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

 /*
  * Live block commit
  *
  * Copyright Red Hat, Inc. 2012
  *
  * Authors:
  *  Jeff Cody   <jcody@redhat.com>
  *  Based on stream.c by Stefan Hajnoczi
  *
  * This work is licensed under the terms of the GNU LGPL, version 2 or later.
  * See the COPYING.LIB file in the top-level directory.
  *
  */

 #include "qemu/osdep.h"
 #include "qemu/cutils.h"
 #include "trace.h"
 #include "block/block_int.h"
 #include "block/blockjob_int.h"
 #include "qapi/error.h"
 #include "qapi/qmp/qerror.h"
 #include "qemu/ratelimit.h"
 #include "qemu/memalign.h"
 #include "sysemu/block-backend.h"

 enum {
     /*
      * Size of data buffer for populating the image file.  This should be large
      * enough to process multiple clusters in a single call, so that populating
      * contiguous regions of the image is efficient.
      */
     COMMIT_BUFFER_SIZE = 512 * 1024, /* in bytes */
 };

 typedef struct CommitBlockJob {
     BlockJob common;
     BlockDriverState *commit_top_bs;
     BlockBackend *top;
     BlockBackend *base;
     BlockDriverState *base_bs;
     BlockDriverState *base_overlay;
     BlockdevOnError on_error;
     bool base_read_only;
     bool chain_frozen;
     char *backing_file_str;
 } CommitBlockJob;

 static int commit_prepare(Job *job)
 {
     CommitBlockJob *s = container_of(job, CommitBlockJob, common.job);

     bdrv_unfreeze_backing_chain(s->commit_top_bs, s->base_bs);
     s->chain_frozen = false;

     /* Remove base node parent that still uses BLK_PERM_WRITE/RESIZE before
      * the normal backing chain can be restored. */
     blk_unref(s->base);
     s->base = NULL;

     /* FIXME: bdrv_drop_intermediate treats total failures and partial failures
      * identically. Further work is needed to disambiguate these cases. */
     return bdrv_drop_intermediate(s->commit_top_bs, s->base_bs,
                                   s->backing_file_str);
 }

 static void commit_abort(Job *job)
 {
     CommitBlockJob *s = container_of(job, CommitBlockJob, common.job);
     BlockDriverState *top_bs = blk_bs(s->top);

     if (s->chain_frozen) {
         bdrv_unfreeze_backing_chain(s->commit_top_bs, s->base_bs);
     }

     /* Make sure commit_top_bs and top stay around until bdrv_replace_node() */
     bdrv_ref(top_bs);
     bdrv_ref(s->commit_top_bs);

     if (s->base) {
         blk_unref(s->base);
     }

     /* free the blockers on the intermediate nodes so that bdrv_replace_nodes
      * can succeed */
     block_job_remove_all_bdrv(&s->common);

     /* If bdrv_drop_intermediate() failed (or was not invoked), remove the
      * commit filter driver from the backing chain now. Do this as the final
      * step so that the 'consistent read' permission can be granted.
      *
      * XXX Can (or should) we somehow keep 'consistent read' blocked even
      * after the failed/cancelled commit job is gone? If we already wrote
      * something to base, the intermediate images aren't valid any more. */
     bdrv_replace_node(s->commit_top_bs, s->commit_top_bs->backing->bs,
                       &error_abort);

     bdrv_unref(s->commit_top_bs);
     bdrv_unref(top_bs);
 }

 static void commit_clean(Job *job)
 {
     CommitBlockJob *s = container_of(job, CommitBlockJob, common.job);

     /* restore base open flags here if appropriate (e.g., change the base back
      * to r/o). These reopens do not need to be atomic, since we won't abort
      * even on failure here */
     if (s->base_read_only) {
         bdrv_reopen_set_read_only(s->base_bs, true, NULL);
     }

     g_free(s->backing_file_str);
     blk_unref(s->top);
 }

 static int coroutine_fn commit_run(Job *job, Error **errp)
 {
     CommitBlockJob *s = container_of(job, CommitBlockJob, common.job);
     int64_t offset;
     uint64_t delay_ns = 0;
     int ret = 0;
     int64_t n = 0; /* bytes */
     QEMU_AUTO_VFREE void *buf = NULL;
     int64_t len, base_len;

     len = blk_getlength(s->top);
     if (len < 0) {
         return len;
     }
     job_progress_set_remaining(&s->common.job, len);

     base_len = blk_getlength(s->base);
     if (base_len < 0) {
         return base_len;
     }

     if (base_len < len) {
         ret = blk_truncate(s->base, len, false, PREALLOC_MODE_OFF, 0, NULL);
         if (ret) {
             return ret;
         }
     }

     buf = blk_blockalign(s->top, COMMIT_BUFFER_SIZE);

     for (offset = 0; offset < len; offset += n) {
         bool copy;
         bool error_in_source = true;

