block/preallocate.c - third_party/qemu - Git at Google

 /*
  * preallocate filter driver
  *
  * The driver performs preallocate operation: it is injected above
  * some node, and before each write over EOF it does additional preallocating
  * write-zeroes request.
  *
  * Copyright (c) 2020 Virtuozzo International GmbH.
  *
  * Author:
  *  Sementsov-Ogievskiy Vladimir <vsementsov@virtuozzo.com>
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
  * the Free Software Foundation; either version 2 of the License, or
  * (at your option) any later version.
  *
  * This program is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  * GNU General Public License for more details.
  *
  * You should have received a copy of the GNU General Public License
  * along with this program. If not, see <http://www.gnu.org/licenses/>.
  */

 #include "qemu/osdep.h"

 #include "qapi/error.h"
 #include "qemu/module.h"
 #include "qemu/option.h"
 #include "qemu/units.h"
 #include "block/block_int.h"


 typedef struct PreallocateOpts {
     int64_t prealloc_size;
     int64_t prealloc_align;
 } PreallocateOpts;

 typedef struct BDRVPreallocateState {
     PreallocateOpts opts;

     /*
      * Track real data end, to crop preallocation on close. If < 0 the status is
      * unknown.
      *
      * @data_end is a maximum of file size on open (or when we get write/resize
      * permissions) and all write request ends after it. So it's safe to
      * truncate to data_end if it is valid.
      */
     int64_t data_end;

     /*
      * Start of trailing preallocated area which reads as zero. May be smaller
      * than data_end, if user does over-EOF write zero operation. If < 0 the
      * status is unknown.
      *
      * If both @zero_start and @file_end are valid, the region
      * [@zero_start, @file_end) is known to be preallocated zeroes. If @file_end
      * is not valid, @zero_start doesn't make much sense.
      */
     int64_t zero_start;

     /*
      * Real end of file. Actually the cache for bdrv_getlength(bs->file->bs),
      * to avoid extra lseek() calls on each write operation. If < 0 the status
      * is unknown.
      */
     int64_t file_end;

     /*
      * All three states @data_end, @zero_start and @file_end are guaranteed to
      * be invalid (< 0) when we don't have both exclusive BLK_PERM_RESIZE and
      * BLK_PERM_WRITE permissions on file child.
      */
 } BDRVPreallocateState;

 #define PREALLOCATE_OPT_PREALLOC_ALIGN "prealloc-align"
 #define PREALLOCATE_OPT_PREALLOC_SIZE "prealloc-size"
 static QemuOptsList runtime_opts = {
     .name = "preallocate",
     .head = QTAILQ_HEAD_INITIALIZER(runtime_opts.head),
     .desc = {
         {
             .name = PREALLOCATE_OPT_PREALLOC_ALIGN,
             .type = QEMU_OPT_SIZE,
             .help = "on preallocation, align file length to this number, "
                 "default 1M",
         },
         {
             .name = PREALLOCATE_OPT_PREALLOC_SIZE,
             .type = QEMU_OPT_SIZE,
             .help = "how much to preallocate, default 128M",
         },
         { /* end of list */ }
     },
 };

 static bool preallocate_absorb_opts(PreallocateOpts *dest, QDict *options,
                                     BlockDriverState *child_bs, Error **errp)
 {
     QemuOpts *opts = qemu_opts_create(&runtime_opts, NULL, 0, &error_abort);

     if (!qemu_opts_absorb_qdict(opts, options, errp)) {
         return false;
     }

     dest->prealloc_align =
         qemu_opt_get_size(opts, PREALLOCATE_OPT_PREALLOC_ALIGN, 1 * MiB);
     dest->prealloc_size =
         qemu_opt_get_size(opts, PREALLOCATE_OPT_PREALLOC_SIZE, 128 * MiB);

     qemu_opts_del(opts);

     if (!QEMU_IS_ALIGNED(dest->prealloc_align, BDRV_SECTOR_SIZE)) {
         error_setg(errp, "prealloc-align parameter of preallocate filter "
                    "is not aligned to %llu", BDRV_SECTOR_SIZE);
         return false;
     }

     if (!QEMU_IS_ALIGNED(dest->prealloc_align,
                          child_bs->bl.request_alignment)) {
         error_setg(errp, "prealloc-align parameter of preallocate filter "
                    "is not aligned to underlying node request alignment "
                    "(%" PRIi32 ")", child_bs->bl.request_alignment);
         return false;
     }

     return true;
 }

 static int preallocate_open(BlockDriverState *bs, QDict *options, int flags,
                             Error **errp)
 {
     BDRVPreallocateState *s = bs->opaque;

