// SPDX-License-Identifier: GPL-2.0
/*
 * (C) 2001 Clemson University and The University of Chicago
 *
 * See COPYING in top-level directory.
 */
#include "protocol.h"
#include "orangefs-kernel.h"
#include "orangefs-bufmap.h"

struct slot_map {
        int c;
        wait_queue_head_t q;
        int count;
        unsigned long *map;
};

static struct slot_map rw_map = {
        .c = -1,
        .q = __WAIT_QUEUE_HEAD_INITIALIZER(rw_map.q)
};
static struct slot_map readdir_map = {
        .c = -1,
        .q = __WAIT_QUEUE_HEAD_INITIALIZER(readdir_map.q)
};


static void install(struct slot_map *m, int count, unsigned long *map)
{
        spin_lock(&m->q.lock);
        m->c = m->count = count;
        m->map = map;
        wake_up_all_locked(&m->q);
        spin_unlock(&m->q.lock);
}

static void mark_killed(struct slot_map *m)
{
        spin_lock(&m->q.lock);
        m->c -= m->count + 1;
        spin_unlock(&m->q.lock);
}

static void run_down(struct slot_map *m)
{
        DEFINE_WAIT(wait);
        spin_lock(&m->q.lock);
        if (m->c != -1) {
                for (;;) {
                        if (likely(list_empty(&wait.entry)))
                                __add_wait_queue_entry_tail(&m->q, &wait);
                        set_current_state(TASK_UNINTERRUPTIBLE);

                        if (m->c == -1)
                                break;

                        spin_unlock(&m->q.lock);
                        schedule();
                        spin_lock(&m->q.lock);
                }
                __remove_wait_queue(&m->q, &wait);
                __set_current_state(TASK_RUNNING);
        }
        m->map = NULL;
        spin_unlock(&m->q.lock);
}

static void put(struct slot_map *m, int slot)
{
        int v;
        spin_lock(&m->q.lock);
        __clear_bit(slot, m->map);
        v = ++m->c;
        if (v > 0)
                wake_up_locked(&m->q);
        if (unlikely(v == -1))     /* finished dying */
                wake_up_all_locked(&m->q);
        spin_unlock(&m->q.lock);
}

static int wait_for_free(struct slot_map *m)
{
        long left = slot_timeout_secs * HZ;
        DEFINE_WAIT(wait);

        do {
                long n = left, t;
                if (likely(list_empty(&wait.entry)))
                        __add_wait_queue_entry_tail_exclusive(&m->q, &wait);
                set_current_state(TASK_INTERRUPTIBLE);

                if (m->c > 0)
                        break;

                if (m->c < 0) {
                        /* we are waiting for map to be installed */
                        /* it would better be there soon, or we go away */
                        if (n > ORANGEFS_BUFMAP_WAIT_TIMEOUT_SECS * HZ)
                                n = ORANGEFS_BUFMAP_WAIT_TIMEOUT_SECS * HZ;
                }
                spin_unlock(&m->q.lock);
                t = schedule_timeout(n);
                spin_lock(&m->q.lock);
                if (unlikely(!t) && n != left && m->c < 0)
                        left = t;
                else
                        left = t + (left - n);
                if (signal_pending(current))
                        left = -EINTR;
        } while (left > 0);

        if (!list_empty(&wait.entry))
                list_del(&wait.entry);
        else if (left <= 0 && waitqueue_active(&m->q))
                __wake_up_locked_key(&m->q, TASK_INTERRUPTIBLE, NULL);
        __set_current_state(TASK_RUNNING);

        if (likely(left > 0))
                return 0;

        return left < 0 ? -EINTR : -ETIMEDOUT;
}

static int get(struct slot_map *m)
{
        int res = 0;
        spin_lock(&m->q.lock);
        if (unlikely(m->c <= 0))
                res = wait_for_free(m);
        if (likely(!res)) {
                m->c--;
                res = find_first_zero_bit(m->map, m->count);
                __set_bit(res, m->map);
        }
        spin_unlock(&m->q.lock);
        return res;
}

