Fleecing scheme works as follows: we want a kind of temporary snapshot
of active drive A. We create temporary image B, with B->backing = A.
Then we start backup(sync=none) from A to B. From this point, B reads
as point-in-time snapshot of A (A continues to be active drive,
accepting guest IO).
This scheme needs some additional synchronization between reads from B
and backup COW operations, otherwise, the following situation is
theoretically possible:
(assume B is qcow2, client is NBD client, reading from B)
1. client starts reading and take qcow2 mutex in qcow2_co_preadv, and
goes up to l2 table loading (assume cache miss)
2) guest write => backup COW => qcow2 write =>
try to take qcow2 mutex => waiting
3. l2 table loaded, we see that cluster is UNALLOCATED, go to
"case QCOW2_CLUSTER_UNALLOCATED" and unlock mutex before
bdrv_co_preadv(bs->backing, ...)
4) aha, mutex unlocked, backup COW continues, and we finally finish
guest write and change cluster in our active disk A
5. actually, do bdrv_co_preadv(bs->backing, ...) and read
_new updated_ data.
To avoid this, let's make backup writes serializing, to not intersect
with reads from B.
Note: we expand range of handled cases from (sync=none and
B->backing = A) to just (A in backing chain of B), to finally allow
safe reading from B during backup for all cases when A in backing chain
of B, i.e. B formally looks like point-in-time snapshot of A.
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsement...@virtuozzo.com>
---
block/backup.c | 20 ++++++++++++++------
1 file changed, 14 insertions(+), 6 deletions(-)
diff --git a/block/backup.c b/block/backup.c
index f3e4e814b6..319fc922e8 100644
--- a/block/backup.c
+++ b/block/backup.c
@@ -47,6 +47,8 @@ typedef struct BackupBlockJob {
HBitmap *copy_bitmap;
bool use_copy_range;
int64_t copy_range_size;
+
+ bool serialize_target_writes;
} BackupBlockJob;
static const BlockJobDriver backup_job_driver;
@@ -102,6 +104,8 @@ static int coroutine_fn
backup_cow_with_bounce_buffer(BackupBlockJob *job,
QEMUIOVector qiov;
BlockBackend *blk = job->common.blk;
int nbytes;
+ int read_flags = is_write_notifier ? BDRV_REQ_NO_SERIALISING : 0;
+ int write_flags = job->serialize_target_writes ? BDRV_REQ_SERIALISING : 0;
hbitmap_reset(job->copy_bitmap, start / job->cluster_size, 1);
nbytes = MIN(job->cluster_size, job->len - start);
@@ -112,8 +116,7 @@ static int coroutine_fn
backup_cow_with_bounce_buffer(BackupBlockJob *job,
iov.iov_len = nbytes;
qemu_iovec_init_external(&qiov, &iov, 1);
- ret = blk_co_preadv(blk, start, qiov.size, &qiov,
- is_write_notifier ? BDRV_REQ_NO_SERIALISING : 0);
+ ret = blk_co_preadv(blk, start, qiov.size, &qiov, read_flags);
if (ret < 0) {
trace_backup_do_cow_read_fail(job, start, ret);
if (error_is_read) {
@@ -124,11 +127,11 @@ static int coroutine_fn
backup_cow_with_bounce_buffer(BackupBlockJob *job,
if (qemu_iovec_is_zero(&qiov)) {
ret = blk_co_pwrite_zeroes(job->target, start,
- qiov.size, BDRV_REQ_MAY_UNMAP);
+ qiov.size, write_flags |
BDRV_REQ_MAY_UNMAP);
} else {
ret = blk_co_pwritev(job->target, start,
- qiov.size, &qiov,
- job->compress ? BDRV_REQ_WRITE_COMPRESSED : 0);
+ qiov.size, &qiov, write_flags |
+ (job->compress ? BDRV_REQ_WRITE_COMPRESSED : 0));
}
if (ret < 0) {
trace_backup_do_cow_write_fail(job, start, ret);
@@ -156,6 +159,8 @@ static int coroutine_fn
backup_cow_with_offload(BackupBlockJob *job,
int nr_clusters;
BlockBackend *blk = job->common.blk;
int nbytes;
+ int read_flags = is_write_notifier ? BDRV_REQ_NO_SERIALISING : 0;
+ int write_flags = job->serialize_target_writes ? BDRV_REQ_SERIALISING : 0;
assert(QEMU_IS_ALIGNED(job->copy_range_size, job->cluster_size));
nbytes = MIN(job->copy_range_size, end - start);
@@ -163,7 +168,7 @@ static int coroutine_fn
backup_cow_with_offload(BackupBlockJob *job,
hbitmap_reset(job->copy_bitmap, start / job->cluster_size,
nr_clusters);
ret = blk_co_copy_range(blk, start, job->target, start, nbytes,
- is_write_notifier ? BDRV_REQ_NO_SERIALISING : 0,
0);
+ read_flags, write_flags);
if (ret < 0) {
trace_backup_do_cow_copy_range_fail(job, start, ret);
hbitmap_set(job->copy_bitmap, start / job->cluster_size,
@@ -701,6 +706,9 @@ BlockJob *backup_job_create(const char *job_id,
BlockDriverState *bs,
sync_bitmap : NULL;
job->compress = compress;
+ /* Detect image-fleecing (and similar) schemes */
+ job->serialize_target_writes = bdrv_chain_contains(target, bs);
+
/* If there is no backing file on the target, we cannot rely on COW if our
* backup cluster size is smaller than the target cluster size. Even for
* targets with a backing file, try to avoid COW if possible. */
--
2.11.1