     /*
      * s->data_end and friends should be initialized on permission update.
      * For this to work, mark them invalid.
      */
     s->file_end = s->zero_start = s->data_end = -EINVAL;

     bs->file = bdrv_open_child(NULL, options, "file", bs, &child_of_bds,
                                BDRV_CHILD_FILTERED | BDRV_CHILD_PRIMARY,
                                false, errp);
     if (!bs->file) {
         return -EINVAL;
     }

     if (!preallocate_absorb_opts(&s->opts, options, bs->file->bs, errp)) {
         return -EINVAL;
     }

     bs->supported_write_flags = BDRV_REQ_WRITE_UNCHANGED |
         (BDRV_REQ_FUA & bs->file->bs->supported_write_flags);

     bs->supported_zero_flags = BDRV_REQ_WRITE_UNCHANGED |
         ((BDRV_REQ_FUA | BDRV_REQ_MAY_UNMAP | BDRV_REQ_NO_FALLBACK) &
             bs->file->bs->supported_zero_flags);

     return 0;
 }

 static void preallocate_close(BlockDriverState *bs)
 {
     int ret;
     BDRVPreallocateState *s = bs->opaque;

     if (s->data_end < 0) {
         return;
     }

     if (s->file_end < 0) {
         s->file_end = bdrv_getlength(bs->file->bs);
         if (s->file_end < 0) {
             return;
         }
     }

     if (s->data_end < s->file_end) {
         ret = bdrv_truncate(bs->file, s->data_end, true, PREALLOC_MODE_OFF, 0,
                             NULL);
         s->file_end = ret < 0 ? ret : s->data_end;
     }
 }


 /*
  * Handle reopen.
  *
  * We must implement reopen handlers, otherwise reopen just don't work. Handle
  * new options and don't care about preallocation state, as it is handled in
  * set/check permission handlers.
  */

 static int preallocate_reopen_prepare(BDRVReopenState *reopen_state,
                                       BlockReopenQueue *queue, Error **errp)
 {
     PreallocateOpts *opts = g_new0(PreallocateOpts, 1);

     if (!preallocate_absorb_opts(opts, reopen_state->options,
                                  reopen_state->bs->file->bs, errp)) {
         g_free(opts);
         return -EINVAL;
     }

     reopen_state->opaque = opts;

     return 0;
 }

 static void preallocate_reopen_commit(BDRVReopenState *state)
 {
     BDRVPreallocateState *s = state->bs->opaque;

     s->opts = *(PreallocateOpts *)state->opaque;

     g_free(state->opaque);
     state->opaque = NULL;
 }

 static void preallocate_reopen_abort(BDRVReopenState *state)
 {
     g_free(state->opaque);
     state->opaque = NULL;
 }

 static coroutine_fn int preallocate_co_preadv_part(
         BlockDriverState *bs, int64_t offset, int64_t bytes,
         QEMUIOVector *qiov, size_t qiov_offset, BdrvRequestFlags flags)
 {
     return bdrv_co_preadv_part(bs->file, offset, bytes, qiov, qiov_offset,
                                flags);
 }

 static int coroutine_fn preallocate_co_pdiscard(BlockDriverState *bs,
                                                int64_t offset, int64_t bytes)
 {
     return bdrv_co_pdiscard(bs->file, offset, bytes);
 }

 static bool can_write_resize(uint64_t perm)
 {
     return (perm & BLK_PERM_WRITE) && (perm & BLK_PERM_RESIZE);
 }

 static bool has_prealloc_perms(BlockDriverState *bs)
 {
     BDRVPreallocateState *s = bs->opaque;

     if (can_write_resize(bs->file->perm)) {
         assert(!(bs->file->shared_perm & BLK_PERM_WRITE));
         assert(!(bs->file->shared_perm & BLK_PERM_RESIZE));
         return true;
     }

     assert(s->data_end < 0);
     assert(s->zero_start < 0);
     assert(s->file_end < 0);
     return false;
 }

 /*
  * Call on each write. Returns true if @want_merge_zero is true and the region
  * [offset, offset + bytes) is zeroed (as a result of this call or earlier
  * preallocation).
  *
  * want_merge_zero is used to merge write-zero request with preallocation in
  * one bdrv_co_pwrite_zeroes() call.
  */
 static bool coroutine_fn handle_write(BlockDriverState *bs, int64_t offset,
                                       int64_t bytes, bool want_merge_zero)
 {
     BDRVPreallocateState *s = bs->opaque;
     int64_t end = offset + bytes;
     int64_t prealloc_start, prealloc_end;
     int ret;
     uint32_t file_align = bs->file->bs->bl.request_alignment;
     uint32_t prealloc_align = MAX(s->opts.prealloc_align, file_align);