         /* Note that even when no rate limit is applied we need to yield
          * with no pending I/O here so that bdrv_drain_all() returns.
          */
         job_sleep_ns(&s->common.job, delay_ns);
         if (job_is_cancelled(&s->common.job)) {
             break;
         }
         /* Copy if allocated above the base */
         ret = bdrv_is_allocated_above(blk_bs(s->top), s->base_overlay, true,
                                       offset, COMMIT_BUFFER_SIZE, &n);
         copy = (ret > 0);
         trace_commit_one_iteration(s, offset, n, ret);
         if (copy) {
             assert(n < SIZE_MAX);

             ret = blk_co_pread(s->top, offset, n, buf, 0);
             if (ret >= 0) {
                 ret = blk_co_pwrite(s->base, offset, n, buf, 0);
                 if (ret < 0) {
                     error_in_source = false;
                 }
             }
         }
         if (ret < 0) {
             BlockErrorAction action =
                 block_job_error_action(&s->common, s->on_error,
                                        error_in_source, -ret);
             if (action == BLOCK_ERROR_ACTION_REPORT) {
                 return ret;
             } else {
                 n = 0;
                 continue;
             }
         }
         /* Publish progress */
         job_progress_update(&s->common.job, n);

         if (copy) {
             delay_ns = block_job_ratelimit_get_delay(&s->common, n);
         } else {
             delay_ns = 0;
         }
     }

     return 0;
 }

 static const BlockJobDriver commit_job_driver = {
     .job_driver = {
         .instance_size = sizeof(CommitBlockJob),
         .job_type      = JOB_TYPE_COMMIT,
         .free          = block_job_free,
         .user_resume   = block_job_user_resume,
         .run           = commit_run,
         .prepare       = commit_prepare,
         .abort         = commit_abort,
         .clean         = commit_clean
     },
 };

 static int coroutine_fn bdrv_commit_top_preadv(BlockDriverState *bs,
     int64_t offset, int64_t bytes, QEMUIOVector *qiov, BdrvRequestFlags flags)
 {
     return bdrv_co_preadv(bs->backing, offset, bytes, qiov, flags);
 }

 static void bdrv_commit_top_refresh_filename(BlockDriverState *bs)
 {
     pstrcpy(bs->exact_filename, sizeof(bs->exact_filename),
             bs->backing->bs->filename);
 }

 static void bdrv_commit_top_child_perm(BlockDriverState *bs, BdrvChild *c,
                                        BdrvChildRole role,
                                        BlockReopenQueue *reopen_queue,
                                        uint64_t perm, uint64_t shared,
                                        uint64_t *nperm, uint64_t *nshared)
 {
     *nperm = 0;
     *nshared = BLK_PERM_ALL;
 }

 /* Dummy node that provides consistent read to its users without requiring it
  * from its backing file and that allows writes on the backing file chain. */
 static BlockDriver bdrv_commit_top = {
     .format_name                = "commit_top",
     .bdrv_co_preadv             = bdrv_commit_top_preadv,
     .bdrv_refresh_filename      = bdrv_commit_top_refresh_filename,
     .bdrv_child_perm            = bdrv_commit_top_child_perm,

     .is_filter                  = true,
 };

 void commit_start(const char *job_id, BlockDriverState *bs,
                   BlockDriverState *base, BlockDriverState *top,
                   int creation_flags, int64_t speed,
                   BlockdevOnError on_error, const char *backing_file_str,
                   const char *filter_node_name, Error **errp)
 {
     CommitBlockJob *s;
     BlockDriverState *iter;
     BlockDriverState *commit_top_bs = NULL;
     BlockDriverState *filtered_base;
     int64_t base_size, top_size;
     uint64_t base_perms, iter_shared_perms;
     int ret;

     GLOBAL_STATE_CODE();

     assert(top != bs);
     if (bdrv_skip_filters(top) == bdrv_skip_filters(base)) {
         error_setg(errp, "Invalid files for merge: top and base are the same");
         return;
     }

     base_size = bdrv_getlength(base);
     if (base_size < 0) {
         error_setg_errno(errp, -base_size, "Could not inquire base image size");
         return;
     }

     top_size = bdrv_getlength(top);
     if (top_size < 0) {
         error_setg_errno(errp, -top_size, "Could not inquire top image size");
         return;
     }

     base_perms = BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE;
     if (base_size < top_size) {
         base_perms |= BLK_PERM_RESIZE;
     }

     s = block_job_create(job_id, &commit_job_driver, NULL, bs, 0, BLK_PERM_ALL,
                          speed, creation_flags, NULL, NULL, errp);
     if (!s) {
         return;
     }