/* used to describe mapped buffers */
struct orangefs_bufmap_desc {
        void __user *uaddr;             /* user space address pointer */
        struct folio **folio_array;
        /*
         * folio_offsets could be needed when userspace sets custom
         * sizes in user_desc, or when folios aren't all backed by
         * 2MB THPs.
         */
        size_t *folio_offsets;
        int folio_count;
        bool is_two_2mib_chunks;
};

static struct orangefs_bufmap {
        int desc_size;
        int desc_shift;
        int desc_count;
        int total_size;
        int page_count;
        int folio_count;

        struct page **page_array;
        struct folio **folio_array;
        struct orangefs_bufmap_desc *desc_array;

        /* array to track usage of buffer descriptors */
        unsigned long *buffer_index_array;

        /* array to track usage of buffer descriptors for readdir */
#define N DIV_ROUND_UP(ORANGEFS_READDIR_DEFAULT_DESC_COUNT, BITS_PER_LONG)
        unsigned long readdir_index_array[N];
#undef N
} *__orangefs_bufmap;

static DEFINE_SPINLOCK(orangefs_bufmap_lock);

static void
orangefs_bufmap_unmap(struct orangefs_bufmap *bufmap)
{
        unpin_user_pages(bufmap->page_array, bufmap->page_count);
}

static void
orangefs_bufmap_free(struct orangefs_bufmap *bufmap)
{
        int i;

        if (!bufmap)
                return;

        for (i = 0; i < bufmap->desc_count; i++) {
                kfree(bufmap->desc_array[i].folio_array);
                kfree(bufmap->desc_array[i].folio_offsets);
                bufmap->desc_array[i].folio_array = NULL;
                bufmap->desc_array[i].folio_offsets = NULL;
        }
        kfree(bufmap->page_array);
        kfree(bufmap->desc_array);
        bitmap_free(bufmap->buffer_index_array);
        kfree(bufmap);
}

/*
 * XXX: Can the size and shift change while the caller gives up the
 * XXX: lock between calling this and doing something useful?
 */

int orangefs_bufmap_size_query(void)
{
        struct orangefs_bufmap *bufmap;
        int size = 0;
        spin_lock(&orangefs_bufmap_lock);
        bufmap = __orangefs_bufmap;
        if (bufmap)
                size = bufmap->desc_size;
        spin_unlock(&orangefs_bufmap_lock);
        return size;
}

static DECLARE_WAIT_QUEUE_HEAD(bufmap_waitq);
static DECLARE_WAIT_QUEUE_HEAD(readdir_waitq);

static struct orangefs_bufmap *
orangefs_bufmap_alloc(struct ORANGEFS_dev_map_desc *user_desc)
{
        struct orangefs_bufmap *bufmap;

        bufmap = kzalloc_obj(*bufmap);
        if (!bufmap)
                goto out;

        bufmap->total_size = user_desc->total_size;
        bufmap->desc_count = user_desc->count;
        bufmap->desc_size = user_desc->size;
        bufmap->desc_shift = ilog2(bufmap->desc_size);
        bufmap->page_count = bufmap->total_size / PAGE_SIZE;

        bufmap->buffer_index_array =
                bitmap_zalloc(bufmap->desc_count, GFP_KERNEL);
        if (!bufmap->buffer_index_array)
                goto out_free_bufmap;

        bufmap->desc_array =
                kzalloc_objs(struct orangefs_bufmap_desc, bufmap->desc_count);
        if (!bufmap->desc_array)
                goto out_free_index_array;

        /* allocate storage to track our page mappings */
        bufmap->page_array =
                kzalloc_objs(struct page *, bufmap->page_count);
        if (!bufmap->page_array)
                goto out_free_desc_array;

        /* allocate folio array. */
        bufmap->folio_array = kzalloc_objs(struct folio *, bufmap->page_count);
        if (!bufmap->folio_array)
                goto out_free_page_array;

        return bufmap;

out_free_page_array:
        kfree(bufmap->page_array);
out_free_desc_array:
        kfree(bufmap->desc_array);
out_free_index_array:
        bitmap_free(bufmap->buffer_index_array);
out_free_bufmap:
        kfree(bufmap);
out:
        return NULL;
}

static int orangefs_bufmap_group_folios(struct orangefs_bufmap *bufmap)
{
        int i = 0;
        int f = 0;
        int k;
        int num_pages;
        struct page *page;
        struct folio *folio;

        while (i < bufmap->page_count) {
                page = bufmap->page_array[i];
                folio = page_folio(page);
                num_pages = folio_nr_pages(folio);
                gossip_debug(GOSSIP_BUFMAP_DEBUG,
                        "%s: i:%d: num_pages:%d: \n", __func__, i, num_pages);