     assert(QEMU_IS_ALIGNED(prealloc_align, file_align));

     if (!has_prealloc_perms(bs)) {
         /* We don't have state neither should try to recover it */
         return false;
     }

     if (s->data_end < 0) {
         s->data_end = bdrv_getlength(bs->file->bs);
         if (s->data_end < 0) {
             return false;
         }

         if (s->file_end < 0) {
             s->file_end = s->data_end;
         }
     }

     if (end <= s->data_end) {
         return false;
     }

     /* We have valid s->data_end, and request writes beyond it. */

     s->data_end = end;
     if (s->zero_start < 0 || !want_merge_zero) {
         s->zero_start = end;
     }

     if (s->file_end < 0) {
         s->file_end = bdrv_getlength(bs->file->bs);
         if (s->file_end < 0) {
             return false;
         }
     }

     /* Now s->data_end, s->zero_start and s->file_end are valid. */

     if (end <= s->file_end) {
         /* No preallocation needed. */
         return want_merge_zero && offset >= s->zero_start;
     }

     /* Now we want new preallocation, as request writes beyond s->file_end. */

     prealloc_start = QEMU_ALIGN_UP(
             want_merge_zero ? MIN(offset, s->file_end) : s->file_end,
             file_align);
     prealloc_end = QEMU_ALIGN_UP(
             MAX(prealloc_start, end) + s->opts.prealloc_size,
             prealloc_align);

     want_merge_zero = want_merge_zero && (prealloc_start <= offset);

     ret = bdrv_co_pwrite_zeroes(
             bs->file, prealloc_start, prealloc_end - prealloc_start,
             BDRV_REQ_NO_FALLBACK | BDRV_REQ_SERIALISING | BDRV_REQ_NO_WAIT);
     if (ret < 0) {
         s->file_end = ret;
         return false;
     }

     s->file_end = prealloc_end;
     return want_merge_zero;
 }

 static int coroutine_fn preallocate_co_pwrite_zeroes(BlockDriverState *bs,
         int64_t offset, int64_t bytes, BdrvRequestFlags flags)
 {
     bool want_merge_zero =
         !(flags & ~(BDRV_REQ_ZERO_WRITE | BDRV_REQ_NO_FALLBACK));
     if (handle_write(bs, offset, bytes, want_merge_zero)) {
         return 0;
     }

     return bdrv_co_pwrite_zeroes(bs->file, offset, bytes, flags);
 }

 static coroutine_fn int preallocate_co_pwritev_part(BlockDriverState *bs,
                                                     int64_t offset,
                                                     int64_t bytes,
                                                     QEMUIOVector *qiov,
                                                     size_t qiov_offset,
                                                     BdrvRequestFlags flags)
 {
     handle_write(bs, offset, bytes, false);

     return bdrv_co_pwritev_part(bs->file, offset, bytes, qiov, qiov_offset,
                                 flags);
 }

 static int coroutine_fn
 preallocate_co_truncate(BlockDriverState *bs, int64_t offset,
                         bool exact, PreallocMode prealloc,
                         BdrvRequestFlags flags, Error **errp)
 {
     ERRP_GUARD();
     BDRVPreallocateState *s = bs->opaque;
     int ret;

     if (s->data_end >= 0 && offset > s->data_end) {
         if (s->file_end < 0) {
             s->file_end = bdrv_getlength(bs->file->bs);
             if (s->file_end < 0) {
                 error_setg(errp, "failed to get file length");
                 return s->file_end;
             }
         }

         if (prealloc == PREALLOC_MODE_FALLOC) {
             /*
              * If offset <= s->file_end, the task is already done, just
              * update s->data_end, to move part of "filter preallocation"
              * to "preallocation requested by user".
              * Otherwise just proceed to preallocate missing part.
              */
             if (offset <= s->file_end) {
                 s->data_end = offset;
                 return 0;
             }
         } else {
             /*
              * We have to drop our preallocation, to
              * - avoid "Cannot use preallocation for shrinking files" in
              *   case of offset < file_end
              * - give PREALLOC_MODE_OFF a chance to keep small disk
              *   usage
              * - give PREALLOC_MODE_FULL a chance to actually write the
              *   whole region as user expects
              */
             if (s->file_end > s->data_end) {
                 ret = bdrv_co_truncate(bs->file, s->data_end, true,
                                        PREALLOC_MODE_OFF, 0, errp);
                 if (ret < 0) {
                     s->file_end = ret;
                     error_prepend(errp, "preallocate-filter: failed to drop "
                                   "write-zero preallocation: ");
                     return ret;
                 }
                 s->file_end = s->data_end;
             }
         }

