From d9a55a5815a040032f85c20020b118dda54bba43 Mon Sep 17 00:00:00 2001

From: John Snow <jsnow@redhat.com>

Date: Wed, 18 Jul 2018 22:54:58 +0200

Subject: [PATCH 240/268] block/backup: fix fleecing scheme: use serialized

 writes

RH-Author: John Snow <jsnow@redhat.com>

Message-id: <20180718225511.14878-23-jsnow@redhat.com>

Patchwork-id: 81396

O-Subject: [RHEL-7.6 qemu-kvm-rhev PATCH 22/35] block/backup: fix fleecing scheme: use serialized writes

Bugzilla: 1207657

RH-Acked-by: Eric Blake <eblake@redhat.com>

RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>

RH-Acked-by: Fam Zheng <famz@redhat.com>

From: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

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 <vsementsov@virtuozzo.com>

Reviewed-by: Fam Zheng <famz@redhat.com>

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

(cherry picked from commit f8d59dfb40bbc6f5aeea57c8aac1e68c1d2454ee)

Signed-off-by: John Snow <jsnow@redhat.com>

Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>

---

 block/backup.c | 20 ++++++++++++++------

 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/block/backup.c b/block/backup.c

index 369155a..4ba1a6a 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. */

-- 

1.8.3.1