Currently, mirror disables the block layer's dirty bitmap before its own
replacement is working. This means that during startup, there is a
window in which the allocation status of blocks in the source has
already been checked, but new writes coming in aren't tracked yet,
resulting in a corrupted copy:
1. Dirty bitmap is disabled in mirror_start_job()
2. Some request are started in mirror_top_bs while s->job == NULL
3. mirror_dirty_init() -> bdrv_co_is_allocated_above() runs and because
the request hasn't completed yet, the block isn't allocated
4. The request completes, still sees s->job == NULL and skips the
bitmap, and nothing else will mark it dirty either
One ingredient is that mirror_top_opaque->job is only set after the
job is fully initialized. For the rationale, see commit 32125b1460
("mirror: Fix access of uninitialised fields during start").
Fix this by giving mirror_top_bs access to dirty_bitmap and enabling it
to track writes from the beginning. Disabling the block layer's tracking
and enabling the mirror_top_bs one happens in a drained section, so
there is no danger of races with in-flight requests any more. All of
this happens well before the block allocation status is checked, so we
can be sure that no writes will be missed.
Cc: [email protected]
Closes: https://gitlab.com/qemu-project/qemu/-/issues/3273
Fixes: 32125b14606a ('mirror: Fix access of uninitialised fields during start')
Signed-off-by: Kevin Wolf <[email protected]>
---
Supersedes: <[email protected]>
---
block/mirror.c | 48 +++++++++++++++++++++++++++++-------------------
1 file changed, 29 insertions(+), 19 deletions(-)
diff --git a/block/mirror.c b/block/mirror.c
index b344182c747..f38636e7457 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -99,6 +99,7 @@ typedef struct MirrorBlockJob {
typedef struct MirrorBDSOpaque {
MirrorBlockJob *job;
+ BdrvDirtyBitmap *dirty_bitmap;
bool stop;
bool is_commit;
} MirrorBDSOpaque;
@@ -1672,9 +1673,9 @@ bdrv_mirror_top_do_write(BlockDriverState *bs,
MirrorMethod method,
abort();
}
- if (!copy_to_target && s->job && s->job->dirty_bitmap) {
+ if (!copy_to_target) {
qatomic_set(&s->job->actively_synced, false);
- bdrv_set_dirty_bitmap(s->job->dirty_bitmap, offset, bytes);
+ bdrv_set_dirty_bitmap(s->dirty_bitmap, offset, bytes);
}
if (ret < 0) {
@@ -1901,13 +1902,35 @@ static BlockJob *mirror_start_job(
bdrv_drained_begin(bs);
ret = bdrv_append(mirror_top_bs, bs, errp);
- bdrv_drained_end(bs);
-
if (ret < 0) {
+ bdrv_drained_end(bs);
+ bdrv_unref(mirror_top_bs);
+ return NULL;
+ }
+
+ bs_opaque->dirty_bitmap = bdrv_create_dirty_bitmap(mirror_top_bs,
+ granularity,
+ NULL, errp);
+ if (!bs_opaque->dirty_bitmap) {
+ bdrv_drained_end(bs);
bdrv_unref(mirror_top_bs);
return NULL;
}
+ /*
+ * The mirror job doesn't use the block layer's dirty tracking because it
+ * needs to be able to switch seemlessly between background copy mode
(which
+ * does need dirty tracking) and write blocking mode (which doesn't) and
+ * doing that would require draining the node. Instead, mirror_top_bs takes
+ * care of updating the dirty bitmap as appropriate.
+ *
+ * Note that write blocking mode only becomes effective after mirror_run()
+ * sets mirror_top_opaque->job (see should_copy_to_target()). Until then,
+ * we're still in background copy mode irrespective of @copy_mode.
+ */
+ bdrv_disable_dirty_bitmap(bs_opaque->dirty_bitmap);
+ bdrv_drained_end(bs);
+
/* Make sure that the source is not resized while the job is running */
s = block_job_create(job_id, driver, NULL, mirror_top_bs,
BLK_PERM_CONSISTENT_READ,
@@ -2002,24 +2025,13 @@ static BlockJob *mirror_start_job(
s->base_overlay = bdrv_find_overlay(bs, base);
s->granularity = granularity;
s->buf_size = ROUND_UP(buf_size, granularity);
+ s->dirty_bitmap = bs_opaque->dirty_bitmap;
s->unmap = unmap;
if (auto_complete) {
s->should_complete = true;
}
bdrv_graph_rdunlock_main_loop();
- s->dirty_bitmap = bdrv_create_dirty_bitmap(s->mirror_top_bs, granularity,
- NULL, errp);
- if (!s->dirty_bitmap) {
- goto fail;
- }
-
- /*
- * The dirty bitmap is set by bdrv_mirror_top_do_write() when not in active
- * mode.
- */
- bdrv_disable_dirty_bitmap(s->dirty_bitmap);
-
bdrv_graph_wrlock_drained();
ret = block_job_add_bdrv(&s->common, "source", bs, 0,
BLK_PERM_WRITE_UNCHANGED | BLK_PERM_WRITE |
@@ -2099,9 +2111,6 @@ fail:
g_free(s->replaces);
blk_unref(s->target);
bs_opaque->job = NULL;
- if (s->dirty_bitmap) {
- bdrv_release_dirty_bitmap(s->dirty_bitmap);
- }
job_early_fail(&s->common.job);
}
@@ -2115,6 +2124,7 @@ fail:
bdrv_graph_wrunlock();
bdrv_drained_end(bs);
+ bdrv_release_dirty_bitmap(bs_opaque->dirty_bitmap);
bdrv_unref(mirror_top_bs);
return NULL;
--
2.53.0