         s->data_end = offset;
     }

     ret = bdrv_co_truncate(bs->file, offset, exact, prealloc, flags, errp);
     if (ret < 0) {
         s->file_end = s->zero_start = s->data_end = ret;
         return ret;
     }

     if (has_prealloc_perms(bs)) {
         s->file_end = s->zero_start = s->data_end = offset;
     }
     return 0;
 }

 static int coroutine_fn preallocate_co_flush(BlockDriverState *bs)
 {
     return bdrv_co_flush(bs->file->bs);
 }

 static int64_t preallocate_getlength(BlockDriverState *bs)
 {
     int64_t ret;
     BDRVPreallocateState *s = bs->opaque;

     if (s->data_end >= 0) {
         return s->data_end;
     }

     ret = bdrv_getlength(bs->file->bs);

     if (has_prealloc_perms(bs)) {
         s->file_end = s->zero_start = s->data_end = ret;
     }

     return ret;
 }

 static int preallocate_check_perm(BlockDriverState *bs,
                                   uint64_t perm, uint64_t shared, Error **errp)
 {
     BDRVPreallocateState *s = bs->opaque;

     if (s->data_end >= 0 && !can_write_resize(perm)) {
         /*
          * Lose permissions.
          * We should truncate in check_perm, as in set_perm bs->file->perm will
          * be already changed, and we should not violate it.
          */
         if (s->file_end < 0) {
             s->file_end = bdrv_getlength(bs->file->bs);
             if (s->file_end < 0) {
                 error_setg(errp, "Failed to get file length");
                 return s->file_end;
             }
         }

         if (s->data_end < s->file_end) {
             int ret = bdrv_truncate(bs->file, s->data_end, true,
                                     PREALLOC_MODE_OFF, 0, NULL);
             if (ret < 0) {
                 error_setg(errp, "Failed to drop preallocation");
                 s->file_end = ret;
                 return ret;
             }
             s->file_end = s->data_end;
         }
     }

     return 0;
 }

 static void preallocate_set_perm(BlockDriverState *bs,
                                  uint64_t perm, uint64_t shared)
 {
     BDRVPreallocateState *s = bs->opaque;

     if (can_write_resize(perm)) {
         if (s->data_end < 0) {
             s->data_end = s->file_end = s->zero_start =
                 bdrv_getlength(bs->file->bs);
         }
     } else {
         /*
          * We drop our permissions, as well as allow shared
          * permissions (see preallocate_child_perm), anyone will be able to
          * change the child, so mark all states invalid. We'll regain control if
          * get good permissions back.
          */
         s->data_end = s->file_end = s->zero_start = -EINVAL;
     }
 }

 static void preallocate_child_perm(BlockDriverState *bs, BdrvChild *c,
     BdrvChildRole role, BlockReopenQueue *reopen_queue,
     uint64_t perm, uint64_t shared, uint64_t *nperm, uint64_t *nshared)
 {
     bdrv_default_perms(bs, c, role, reopen_queue, perm, shared, nperm, nshared);

     if (can_write_resize(perm)) {
         /* This should come by default, but let's enforce: */
         *nperm |= BLK_PERM_WRITE | BLK_PERM_RESIZE;

         /*
          * Don't share, to keep our states s->file_end, s->data_end and
          * s->zero_start valid.
          */
         *nshared &= ~(BLK_PERM_WRITE | BLK_PERM_RESIZE);
     }
 }

 BlockDriver bdrv_preallocate_filter = {
     .format_name = "preallocate",
     .instance_size = sizeof(BDRVPreallocateState),

     .bdrv_getlength = preallocate_getlength,
     .bdrv_open = preallocate_open,
     .bdrv_close = preallocate_close,

     .bdrv_reopen_prepare  = preallocate_reopen_prepare,
     .bdrv_reopen_commit   = preallocate_reopen_commit,
     .bdrv_reopen_abort    = preallocate_reopen_abort,

     .bdrv_co_preadv_part = preallocate_co_preadv_part,
     .bdrv_co_pwritev_part = preallocate_co_pwritev_part,
     .bdrv_co_pwrite_zeroes = preallocate_co_pwrite_zeroes,
     .bdrv_co_pdiscard = preallocate_co_pdiscard,
     .bdrv_co_flush = preallocate_co_flush,
     .bdrv_co_truncate = preallocate_co_truncate,

     .bdrv_check_perm = preallocate_check_perm,
     .bdrv_set_perm = preallocate_set_perm,
     .bdrv_child_perm = preallocate_child_perm,