     /* convert base to r/w, if necessary */
     s->base_read_only = bdrv_is_read_only(base);
     if (s->base_read_only) {
         if (bdrv_reopen_set_read_only(base, false, errp) != 0) {
             goto fail;
         }
     }

     /* Insert commit_top block node above top, so we can block consistent read
      * on the backing chain below it */
     commit_top_bs = bdrv_new_open_driver(&bdrv_commit_top, filter_node_name, 0,
                                          errp);
     if (commit_top_bs == NULL) {
         goto fail;
     }
     if (!filter_node_name) {
         commit_top_bs->implicit = true;
     }

     /* So that we can always drop this node */
     commit_top_bs->never_freeze = true;

     commit_top_bs->total_sectors = top->total_sectors;

     ret = bdrv_append(commit_top_bs, top, errp);
     bdrv_unref(commit_top_bs); /* referenced by new parents or failed */
     if (ret < 0) {
         commit_top_bs = NULL;
         goto fail;
     }

     s->commit_top_bs = commit_top_bs;

     /*
      * Block all nodes between top and base, because they will
      * disappear from the chain after this operation.
      * Note that this assumes that the user is fine with removing all
      * nodes (including R/W filters) between top and base.  Assuring
      * this is the responsibility of the interface (i.e. whoever calls
      * commit_start()).
      */
     s->base_overlay = bdrv_find_overlay(top, base);
     assert(s->base_overlay);

     /*
      * The topmost node with
      * bdrv_skip_filters(filtered_base) == bdrv_skip_filters(base)
      */
     filtered_base = bdrv_cow_bs(s->base_overlay);
     assert(bdrv_skip_filters(filtered_base) == bdrv_skip_filters(base));

     /*
      * XXX BLK_PERM_WRITE needs to be allowed so we don't block ourselves
      * at s->base (if writes are blocked for a node, they are also blocked
      * for its backing file). The other options would be a second filter
      * driver above s->base.
      */
     iter_shared_perms = BLK_PERM_WRITE_UNCHANGED | BLK_PERM_WRITE;

     for (iter = top; iter != base; iter = bdrv_filter_or_cow_bs(iter)) {
         if (iter == filtered_base) {
             /*
              * From here on, all nodes are filters on the base.  This
              * allows us to share BLK_PERM_CONSISTENT_READ.
              */
             iter_shared_perms |= BLK_PERM_CONSISTENT_READ;
         }

         ret = block_job_add_bdrv(&s->common, "intermediate node", iter, 0,
                                  iter_shared_perms, errp);
         if (ret < 0) {
             goto fail;
         }
     }

     if (bdrv_freeze_backing_chain(commit_top_bs, base, errp) < 0) {
         goto fail;
     }
     s->chain_frozen = true;

     ret = block_job_add_bdrv(&s->common, "base", base, 0, BLK_PERM_ALL, errp);
     if (ret < 0) {
         goto fail;
     }

     s->base = blk_new(s->common.job.aio_context,
                       base_perms,
                       BLK_PERM_CONSISTENT_READ
                       | BLK_PERM_WRITE_UNCHANGED);
     ret = blk_insert_bs(s->base, base, errp);
     if (ret < 0) {
         goto fail;
     }
     blk_set_disable_request_queuing(s->base, true);
     s->base_bs = base;

     /* Required permissions are already taken with block_job_add_bdrv() */
     s->top = blk_new(s->common.job.aio_context, 0, BLK_PERM_ALL);
     ret = blk_insert_bs(s->top, top, errp);
     if (ret < 0) {
         goto fail;
     }
     blk_set_disable_request_queuing(s->top, true);

     s->backing_file_str = g_strdup(backing_file_str);
     s->on_error = on_error;

     trace_commit_start(bs, base, top, s);
     job_start(&s->common.job);
     return;

 fail:
     if (s->chain_frozen) {
         bdrv_unfreeze_backing_chain(commit_top_bs, base);
     }
     if (s->base) {
         blk_unref(s->base);
     }
     if (s->top) {
         blk_unref(s->top);
     }
     if (s->base_read_only) {
         bdrv_reopen_set_read_only(base, true, NULL);
     }
     job_early_fail(&s->common.job);
     /* commit_top_bs has to be replaced after deleting the block job,
      * otherwise this would fail because of lack of permissions. */
     if (commit_top_bs) {
         bdrv_replace_node(commit_top_bs, top, &error_abort);
     }
 }


 #define COMMIT_BUF_SIZE (2048 * BDRV_SECTOR_SIZE)

