know about.  Those don't pose
                                 * any problem.
                                 * XXX: we don't currently track whether dirty
                                 * writes are cached beyond EOF, before EOF, or
                                 * both.
                                 */
                                SDT_PROBE2(fusefs, , io, trace, 1,
                                        "Short read of a dirty file");
                                uiop->uio_resid = 0;
                        }
                        CACHED_ATTR_UNLOCK(vp);
                }
                if (error) {
                        bp->b_ioflags |= BIO_ERROR;
                        bp->b_error = error;
                }
        } else {
                /*
                 * Setup for actual write
                 */
                /*
                 * If the file's size is cached, use that value, even if the
                 * cache is expired.  At this point we're already committed to
                 * writing something.  If the FUSE server has changed the
                 * file's size behind our back, it's too late for us to do
                 * anything about it.  In particular, we can't invalidate any
                 * part of the file's buffers because VOP_STRATEGY is called
                 * with them already locked.
                 *
                 * Normally the vnode should be exclusively locked at this
                 * point.  However, if clustered reads are in use, then in a
                 * mixed read-write workload getblkx may need to flush a
                 * partially written buffer to disk during a read.  In such a
                 * case, the vnode may only have a shared lock at this point.
                 */
                CACHED_ATTR_LOCK(vp);
                filesize = fvdat->cached_attrs.va_size;
                /* filesize must've been cached by fuse_vnop_open.  */
                KASSERT(filesize != VNOVAL, ("filesize should've been cached"));
                CACHED_ATTR_UNLOCK(vp);

                if ((off_t)bp->b_lblkno * biosize + bp->b_dirtyend > filesize)
                        bp->b_dirtyend = filesize - 
                                (off_t)bp->b_lblkno * biosize;

                if (bp->b_dirtyend > bp->b_dirtyoff) {
                        io.iov_len = uiop->uio_resid = bp->b_dirtyend
                            - bp->b_dirtyoff;
                        uiop->uio_offset = (off_t)bp->b_lblkno * biosize
                            + bp->b_dirtyoff;
                        io.iov_base = (char *)bp->b_data + bp->b_dirtyoff;
                        uiop->uio_rw = UIO_WRITE;

                        bool pages = bp->b_flags & B_FUSEFS_WRITE_CACHE;
                        error = fuse_write_directbackend(vp, uiop, cred, fufh,
                                filesize, 0, pages);

                        if (error == EINTR || error == ETIMEDOUT) {
                                bp->b_flags &= ~(B_INVAL | B_NOCACHE);
                                if ((bp->b_flags & B_PAGING) == 0) {
                                        bdirty(bp);
                                        bp->b_flags &= ~B_DONE;
                                }
                                if ((error == EINTR || error == ETIMEDOUT) &&
                                    (bp->b_flags & B_ASYNC) == 0)
                                        bp->b_flags |= B_EINTR;
                        } else {
                                if (error) {
                                        bp->b_ioflags |= BIO_ERROR;
                                        bp->b_flags |= B_INVAL;
                                        bp->b_error = error;
                                }
                                bp->b_dirtyoff = bp->b_dirtyend = 0;
                        }
                } else {
                        bp->b_resid = 0;
                        bufdone(bp);
                        return (0);
                }
        }
        bp->b_resid = uiop->uio_resid;
        bufdone(bp);
        return (error);
}

int
fuse_io_flushbuf(struct vnode *vp, int waitfor, struct thread *td)
{

        return (vn_fsync_buf(vp, waitfor));
}

/* Flush and invalidate all dirty buffers. */
int
fuse_io_invalbuf(struct vnode *vp, struct thread *td)
{
        if (VN_IS_DOOMED(vp))
                return 0;

        vnode_pager_clean_sync(vp);
        return (vinvalbuf(vp, V_SAVE, PCATCH, 0));
}