     .has_variable_length = true,
     .is_filter = true,
 };

 static void bdrv_preallocate_init(void)
 {
     bdrv_register(&bdrv_preallocate_filter);
 }

 block_init(bdrv_preallocate_init);
	/*
	* preallocate filter driver
	*
	* The driver performs preallocate operation: it is injected above
	* some node, and before each write over EOF it does additional preallocating
	* write-zeroes request.
	*
	* Copyright (c) 2020 Virtuozzo International GmbH.
	*
	* Author:
	* Sementsov-Ogievskiy Vladimir <vsementsov@virtuozzo.com>
	*
	* This program is free software; you can redistribute it and/or modify
	* it under the terms of the GNU General Public License as published by
	* the Free Software Foundation; either version 2 of the License, or
	* (at your option) any later version.
	*
	* This program is distributed in the hope that it will be useful,
	* but WITHOUT ANY WARRANTY; without even the implied warranty of
	* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
	* GNU General Public License for more details.
	*
	* You should have received a copy of the GNU General Public License
	* along with this program. If not, see <http://www.gnu.org/licenses/>.
	*/

	#include "qemu/osdep.h"

	#include "qapi/error.h"
	#include "qemu/module.h"
	#include "qemu/option.h"
	#include "qemu/units.h"
	#include "block/block_int.h"


	typedef struct PreallocateOpts {
	int64_t prealloc_size;
	int64_t prealloc_align;
	} PreallocateOpts;

	typedef struct BDRVPreallocateState {
	PreallocateOpts opts;

	/*
	* Track real data end, to crop preallocation on close. If < 0 the status is
	* unknown.
	*
	* @data_end is a maximum of file size on open (or when we get write/resize
	* permissions) and all write request ends after it. So it's safe to
	* truncate to data_end if it is valid.
	*/
	int64_t data_end;

	/*
	* Start of trailing preallocated area which reads as zero. May be smaller
	* than data_end, if user does over-EOF write zero operation. If < 0 the
	* status is unknown.
	*
	* If both @zero_start and @file_end are valid, the region
	* [@zero_start, @file_end) is known to be preallocated zeroes. If @file_end
	* is not valid, @zero_start doesn't make much sense.
	*/
	int64_t zero_start;

	/*
	* Real end of file. Actually the cache for bdrv_getlength(bs->file->bs),
	* to avoid extra lseek() calls on each write operation. If < 0 the status
	* is unknown.
	*/
	int64_t file_end;

	/*
	* All three states @data_end, @zero_start and @file_end are guaranteed to
	* be invalid (< 0) when we don't have both exclusive BLK_PERM_RESIZE and
	* BLK_PERM_WRITE permissions on file child.
	*/
	} BDRVPreallocateState;

	#define PREALLOCATE_OPT_PREALLOC_ALIGN "prealloc-align"
	#define PREALLOCATE_OPT_PREALLOC_SIZE "prealloc-size"
	static QemuOptsList runtime_opts = {
	.name = "preallocate",
	.head = QTAILQ_HEAD_INITIALIZER(runtime_opts.head),
	.desc = {
	{
	.name = PREALLOCATE_OPT_PREALLOC_ALIGN,
	.type = QEMU_OPT_SIZE,
	.help = "on preallocation, align file length to this number, "
	"default 1M",
	},
	{
	.name = PREALLOCATE_OPT_PREALLOC_SIZE,
	.type = QEMU_OPT_SIZE,
	.help = "how much to preallocate, default 128M",
	},
	{ /* end of list */ }
	},
	};

	static bool preallocate_absorb_opts(PreallocateOpts dest, QDict options,
	BlockDriverState child_bs, Error *errp)
	{
	QemuOpts *opts = qemu_opts_create(&runtime_opts, NULL, 0, &error_abort);

	if (!qemu_opts_absorb_qdict(opts, options, errp)) {
	return false;
	}

	dest->prealloc_align =
	qemu_opt_get_size(opts, PREALLOCATE_OPT_PREALLOC_ALIGN, 1 * MiB);
	dest->prealloc_size =
	qemu_opt_get_size(opts, PREALLOCATE_OPT_PREALLOC_SIZE, 128 * MiB);

	qemu_opts_del(opts);

	if (!QEMU_IS_ALIGNED(dest->prealloc_align, BDRV_SECTOR_SIZE)) {
	error_setg(errp, "prealloc-align parameter of preallocate filter "
	"is not aligned to %llu", BDRV_SECTOR_SIZE);
	return false;
	}

	if (!QEMU_IS_ALIGNED(dest->prealloc_align,
	child_bs->bl.request_alignment)) {
	error_setg(errp, "prealloc-align parameter of preallocate filter "
	"is not aligned to underlying node request alignment "
	"(%" PRIi32 ")", child_bs->bl.request_alignment);
	return false;
	}

	return true;
	}

	static int preallocate_open(BlockDriverState bs, QDict options, int flags,
	Error **errp)
	{
	BDRVPreallocateState *s = bs->opaque;