 /* commit COW file into the raw image */
 int bdrv_commit(BlockDriverState *bs)
 {
     BlockBackend *src, *backing;
     BlockDriverState *backing_file_bs = NULL;
     BlockDriverState *commit_top_bs = NULL;
     BlockDriver *drv = bs->drv;
     AioContext *ctx;
     int64_t offset, length, backing_length;
     int ro;
     int64_t n;
     int ret = 0;
     QEMU_AUTO_VFREE uint8_t *buf = NULL;
     Error *local_err = NULL;

     GLOBAL_STATE_CODE();

     if (!drv)
         return -ENOMEDIUM;

     backing_file_bs = bdrv_cow_bs(bs);

     if (!backing_file_bs) {
         return -ENOTSUP;
     }

     if (bdrv_op_is_blocked(bs, BLOCK_OP_TYPE_COMMIT_SOURCE, NULL) ||
         bdrv_op_is_blocked(backing_file_bs, BLOCK_OP_TYPE_COMMIT_TARGET, NULL))
     {
         return -EBUSY;
     }

     ro = bdrv_is_read_only(backing_file_bs);

     if (ro) {
         if (bdrv_reopen_set_read_only(backing_file_bs, false, NULL)) {
             return -EACCES;
         }
     }

     ctx = bdrv_get_aio_context(bs);
     /* WRITE_UNCHANGED is required for bdrv_make_empty() */
     src = blk_new(ctx, BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE_UNCHANGED,
                   BLK_PERM_ALL);
     backing = blk_new(ctx, BLK_PERM_WRITE | BLK_PERM_RESIZE, BLK_PERM_ALL);

     ret = blk_insert_bs(src, bs, &local_err);
     if (ret < 0) {
         error_report_err(local_err);
         goto ro_cleanup;
     }

     /* Insert commit_top block node above backing, so we can write to it */
     commit_top_bs = bdrv_new_open_driver(&bdrv_commit_top, NULL, BDRV_O_RDWR,
                                          &local_err);
     if (commit_top_bs == NULL) {
         error_report_err(local_err);
         goto ro_cleanup;
     }

     bdrv_set_backing_hd(commit_top_bs, backing_file_bs, &error_abort);
     bdrv_set_backing_hd(bs, commit_top_bs, &error_abort);

     ret = blk_insert_bs(backing, backing_file_bs, &local_err);
     if (ret < 0) {
         error_report_err(local_err);
         goto ro_cleanup;
     }

     length = blk_getlength(src);
     if (length < 0) {
         ret = length;
         goto ro_cleanup;
     }

     backing_length = blk_getlength(backing);
     if (backing_length < 0) {
         ret = backing_length;
         goto ro_cleanup;
     }

     /* If our top snapshot is larger than the backing file image,
      * grow the backing file image if possible.  If not possible,
      * we must return an error */
     if (length > backing_length) {
         ret = blk_truncate(backing, length, false, PREALLOC_MODE_OFF, 0,
                            &local_err);
         if (ret < 0) {
             error_report_err(local_err);
             goto ro_cleanup;
         }
     }

     /* blk_try_blockalign() for src will choose an alignment that works for
      * backing as well, so no need to compare the alignment manually. */
     buf = blk_try_blockalign(src, COMMIT_BUF_SIZE);
     if (buf == NULL) {
         ret = -ENOMEM;
         goto ro_cleanup;
     }

     for (offset = 0; offset < length; offset += n) {
         ret = bdrv_is_allocated(bs, offset, COMMIT_BUF_SIZE, &n);
         if (ret < 0) {
             goto ro_cleanup;
         }
         if (ret) {
             ret = blk_pread(src, offset, buf, n);
             if (ret < 0) {
                 goto ro_cleanup;
             }

             ret = blk_pwrite(backing, offset, buf, n, 0);
             if (ret < 0) {
                 goto ro_cleanup;
             }
         }
     }

     ret = blk_make_empty(src, NULL);
     /* Ignore -ENOTSUP */
     if (ret < 0 && ret != -ENOTSUP) {
         goto ro_cleanup;
     }

     blk_flush(src);

     /*
      * Make sure all data we wrote to the backing device is actually
      * stable on disk.
      */
     blk_flush(backing);

     ret = 0;
 ro_cleanup:
     blk_unref(backing);
     if (bdrv_cow_bs(bs) != backing_file_bs) {
         bdrv_set_backing_hd(bs, backing_file_bs, &error_abort);
     }
     bdrv_unref(commit_top_bs);
     blk_unref(src);

     if (ro) {
         /* ignoring error return here */
         bdrv_reopen_set_read_only(backing_file_bs, true, NULL);
     }

     return ret;
 }
	/*
	* Live block commit
	*
	* Copyright Red Hat, Inc. 2012
	*
	* Authors:
	* Jeff Cody <jcody@redhat.com>
	* Based on stream.c by Stefan Hajnoczi
	*
	* This work is licensed under the terms of the GNU LGPL, version 2 or later.
	* See the COPYING.LIB file in the top-level directory.
	*
	*/