                for (k = 1; k < num_pages; k++) {
                        if (bufmap->page_array[i + k] != folio_page(folio, k)) {
                                gossip_err("%s: bad match,  i:%d: k:%d:\n",
                                        __func__, i, k);
                                return -EINVAL;
                        }
                }

                bufmap->folio_array[f++] = folio;
                i += num_pages;
        }

        bufmap->folio_count = f;
        pr_info("%s: Grouped %d folios from %d pages.\n",
                __func__,
                bufmap->folio_count,
                bufmap->page_count);
        return 0;
}

static int orangefs_bufmap_map(struct orangefs_bufmap *bufmap,
                                struct ORANGEFS_dev_map_desc *user_desc)
{
        int pages_per_desc = bufmap->desc_size / PAGE_SIZE;
        int ret;
        int i;
        int j;
        int current_folio;
        int desc_pages_needed;
        int desc_folio_count;
        int remaining_pages;
        int need_avail_min;
        int pages_assigned_to_this_desc;
        int allocated_descs = 0;
        size_t current_offset;
        size_t adjust_offset;
        struct folio *folio;

        /* map the pages */
        ret = pin_user_pages_fast((unsigned long)user_desc->ptr,
                bufmap->page_count,
                FOLL_WRITE,
                bufmap->page_array);

        if (ret < 0)
                return ret;

        if (ret != bufmap->page_count) {
                gossip_err("orangefs error: asked for %d pages, only got %d.\n",
                                bufmap->page_count, ret);
                for (i = 0; i < ret; i++)
                        unpin_user_page(bufmap->page_array[i]);
                return -ENOMEM;
        }

        /*
         * ideally we want to get kernel space pointers for each page, but
         * we can't kmap that many pages at once if highmem is being used.
         * so instead, we just kmap/kunmap the page address each time the
         * kaddr is needed.
         */
        for (i = 0; i < bufmap->page_count; i++)
                flush_dcache_page(bufmap->page_array[i]);

        /*
         * Group pages into folios.
         */
        ret = orangefs_bufmap_group_folios(bufmap);
        if (ret)
                goto unpin;

        pr_info("%s: desc_size=%d bytes (%d pages per desc), total folios=%d\n",
                        __func__, bufmap->desc_size, pages_per_desc,
                        bufmap->folio_count);

        current_folio = 0;
        remaining_pages = 0;
        current_offset = 0;
        for (i = 0; i < bufmap->desc_count; i++) {
                desc_pages_needed = pages_per_desc;
                desc_folio_count = 0;
                pages_assigned_to_this_desc = 0;
                bufmap->desc_array[i].is_two_2mib_chunks = false;

                /*
                 * We hope there was enough memory that each desc is
                 * covered by two THPs/folios, if not we want to keep on
                 * working even if there's only one page per folio.
                 */
                bufmap->desc_array[i].folio_array =
                        kzalloc_objs(struct folio *, pages_per_desc);
                if (!bufmap->desc_array[i].folio_array) {
                        ret = -ENOMEM;
                        goto unpin;
                }

                bufmap->desc_array[i].folio_offsets =
                        kzalloc_objs(size_t, pages_per_desc);
                if (!bufmap->desc_array[i].folio_offsets) {
                        ret = -ENOMEM;
                        kfree(bufmap->desc_array[i].folio_array);
                        bufmap->desc_array[i].folio_array = NULL;
                        goto unpin;
                }

                bufmap->desc_array[i].uaddr =
                        user_desc->ptr + (size_t)i * bufmap->desc_size;

                /*
                 * Accumulate folios until desc is full.
                 */
                while (desc_pages_needed > 0) {
                        if (remaining_pages == 0) {
                                /* shouldn't happen. */
                                if (current_folio >= bufmap->folio_count) {
                                        ret = -EINVAL;
                                        goto unpin;
                                }
                                folio = bufmap->folio_array[current_folio++];
                                remaining_pages = folio_nr_pages(folio);
                                current_offset = 0;
                        } else {
                                folio = bufmap->folio_array[current_folio - 1];
                        }

                        need_avail_min =
                                min(desc_pages_needed, remaining_pages);
                        adjust_offset = need_avail_min * PAGE_SIZE;