	/*
	* s->data_end and friends should be initialized on permission update.
	* For this to work, mark them invalid.
	*/
	s->file_end = s->zero_start = s->data_end = -EINVAL;

	bs->file = bdrv_open_child(NULL, options, "file", bs, &child_of_bds,
	BDRV_CHILD_FILTERED \| BDRV_CHILD_PRIMARY,
	false, errp);
	if (!bs->file) {
	return -EINVAL;
	}

	if (!preallocate_absorb_opts(&s->opts, options, bs->file->bs, errp)) {
	return -EINVAL;
	}

	bs->supported_write_flags = BDRV_REQ_WRITE_UNCHANGED \|
	(BDRV_REQ_FUA & bs->file->bs->supported_write_flags);

	bs->supported_zero_flags = BDRV_REQ_WRITE_UNCHANGED \|
	((BDRV_REQ_FUA \| BDRV_REQ_MAY_UNMAP \| BDRV_REQ_NO_FALLBACK) &
	bs->file->bs->supported_zero_flags);

	return 0;
	}

	static void preallocate_close(BlockDriverState *bs)
	{
	int ret;
	BDRVPreallocateState *s = bs->opaque;

	if (s->data_end < 0) {
	return;
	}

	if (s->file_end < 0) {
	s->file_end = bdrv_getlength(bs->file->bs);
	if (s->file_end < 0) {
	return;
	}
	}

	if (s->data_end < s->file_end) {
	ret = bdrv_truncate(bs->file, s->data_end, true, PREALLOC_MODE_OFF, 0,
	NULL);
	s->file_end = ret < 0 ? ret : s->data_end;
	}
	}


	/*
	* Handle reopen.
	*
	* We must implement reopen handlers, otherwise reopen just don't work. Handle
	* new options and don't care about preallocation state, as it is handled in
	* set/check permission handlers.
	*/

	static int preallocate_reopen_prepare(BDRVReopenState *reopen_state,
	BlockReopenQueue queue, Error *errp)
	{
	PreallocateOpts *opts = g_new0(PreallocateOpts, 1);

	if (!preallocate_absorb_opts(opts, reopen_state->options,
	reopen_state->bs->file->bs, errp)) {
	g_free(opts);
	return -EINVAL;
	}

	reopen_state->opaque = opts;

	return 0;
	}

	static void preallocate_reopen_commit(BDRVReopenState *state)
	{
	BDRVPreallocateState *s = state->bs->opaque;

	s->opts = (PreallocateOpts )state->opaque;

	g_free(state->opaque);
	state->opaque = NULL;
	}

	static void preallocate_reopen_abort(BDRVReopenState *state)
	{
	g_free(state->opaque);
	state->opaque = NULL;
	}

	static coroutine_fn int preallocate_co_preadv_part(
	BlockDriverState *bs, int64_t offset, int64_t bytes,
	QEMUIOVector *qiov, size_t qiov_offset, BdrvRequestFlags flags)
	{
	return bdrv_co_preadv_part(bs->file, offset, bytes, qiov, qiov_offset,
	flags);
	}

	static int coroutine_fn preallocate_co_pdiscard(BlockDriverState *bs,
	int64_t offset, int64_t bytes)
	{
	return bdrv_co_pdiscard(bs->file, offset, bytes);
	}

	static bool can_write_resize(uint64_t perm)
	{
	return (perm & BLK_PERM_WRITE) && (perm & BLK_PERM_RESIZE);
	}

	static bool has_prealloc_perms(BlockDriverState *bs)
	{
	BDRVPreallocateState *s = bs->opaque;

	if (can_write_resize(bs->file->perm)) {
	assert(!(bs->file->shared_perm & BLK_PERM_WRITE));
	assert(!(bs->file->shared_perm & BLK_PERM_RESIZE));
	return true;
	}

	assert(s->data_end < 0);
	assert(s->zero_start < 0);
	assert(s->file_end < 0);
	return false;
	}