	#include "qemu/osdep.h"
	#include "qemu/cutils.h"
	#include "trace.h"
	#include "block/block_int.h"
	#include "block/blockjob_int.h"
	#include "qapi/error.h"
	#include "qapi/qmp/qerror.h"
	#include "qemu/ratelimit.h"
	#include "qemu/memalign.h"
	#include "sysemu/block-backend.h"

	enum {
	/*
	* Size of data buffer for populating the image file. This should be large
	* enough to process multiple clusters in a single call, so that populating
	* contiguous regions of the image is efficient.
	*/
	COMMIT_BUFFER_SIZE = 512 * 1024, /* in bytes */
	};

	typedef struct CommitBlockJob {
	BlockJob common;
	BlockDriverState *commit_top_bs;
	BlockBackend *top;
	BlockBackend *base;
	BlockDriverState *base_bs;
	BlockDriverState *base_overlay;
	BlockdevOnError on_error;
	bool base_read_only;
	bool chain_frozen;
	char *backing_file_str;
	} CommitBlockJob;

	static int commit_prepare(Job *job)
	{
	CommitBlockJob *s = container_of(job, CommitBlockJob, common.job);

	bdrv_unfreeze_backing_chain(s->commit_top_bs, s->base_bs);
	s->chain_frozen = false;

	/* Remove base node parent that still uses BLK_PERM_WRITE/RESIZE before
	* the normal backing chain can be restored. */
	blk_unref(s->base);
	s->base = NULL;

	/* FIXME: bdrv_drop_intermediate treats total failures and partial failures
	* identically. Further work is needed to disambiguate these cases. */
	return bdrv_drop_intermediate(s->commit_top_bs, s->base_bs,
	s->backing_file_str);
	}

	static void commit_abort(Job *job)
	{
	CommitBlockJob *s = container_of(job, CommitBlockJob, common.job);
	BlockDriverState *top_bs = blk_bs(s->top);

	if (s->chain_frozen) {
	bdrv_unfreeze_backing_chain(s->commit_top_bs, s->base_bs);
	}

	/* Make sure commit_top_bs and top stay around until bdrv_replace_node() */
	bdrv_ref(top_bs);
	bdrv_ref(s->commit_top_bs);

	if (s->base) {
	blk_unref(s->base);
	}

	/* free the blockers on the intermediate nodes so that bdrv_replace_nodes
	* can succeed */
	block_job_remove_all_bdrv(&s->common);

	/* If bdrv_drop_intermediate() failed (or was not invoked), remove the
	* commit filter driver from the backing chain now. Do this as the final
	* step so that the 'consistent read' permission can be granted.
	*
	* XXX Can (or should) we somehow keep 'consistent read' blocked even
	* after the failed/cancelled commit job is gone? If we already wrote
	* something to base, the intermediate images aren't valid any more. */
	bdrv_replace_node(s->commit_top_bs, s->commit_top_bs->backing->bs,
	&error_abort);

	bdrv_unref(s->commit_top_bs);
	bdrv_unref(top_bs);
	}

	static void commit_clean(Job *job)
	{
	CommitBlockJob *s = container_of(job, CommitBlockJob, common.job);

	/* restore base open flags here if appropriate (e.g., change the base back
	* to r/o). These reopens do not need to be atomic, since we won't abort
	* even on failure here */
	if (s->base_read_only) {
	bdrv_reopen_set_read_only(s->base_bs, true, NULL);
	}

	g_free(s->backing_file_str);
	blk_unref(s->top);
	}

	static int coroutine_fn commit_run(Job job, Error *errp)
	{
	CommitBlockJob *s = container_of(job, CommitBlockJob, common.job);
	int64_t offset;
	uint64_t delay_ns = 0;
	int ret = 0;
	int64_t n = 0; /* bytes */
	QEMU_AUTO_VFREE void *buf = NULL;
	int64_t len, base_len;

	len = blk_getlength(s->top);
	if (len < 0) {
	return len;
	}
	job_progress_set_remaining(&s->common.job, len);

	base_len = blk_getlength(s->base);
	if (base_len < 0) {
	return base_len;
	}

	if (base_len < len) {
	ret = blk_truncate(s->base, len, false, PREALLOC_MODE_OFF, 0, NULL);
	if (ret) {
	return ret;
	}
	}

	buf = blk_blockalign(s->top, COMMIT_BUFFER_SIZE);

	for (offset = 0; offset < len; offset += n) {
	bool copy;
	bool error_in_source = true;