                        bufmap->desc_array[i].folio_array[desc_folio_count] =
                                folio;
                        bufmap->desc_array[i].folio_offsets[desc_folio_count] =
                                current_offset;
                        desc_folio_count++;
                        pages_assigned_to_this_desc += need_avail_min;
                        desc_pages_needed -= need_avail_min;
                        remaining_pages -= need_avail_min;
                        current_offset += adjust_offset;
                }

                /* Detect optimal case: two 2MiB folios per 4MiB slot. */
                if (desc_folio_count == 2 &&
                  folio_nr_pages(bufmap->desc_array[i].folio_array[0]) == 512 &&
                  folio_nr_pages(bufmap->desc_array[i].folio_array[1]) == 512) {
                        bufmap->desc_array[i].is_two_2mib_chunks = true;
                        gossip_debug(GOSSIP_BUFMAP_DEBUG, "%s: descriptor :%d: "
                                "optimal folio/page ratio.\n", __func__, i);
                }

                bufmap->desc_array[i].folio_count = desc_folio_count;
                gossip_debug(GOSSIP_BUFMAP_DEBUG,
                        " descriptor %d: folio_count=%d, "
                        "pages_assigned=%d (should be %d)\n",
                        i, desc_folio_count, pages_assigned_to_this_desc,
                        pages_per_desc);

                allocated_descs = i + 1;
        }

        return 0;
unpin:
        /*
         * rollback any allocations we got so far...
         * Memory pressure, like in generic/340, led me
         * to write the rollback this way.
         */
        for (j = 0; j < allocated_descs; j++) {
                if (bufmap->desc_array[j].folio_array) {
                        kfree(bufmap->desc_array[j].folio_array);
                        bufmap->desc_array[j].folio_array = NULL;
                }
                if (bufmap->desc_array[j].folio_offsets) {
                        kfree(bufmap->desc_array[j].folio_offsets);
                        bufmap->desc_array[j].folio_offsets = NULL;
                }
        }
        unpin_user_pages(bufmap->page_array, bufmap->page_count);
        return ret;
}

/*
 * orangefs_bufmap_initialize()
 *
 * initializes the mapped buffer interface
 *
 * user_desc is the parameters provided by userspace for the bufmap.
 *
 * returns 0 on success, -errno on failure
 */
int orangefs_bufmap_initialize(struct ORANGEFS_dev_map_desc *user_desc)
{
        struct orangefs_bufmap *bufmap;
        int ret = -EINVAL;

        gossip_debug(GOSSIP_BUFMAP_DEBUG,
                     "%s: called (ptr (" "%p) sz (%d) cnt(%d).\n",
                     __func__,
                     user_desc->ptr,
                     user_desc->size,
                     user_desc->count);

        if (user_desc->total_size < 0 ||
            user_desc->size < 0 ||
            user_desc->count < 0)
                goto out;

        /*
         * sanity check alignment and size of buffer that caller wants to
         * work with
         */
        if (PAGE_ALIGN((unsigned long)user_desc->ptr) !=
            (unsigned long)user_desc->ptr) {
                gossip_err("orangefs error: memory alignment (front). %p\n",
                           user_desc->ptr);
                goto out;
        }

        if (PAGE_ALIGN(((unsigned long)user_desc->ptr + user_desc->total_size))
            != (unsigned long)(user_desc->ptr + user_desc->total_size)) {
                gossip_err("orangefs error: memory alignment (back).(%p + %d)\n",
                           user_desc->ptr,
                           user_desc->total_size);
                goto out;
        }

        if (user_desc->total_size != (user_desc->size * user_desc->count)) {
                gossip_err("orangefs error: user provided an oddly sized buffer: (%d, %d, %d)\n",
                           user_desc->total_size,
                           user_desc->size,
                           user_desc->count);
                goto out;
        }

        if ((user_desc->size % PAGE_SIZE) != 0) {
                gossip_err("orangefs error: bufmap size not page size divisible (%d).\n",
                           user_desc->size);
                goto out;
        }

        ret = -ENOMEM;
        bufmap = orangefs_bufmap_alloc(user_desc);
        if (!bufmap)
                goto out;

        ret = orangefs_bufmap_map(bufmap, user_desc);
        if (ret)
                goto out_free_bufmap;