	/*
	* Call on each write. Returns true if @want_merge_zero is true and the region
	* [offset, offset + bytes) is zeroed (as a result of this call or earlier
	* preallocation).
	*
	* want_merge_zero is used to merge write-zero request with preallocation in
	* one bdrv_co_pwrite_zeroes() call.
	*/
	static bool coroutine_fn handle_write(BlockDriverState *bs, int64_t offset,
	int64_t bytes, bool want_merge_zero)
	{
	BDRVPreallocateState *s = bs->opaque;
	int64_t end = offset + bytes;
	int64_t prealloc_start, prealloc_end;
	int ret;
	uint32_t file_align = bs->file->bs->bl.request_alignment;
	uint32_t prealloc_align = MAX(s->opts.prealloc_align, file_align);

	assert(QEMU_IS_ALIGNED(prealloc_align, file_align));

	if (!has_prealloc_perms(bs)) {
	/* We don't have state neither should try to recover it */
	return false;
	}

	if (s->data_end < 0) {
	s->data_end = bdrv_getlength(bs->file->bs);
	if (s->data_end < 0) {
	return false;
	}

	if (s->file_end < 0) {
	s->file_end = s->data_end;
	}
	}

	if (end <= s->data_end) {
	return false;
	}

	/* We have valid s->data_end, and request writes beyond it. */

	s->data_end = end;
	if (s->zero_start < 0 \|\| !want_merge_zero) {
	s->zero_start = end;
	}

	if (s->file_end < 0) {
	s->file_end = bdrv_getlength(bs->file->bs);
	if (s->file_end < 0) {
	return false;
	}
	}

	/* Now s->data_end, s->zero_start and s->file_end are valid. */

	if (end <= s->file_end) {
	/* No preallocation needed. */
	return want_merge_zero && offset >= s->zero_start;
	}

	/* Now we want new preallocation, as request writes beyond s->file_end. */

	prealloc_start = QEMU_ALIGN_UP(
	want_merge_zero ? MIN(offset, s->file_end) : s->file_end,
	file_align);
	prealloc_end = QEMU_ALIGN_UP(
	MAX(prealloc_start, end) + s->opts.prealloc_size,
	prealloc_align);

	want_merge_zero = want_merge_zero && (prealloc_start <= offset);

	ret = bdrv_co_pwrite_zeroes(
	bs->file, prealloc_start, prealloc_end - prealloc_start,
	BDRV_REQ_NO_FALLBACK \| BDRV_REQ_SERIALISING \| BDRV_REQ_NO_WAIT);
	if (ret < 0) {
	s->file_end = ret;
	return false;
	}

	s->file_end = prealloc_end;
	return want_merge_zero;
	}

	static int coroutine_fn preallocate_co_pwrite_zeroes(BlockDriverState *bs,
	int64_t offset, int64_t bytes, BdrvRequestFlags flags)
	{
	bool want_merge_zero =
	!(flags & ~(BDRV_REQ_ZERO_WRITE \| BDRV_REQ_NO_FALLBACK));
	if (handle_write(bs, offset, bytes, want_merge_zero)) {
	return 0;
	}

	return bdrv_co_pwrite_zeroes(bs->file, offset, bytes, flags);
	}

	static coroutine_fn int preallocate_co_pwritev_part(BlockDriverState *bs,
	int64_t offset,
	int64_t bytes,
	QEMUIOVector *qiov,
	size_t qiov_offset,
	BdrvRequestFlags flags)
	{
	handle_write(bs, offset, bytes, false);

	return bdrv_co_pwritev_part(bs->file, offset, bytes, qiov, qiov_offset,
	flags);
	}

	static int coroutine_fn
	preallocate_co_truncate(BlockDriverState *bs, int64_t offset,
	bool exact, PreallocMode prealloc,
	BdrvRequestFlags flags, Error **errp)
	{
	ERRP_GUARD();
	BDRVPreallocateState *s = bs->opaque;
	int ret;

	if (s->data_end >= 0 && offset > s->data_end) {
	if (s->file_end < 0) {
	s->file_end = bdrv_getlength(bs->file->bs);
	if (s->file_end < 0) {
	error_setg(errp, "failed to get file length");
	return s->file_end;
	}
	}

	if (prealloc == PREALLOC_MODE_FALLOC) {
	/*
	* If offset <= s->file_end, the task is already done, just
	* update s->data_end, to move part of "filter preallocation"
	* to "preallocation requested by user".
	* Otherwise just proceed to preallocate missing part.
	*/
	if (offset <= s->file_end) {
	s->data_end = offset;
	return 0;
	}
	} else {
	/*
	* We have to drop our preallocation, to
	* - avoid "Cannot use preallocation for shrinking files" in
	* case of offset < file_end
	* - give PREALLOC_MODE_OFF a chance to keep small disk
	* usage
	* - give PREALLOC_MODE_FULL a chance to actually write the
	* whole region as user expects
	*/
	if (s->file_end > s->data_end) {
	ret = bdrv_co_truncate(bs->file, s->data_end, true,
	PREALLOC_MODE_OFF, 0, errp);
	if (ret < 0) {
	s->file_end = ret;
	error_prepend(errp, "preallocate-filter: failed to drop "
	"write-zero preallocation: ");
	return ret;
	}
	s->file_end = s->data_end;
	}
	}