	/* Note that even when no rate limit is applied we need to yield
	* with no pending I/O here so that bdrv_drain_all() returns.
	*/
	job_sleep_ns(&s->common.job, delay_ns);
	if (job_is_cancelled(&s->common.job)) {
	break;
	}
	/* Copy if allocated above the base */
	ret = bdrv_is_allocated_above(blk_bs(s->top), s->base_overlay, true,
	offset, COMMIT_BUFFER_SIZE, &n);
	copy = (ret > 0);
	trace_commit_one_iteration(s, offset, n, ret);
	if (copy) {
	assert(n < SIZE_MAX);

	ret = blk_co_pread(s->top, offset, n, buf, 0);
	if (ret >= 0) {
	ret = blk_co_pwrite(s->base, offset, n, buf, 0);
	if (ret < 0) {
	error_in_source = false;
	}
	}
	}
	if (ret < 0) {
	BlockErrorAction action =
	block_job_error_action(&s->common, s->on_error,
	error_in_source, -ret);
	if (action == BLOCK_ERROR_ACTION_REPORT) {
	return ret;
	} else {
	n = 0;
	continue;
	}
	}
	/* Publish progress */
	job_progress_update(&s->common.job, n);

	if (copy) {
	delay_ns = block_job_ratelimit_get_delay(&s->common, n);
	} else {
	delay_ns = 0;
	}
	}

	return 0;
	}

	static const BlockJobDriver commit_job_driver = {
	.job_driver = {
	.instance_size = sizeof(CommitBlockJob),
	.job_type = JOB_TYPE_COMMIT,
	.free = block_job_free,
	.user_resume = block_job_user_resume,
	.run = commit_run,
	.prepare = commit_prepare,
	.abort = commit_abort,
	.clean = commit_clean
	},
	};

	static int coroutine_fn bdrv_commit_top_preadv(BlockDriverState *bs,
	int64_t offset, int64_t bytes, QEMUIOVector *qiov, BdrvRequestFlags flags)
	{
	return bdrv_co_preadv(bs->backing, offset, bytes, qiov, flags);
	}

	static void bdrv_commit_top_refresh_filename(BlockDriverState *bs)
	{
	pstrcpy(bs->exact_filename, sizeof(bs->exact_filename),
	bs->backing->bs->filename);
	}

	static void bdrv_commit_top_child_perm(BlockDriverState bs, BdrvChild c,
	BdrvChildRole role,
	BlockReopenQueue *reopen_queue,
	uint64_t perm, uint64_t shared,
	uint64_t nperm, uint64_t nshared)
	{
	*nperm = 0;
	*nshared = BLK_PERM_ALL;
	}

	/* Dummy node that provides consistent read to its users without requiring it
	* from its backing file and that allows writes on the backing file chain. */
	static BlockDriver bdrv_commit_top = {
	.format_name = "commit_top",
	.bdrv_co_preadv = bdrv_commit_top_preadv,
	.bdrv_refresh_filename = bdrv_commit_top_refresh_filename,
	.bdrv_child_perm = bdrv_commit_top_child_perm,

	.is_filter = true,
	};

	void commit_start(const char job_id, BlockDriverState bs,
	BlockDriverState base, BlockDriverState top,
	int creation_flags, int64_t speed,
	BlockdevOnError on_error, const char *backing_file_str,
	const char filter_node_name, Error *errp)
	{
	CommitBlockJob *s;
	BlockDriverState *iter;
	BlockDriverState *commit_top_bs = NULL;
	BlockDriverState *filtered_base;
	int64_t base_size, top_size;
	uint64_t base_perms, iter_shared_perms;
	int ret;

	GLOBAL_STATE_CODE();

	assert(top != bs);
	if (bdrv_skip_filters(top) == bdrv_skip_filters(base)) {
	error_setg(errp, "Invalid files for merge: top and base are the same");
	return;
	}

	base_size = bdrv_getlength(base);
	if (base_size < 0) {
	error_setg_errno(errp, -base_size, "Could not inquire base image size");
	return;
	}

	top_size = bdrv_getlength(top);
	if (top_size < 0) {
	error_setg_errno(errp, -top_size, "Could not inquire top image size");
	return;
	}

	base_perms = BLK_PERM_CONSISTENT_READ \| BLK_PERM_WRITE;
	if (base_size < top_size) {
	base_perms \|= BLK_PERM_RESIZE;
	}

	s = block_job_create(job_id, &commit_job_driver, NULL, bs, 0, BLK_PERM_ALL,
	speed, creation_flags, NULL, NULL, errp);
	if (!s) {
	return;
	}