        spin_lock(&orangefs_bufmap_lock);
        if (__orangefs_bufmap) {
                spin_unlock(&orangefs_bufmap_lock);
                gossip_err("orangefs: error: bufmap already initialized.\n");
                ret = -EINVAL;
                goto out_unmap_bufmap;
        }
        __orangefs_bufmap = bufmap;
        install(&rw_map,
                bufmap->desc_count,
                bufmap->buffer_index_array);
        install(&readdir_map,
                ORANGEFS_READDIR_DEFAULT_DESC_COUNT,
                bufmap->readdir_index_array);
        spin_unlock(&orangefs_bufmap_lock);

        gossip_debug(GOSSIP_BUFMAP_DEBUG,
                     "%s: exiting normally\n", __func__);
        return 0;

out_unmap_bufmap:
        orangefs_bufmap_unmap(bufmap);
out_free_bufmap:
        orangefs_bufmap_free(bufmap);
out:
        return ret;
}

/*
 * orangefs_bufmap_finalize()
 *
 * shuts down the mapped buffer interface and releases any resources
 * associated with it
 *
 * no return value
 */
void orangefs_bufmap_finalize(void)
{
        struct orangefs_bufmap *bufmap = __orangefs_bufmap;
        if (!bufmap)
                return;
        gossip_debug(GOSSIP_BUFMAP_DEBUG, "orangefs_bufmap_finalize: called\n");
        mark_killed(&rw_map);
        mark_killed(&readdir_map);
        gossip_debug(GOSSIP_BUFMAP_DEBUG,
                     "orangefs_bufmap_finalize: exiting normally\n");
}

void orangefs_bufmap_run_down(void)
{
        struct orangefs_bufmap *bufmap = __orangefs_bufmap;
        if (!bufmap)
                return;
        run_down(&rw_map);
        run_down(&readdir_map);
        spin_lock(&orangefs_bufmap_lock);
        __orangefs_bufmap = NULL;
        spin_unlock(&orangefs_bufmap_lock);
        orangefs_bufmap_unmap(bufmap);
        orangefs_bufmap_free(bufmap);
}

/*
 * orangefs_bufmap_get()
 *
 * gets a free mapped buffer descriptor, will sleep until one becomes
 * available if necessary
 *
 * returns slot on success, -errno on failure
 */
int orangefs_bufmap_get(void)
{
        return get(&rw_map);
}

/*
 * orangefs_bufmap_put()
 *
 * returns a mapped buffer descriptor to the collection
 *
 * no return value
 */
void orangefs_bufmap_put(int buffer_index)
{
        put(&rw_map, buffer_index);
}

/*
 * orangefs_readdir_index_get()
 *
 * gets a free descriptor, will sleep until one becomes
 * available if necessary.
 * Although the readdir buffers are not mapped into kernel space
 * we could do that at a later point of time. Regardless, these
 * indices are used by the client-core.
 *
 * returns slot on success, -errno on failure
 */
int orangefs_readdir_index_get(void)
{
        return get(&readdir_map);
}

void orangefs_readdir_index_put(int buffer_index)
{
        put(&readdir_map, buffer_index);
}

/*
 * we've been handed an iovec, we need to copy it to
 * the shared memory descriptor at "buffer_index".
 */
int orangefs_bufmap_copy_from_iovec(struct iov_iter *iter,
                                int buffer_index,
                                size_t size)
{
        struct orangefs_bufmap_desc *to;
        size_t remaining = size;
        int folio_index = 0;
        struct folio *folio;
        size_t folio_offset;
        size_t folio_avail;
        size_t copy_amount;
        size_t copied;
        void *kaddr;
        size_t half;
        size_t first;
        size_t second;

        to = &__orangefs_bufmap->desc_array[buffer_index];

        /* shouldn't happen... */
        if (size > 4194304)
                pr_info("%s: size:%zu\n", __func__, size);

        gossip_debug(GOSSIP_BUFMAP_DEBUG,
                "%s: buffer_index:%d size:%zu folio_count:%d\n",
                __func__,
                buffer_index,
                size,
                to->folio_count);

        /* Fast path: exactly two 2 MiB folios */
        if (to->is_two_2mib_chunks && size <= 4194304) {
                gossip_debug(GOSSIP_BUFMAP_DEBUG,
                        "%s: fastpath hit.\n", __func__);
                half = 2097152;         /* 2 MiB */
                first = min(size, half);
                second = (size > half) ? size - half : 0;