	s->data_end = offset;
	}

	ret = bdrv_co_truncate(bs->file, offset, exact, prealloc, flags, errp);
	if (ret < 0) {
	s->file_end = s->zero_start = s->data_end = ret;
	return ret;
	}

	if (has_prealloc_perms(bs)) {
	s->file_end = s->zero_start = s->data_end = offset;
	}
	return 0;
	}

	static int coroutine_fn preallocate_co_flush(BlockDriverState *bs)
	{
	return bdrv_co_flush(bs->file->bs);
	}

	static int64_t preallocate_getlength(BlockDriverState *bs)
	{
	int64_t ret;
	BDRVPreallocateState *s = bs->opaque;

	if (s->data_end >= 0) {
	return s->data_end;
	}

	ret = bdrv_getlength(bs->file->bs);

	if (has_prealloc_perms(bs)) {
	s->file_end = s->zero_start = s->data_end = ret;
	}

	return ret;
	}

	static int preallocate_check_perm(BlockDriverState *bs,
	uint64_t perm, uint64_t shared, Error **errp)
	{
	BDRVPreallocateState *s = bs->opaque;

	if (s->data_end >= 0 && !can_write_resize(perm)) {
	/*
	* Lose permissions.
	* We should truncate in check_perm, as in set_perm bs->file->perm will
	* be already changed, and we should not violate it.
	*/
	if (s->file_end < 0) {
	s->file_end = bdrv_getlength(bs->file->bs);
	if (s->file_end < 0) {
	error_setg(errp, "Failed to get file length");
	return s->file_end;
	}
	}

	if (s->data_end < s->file_end) {
	int ret = bdrv_truncate(bs->file, s->data_end, true,
	PREALLOC_MODE_OFF, 0, NULL);
	if (ret < 0) {
	error_setg(errp, "Failed to drop preallocation");
	s->file_end = ret;
	return ret;
	}
	s->file_end = s->data_end;
	}
	}

	return 0;
	}

	static void preallocate_set_perm(BlockDriverState *bs,
	uint64_t perm, uint64_t shared)
	{
	BDRVPreallocateState *s = bs->opaque;

	if (can_write_resize(perm)) {
	if (s->data_end < 0) {
	s->data_end = s->file_end = s->zero_start =
	bdrv_getlength(bs->file->bs);
	}
	} else {
	/*
	* We drop our permissions, as well as allow shared
	* permissions (see preallocate_child_perm), anyone will be able to
	* change the child, so mark all states invalid. We'll regain control if
	* get good permissions back.
	*/
	s->data_end = s->file_end = s->zero_start = -EINVAL;
	}
	}

	static void preallocate_child_perm(BlockDriverState bs, BdrvChild c,
	BdrvChildRole role, BlockReopenQueue *reopen_queue,
	uint64_t perm, uint64_t shared, uint64_t nperm, uint64_t nshared)
	{
	bdrv_default_perms(bs, c, role, reopen_queue, perm, shared, nperm, nshared);

	if (can_write_resize(perm)) {
	/* This should come by default, but let's enforce: */
	*nperm \|= BLK_PERM_WRITE \| BLK_PERM_RESIZE;

	/*
	* Don't share, to keep our states s->file_end, s->data_end and
	* s->zero_start valid.
	*/
	*nshared &= ~(BLK_PERM_WRITE \| BLK_PERM_RESIZE);
	}
	}

	BlockDriver bdrv_preallocate_filter = {
	.format_name = "preallocate",
	.instance_size = sizeof(BDRVPreallocateState),

	.bdrv_getlength = preallocate_getlength,
	.bdrv_open = preallocate_open,
	.bdrv_close = preallocate_close,

	.bdrv_reopen_prepare = preallocate_reopen_prepare,
	.bdrv_reopen_commit = preallocate_reopen_commit,
	.bdrv_reopen_abort = preallocate_reopen_abort,

	.bdrv_co_preadv_part = preallocate_co_preadv_part,
	.bdrv_co_pwritev_part = preallocate_co_pwritev_part,
	.bdrv_co_pwrite_zeroes = preallocate_co_pwrite_zeroes,
	.bdrv_co_pdiscard = preallocate_co_pdiscard,
	.bdrv_co_flush = preallocate_co_flush,
	.bdrv_co_truncate = preallocate_co_truncate,

	.bdrv_check_perm = preallocate_check_perm,
	.bdrv_set_perm = preallocate_set_perm,
	.bdrv_child_perm = preallocate_child_perm,

	.has_variable_length = true,
	.is_filter = true,
	};

	static void bdrv_preallocate_init(void)
	{
	bdrv_register(&bdrv_preallocate_filter);
	}

	block_init(bdrv_preallocate_init);