	/* convert base to r/w, if necessary */
	s->base_read_only = bdrv_is_read_only(base);
	if (s->base_read_only) {
	if (bdrv_reopen_set_read_only(base, false, errp) != 0) {
	goto fail;
	}
	}

	/* Insert commit_top block node above top, so we can block consistent read
	* on the backing chain below it */
	commit_top_bs = bdrv_new_open_driver(&bdrv_commit_top, filter_node_name, 0,
	errp);
	if (commit_top_bs == NULL) {
	goto fail;
	}
	if (!filter_node_name) {
	commit_top_bs->implicit = true;
	}

	/* So that we can always drop this node */
	commit_top_bs->never_freeze = true;

	commit_top_bs->total_sectors = top->total_sectors;

	ret = bdrv_append(commit_top_bs, top, errp);
	bdrv_unref(commit_top_bs); /* referenced by new parents or failed */
	if (ret < 0) {
	commit_top_bs = NULL;
	goto fail;
	}

	s->commit_top_bs = commit_top_bs;

	/*
	* Block all nodes between top and base, because they will
	* disappear from the chain after this operation.
	* Note that this assumes that the user is fine with removing all
	* nodes (including R/W filters) between top and base. Assuring
	* this is the responsibility of the interface (i.e. whoever calls
	* commit_start()).
	*/
	s->base_overlay = bdrv_find_overlay(top, base);
	assert(s->base_overlay);

	/*
	* The topmost node with
	* bdrv_skip_filters(filtered_base) == bdrv_skip_filters(base)
	*/
	filtered_base = bdrv_cow_bs(s->base_overlay);
	assert(bdrv_skip_filters(filtered_base) == bdrv_skip_filters(base));

	/*
	* XXX BLK_PERM_WRITE needs to be allowed so we don't block ourselves
	* at s->base (if writes are blocked for a node, they are also blocked
	* for its backing file). The other options would be a second filter
	* driver above s->base.
	*/
	iter_shared_perms = BLK_PERM_WRITE_UNCHANGED \| BLK_PERM_WRITE;

	for (iter = top; iter != base; iter = bdrv_filter_or_cow_bs(iter)) {
	if (iter == filtered_base) {
	/*
	* From here on, all nodes are filters on the base. This
	* allows us to share BLK_PERM_CONSISTENT_READ.
	*/
	iter_shared_perms \|= BLK_PERM_CONSISTENT_READ;
	}

	ret = block_job_add_bdrv(&s->common, "intermediate node", iter, 0,
	iter_shared_perms, errp);
	if (ret < 0) {
	goto fail;
	}
	}

	if (bdrv_freeze_backing_chain(commit_top_bs, base, errp) < 0) {
	goto fail;
	}
	s->chain_frozen = true;

	ret = block_job_add_bdrv(&s->common, "base", base, 0, BLK_PERM_ALL, errp);
	if (ret < 0) {
	goto fail;
	}

	s->base = blk_new(s->common.job.aio_context,
	base_perms,
	BLK_PERM_CONSISTENT_READ
	\| BLK_PERM_WRITE_UNCHANGED);
	ret = blk_insert_bs(s->base, base, errp);
	if (ret < 0) {
	goto fail;
	}
	blk_set_disable_request_queuing(s->base, true);
	s->base_bs = base;

	/* Required permissions are already taken with block_job_add_bdrv() */
	s->top = blk_new(s->common.job.aio_context, 0, BLK_PERM_ALL);
	ret = blk_insert_bs(s->top, top, errp);
	if (ret < 0) {
	goto fail;
	}
	blk_set_disable_request_queuing(s->top, true);

	s->backing_file_str = g_strdup(backing_file_str);
	s->on_error = on_error;

	trace_commit_start(bs, base, top, s);
	job_start(&s->common.job);
	return;

	fail:
	if (s->chain_frozen) {
	bdrv_unfreeze_backing_chain(commit_top_bs, base);
	}
	if (s->base) {
	blk_unref(s->base);
	}
	if (s->top) {
	blk_unref(s->top);
	}
	if (s->base_read_only) {
	bdrv_reopen_set_read_only(base, true, NULL);
	}
	job_early_fail(&s->common.job);
	/* commit_top_bs has to be replaced after deleting the block job,
	* otherwise this would fail because of lack of permissions. */
	if (commit_top_bs) {
	bdrv_replace_node(commit_top_bs, top, &error_abort);
	}
	}


	#define COMMIT_BUF_SIZE (2048 * BDRV_SECTOR_SIZE)