                /* First 2 MiB chunk */
                kaddr = kmap_local_folio(to->folio_array[0], 0);
                copied = copy_from_iter(kaddr, first, iter);
                kunmap_local(kaddr);
                if (copied != first)
                        return -EFAULT;

                if (second == 0)
                        return 0;

                /* Second 2 MiB chunk */
                kaddr = kmap_local_folio(to->folio_array[1], 0);
                copied = copy_from_iter(kaddr, second, iter);
                kunmap_local(kaddr);
                if (copied != second)
                        return -EFAULT;

                return 0;
        }

        while (remaining > 0) {

                if (unlikely(folio_index >= to->folio_count ||
                        to->folio_array[folio_index] == NULL)) {
                                gossip_err("%s: "
                                   "folio_index:%d: >= folio_count:%d: "
                                   "(size %zu, buffer %d)\n",
                                        __func__,
                                        folio_index,
                                        to->folio_count,
                                        size,
                                        buffer_index);
                                return -EFAULT;
                }

                folio = to->folio_array[folio_index];
                folio_offset = to->folio_offsets[folio_index];
                folio_avail = folio_nr_pages(folio) * PAGE_SIZE - folio_offset;
                copy_amount = min(remaining, folio_avail);
                kaddr = kmap_local_folio(folio, folio_offset);
                copied = copy_from_iter(kaddr, copy_amount, iter);
                kunmap_local(kaddr);

                if (copied != copy_amount)
                        return -EFAULT;

                remaining -= copied;
                folio_index++;
        }

        return 0;
}

/*
 * we've been handed an iovec, we need to fill it from
 * the shared memory descriptor at "buffer_index".
 */
int orangefs_bufmap_copy_to_iovec(struct iov_iter *iter,
                                    int buffer_index,
                                    size_t size)
{
        struct orangefs_bufmap_desc *from;
        size_t remaining = size;
        int folio_index = 0;
        struct folio *folio;
        size_t folio_offset;
        size_t folio_avail;
        size_t copy_amount;
        size_t copied;
        void *kaddr;
        size_t half;
        size_t first;
        size_t second;

        from = &__orangefs_bufmap->desc_array[buffer_index];

        /* shouldn't happen... */
        if (size > 4194304)
                pr_info("%s: size:%zu\n", __func__, size);

        gossip_debug(GOSSIP_BUFMAP_DEBUG,
                "%s: buffer_index:%d size:%zu folio_count:%d\n",
                __func__,
                buffer_index,
                size,
                from->folio_count);

        /* Fast path: exactly two 2 MiB folios */
        if (from->is_two_2mib_chunks && size <= 4194304) {
                gossip_debug(GOSSIP_BUFMAP_DEBUG,
                        "%s: fastpath hit.\n", __func__);
                half = 2097152;         /* 2 MiB */
                first = min(size, half);
                second = (size > half) ? size - half : 0;
                void *kaddr;
                size_t copied;

                /* First 2 MiB chunk */
                kaddr = kmap_local_folio(from->folio_array[0], 0);
                copied = copy_to_iter(kaddr, first, iter);
                kunmap_local(kaddr);
                if (copied != first)
                        return -EFAULT;

                if (second == 0)
                        return 0;

                /* Second 2 MiB chunk */
                kaddr = kmap_local_folio(from->folio_array[1], 0);
                copied = copy_to_iter(kaddr, second, iter);
                kunmap_local(kaddr);
                if (copied != second)
                        return -EFAULT;

                return 0;
        }

        while (remaining > 0) {

                if (unlikely(folio_index >= from->folio_count ||
                        from->folio_array[folio_index] == NULL)) {
                                gossip_err("%s: "
                                   "folio_index:%d: >= folio_count:%d: "
                                   "(size %zu, buffer %d)\n",
                                        __func__,
                                        folio_index,
                                        from->folio_count,
                                        size,
                                        buffer_index);
                                return -EFAULT;
                }

                folio = from->folio_array[folio_index];
                folio_offset = from->folio_offsets[folio_index];
                folio_avail = folio_nr_pages(folio) * PAGE_SIZE - folio_offset;
                copy_amount = min(remaining, folio_avail);

                kaddr = kmap_local_folio(folio, folio_offset);
                copied = copy_to_iter(kaddr, copy_amount, iter);
                kunmap_local(kaddr);

                if (copied != copy_amount)
                        return -EFAULT;

                remaining -= copied;
                folio_index++;
        }

        return 0;
}