	/* commit COW file into the raw image */
	int bdrv_commit(BlockDriverState *bs)
	{
	BlockBackend src, backing;
	BlockDriverState *backing_file_bs = NULL;
	BlockDriverState *commit_top_bs = NULL;
	BlockDriver *drv = bs->drv;
	AioContext *ctx;
	int64_t offset, length, backing_length;
	int ro;
	int64_t n;
	int ret = 0;
	QEMU_AUTO_VFREE uint8_t *buf = NULL;
	Error *local_err = NULL;

	GLOBAL_STATE_CODE();

	if (!drv)
	return -ENOMEDIUM;

	backing_file_bs = bdrv_cow_bs(bs);

	if (!backing_file_bs) {
	return -ENOTSUP;
	}

	if (bdrv_op_is_blocked(bs, BLOCK_OP_TYPE_COMMIT_SOURCE, NULL) \|\|
	bdrv_op_is_blocked(backing_file_bs, BLOCK_OP_TYPE_COMMIT_TARGET, NULL))
	{
	return -EBUSY;
	}

	ro = bdrv_is_read_only(backing_file_bs);

	if (ro) {
	if (bdrv_reopen_set_read_only(backing_file_bs, false, NULL)) {
	return -EACCES;
	}
	}

	ctx = bdrv_get_aio_context(bs);
	/* WRITE_UNCHANGED is required for bdrv_make_empty() */
	src = blk_new(ctx, BLK_PERM_CONSISTENT_READ \| BLK_PERM_WRITE_UNCHANGED,
	BLK_PERM_ALL);
	backing = blk_new(ctx, BLK_PERM_WRITE \| BLK_PERM_RESIZE, BLK_PERM_ALL);

	ret = blk_insert_bs(src, bs, &local_err);
	if (ret < 0) {
	error_report_err(local_err);
	goto ro_cleanup;
	}

	/* Insert commit_top block node above backing, so we can write to it */
	commit_top_bs = bdrv_new_open_driver(&bdrv_commit_top, NULL, BDRV_O_RDWR,
	&local_err);
	if (commit_top_bs == NULL) {
	error_report_err(local_err);
	goto ro_cleanup;
	}

	bdrv_set_backing_hd(commit_top_bs, backing_file_bs, &error_abort);
	bdrv_set_backing_hd(bs, commit_top_bs, &error_abort);

	ret = blk_insert_bs(backing, backing_file_bs, &local_err);
	if (ret < 0) {
	error_report_err(local_err);
	goto ro_cleanup;
	}

	length = blk_getlength(src);
	if (length < 0) {
	ret = length;
	goto ro_cleanup;
	}

	backing_length = blk_getlength(backing);
	if (backing_length < 0) {
	ret = backing_length;
	goto ro_cleanup;
	}

	/* If our top snapshot is larger than the backing file image,
	* grow the backing file image if possible. If not possible,
	* we must return an error */
	if (length > backing_length) {
	ret = blk_truncate(backing, length, false, PREALLOC_MODE_OFF, 0,
	&local_err);
	if (ret < 0) {
	error_report_err(local_err);
	goto ro_cleanup;
	}
	}

	/* blk_try_blockalign() for src will choose an alignment that works for
	* backing as well, so no need to compare the alignment manually. */
	buf = blk_try_blockalign(src, COMMIT_BUF_SIZE);
	if (buf == NULL) {
	ret = -ENOMEM;
	goto ro_cleanup;
	}

	for (offset = 0; offset < length; offset += n) {
	ret = bdrv_is_allocated(bs, offset, COMMIT_BUF_SIZE, &n);
	if (ret < 0) {
	goto ro_cleanup;
	}
	if (ret) {
	ret = blk_pread(src, offset, buf, n);
	if (ret < 0) {
	goto ro_cleanup;
	}

	ret = blk_pwrite(backing, offset, buf, n, 0);
	if (ret < 0) {
	goto ro_cleanup;
	}
	}
	}

	ret = blk_make_empty(src, NULL);
	/* Ignore -ENOTSUP */
	if (ret < 0 && ret != -ENOTSUP) {
	goto ro_cleanup;
	}

	blk_flush(src);

	/*
	* Make sure all data we wrote to the backing device is actually
	* stable on disk.
	*/
	blk_flush(backing);

	ret = 0;
	ro_cleanup:
	blk_unref(backing);
	if (bdrv_cow_bs(bs) != backing_file_bs) {
	bdrv_set_backing_hd(bs, backing_file_bs, &error_abort);
	}
	bdrv_unref(commit_top_bs);
	blk_unref(src);

	if (ro) {
	/* ignoring error return here */
	bdrv_reopen_set_read_only(backing_file_bs, true, NULL);
	}

	return ret;
	}