[pve-devel] [PATCH qemu] PVE backup: improve error when copy-before-write fails for fleecing

[Fiona Ebner]

With fleecing, failure for copy-before-write does not fail the guest
write, but only sets the snapshot error that is associated to the
copy-before-write filter, making further requests to the snapshot
access fail with EACCES, which then also fails the job. But that error
code is not the root cause of why the backup failed, so bubble up the
original snapshot error instead.

Reported-by: Friedrich Weber <f.weber at proxmox.com>
Signed-off-by: Fiona Ebner <f.ebner at proxmox.com>
Tested-by: Friedrich Weber <f.weber at proxmox.com>
---

Should have a "fixes #5409:" prefix when applied in the parent git
module. Please consider applying this after the update to QEMU 9.0.0.
Otherwise, I'll have to adapt and re-send that.

 block/copy-before-write.c | 18 ++++++++++++------
 block/copy-before-write.h |  1 +
 pve-backup.c              |  9 +++++++++
 3 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/block/copy-before-write.c b/block/copy-before-write.c
index 9ca5ec5e5c..7d70f221ca 100644
--- a/block/copy-before-write.c
+++ b/block/copy-before-write.c
@@ -27,6 +27,7 @@
 #include "qapi/qmp/qjson.h"
 
 #include "sysemu/block-backend.h"
+#include "qemu/atomic.h"
 #include "qemu/cutils.h"
 #include "qapi/error.h"
 #include "block/block_int.h"
@@ -74,7 +75,8 @@ typedef struct BDRVCopyBeforeWriteState {
      * @snapshot_error is normally zero. But on first copy-before-write failure
      * when @on_cbw_error == ON_CBW_ERROR_BREAK_SNAPSHOT, @snapshot_error takes
      * value of this error (<0). After that all in-flight and further
-     * snapshot-API requests will fail with that error.
+     * snapshot-API requests will fail with that error. To be accessed with
+     * atomics.
      */
     int snapshot_error;
 } BDRVCopyBeforeWriteState;
@@ -114,7 +116,7 @@ static coroutine_fn int cbw_do_copy_before_write(BlockDriverState *bs,
         return 0;
     }
 
-    if (s->snapshot_error) {
+    if (qatomic_read(&s->snapshot_error)) {
         return 0;
     }
 
@@ -138,9 +140,7 @@ static coroutine_fn int cbw_do_copy_before_write(BlockDriverState *bs,
     WITH_QEMU_LOCK_GUARD(&s->lock) {
         if (ret < 0) {
             assert(s->on_cbw_error == ON_CBW_ERROR_BREAK_SNAPSHOT);
-            if (!s->snapshot_error) {
-                s->snapshot_error = ret;
-            }
+            qatomic_cmpxchg(&s->snapshot_error, 0, ret);
         } else {
             bdrv_set_dirty_bitmap(s->done_bitmap, off, end - off);
         }
@@ -214,7 +214,7 @@ cbw_snapshot_read_lock(BlockDriverState *bs, int64_t offset, int64_t bytes,
 
     QEMU_LOCK_GUARD(&s->lock);
 
-    if (s->snapshot_error) {
+    if (qatomic_read(&s->snapshot_error)) {
         g_free(req);
         return NULL;
     }
@@ -594,6 +594,12 @@ void bdrv_cbw_drop(BlockDriverState *bs)
     bdrv_unref(bs);
 }
 
+int bdrv_cbw_snapshot_error(BlockDriverState *bs)
+{
+    BDRVCopyBeforeWriteState *s = bs->opaque;
+    return qatomic_read(&s->snapshot_error);
+}
+
 static void cbw_init(void)
 {
     bdrv_register(&bdrv_cbw_filter);
diff --git a/block/copy-before-write.h b/block/copy-before-write.h
index dc6cafe7fa..a27d2d7d9f 100644
--- a/block/copy-before-write.h
+++ b/block/copy-before-write.h
@@ -44,5 +44,6 @@ BlockDriverState *bdrv_cbw_append(BlockDriverState *source,
                                   BlockCopyState **bcs,
                                   Error **errp);
 void bdrv_cbw_drop(BlockDriverState *bs);
+int bdrv_cbw_snapshot_error(BlockDriverState *bs);
 
 #endif /* COPY_BEFORE_WRITE_H */
diff --git a/pve-backup.c b/pve-backup.c
index 00aaff6509..f8fa8e068b 100644
--- a/pve-backup.c
+++ b/pve-backup.c
@@ -385,6 +385,15 @@ static void pvebackup_complete_cb(void *opaque, int ret)
         di->fleecing.snapshot_access = NULL;
     }
     if (di->fleecing.cbw) {
+        /*
+         * With fleecing, failure for cbw does not fail the guest write, but only sets the snapshot
+         * error, making further requests to the snapshot fail with EACCES, which then also fail the
+         * job. But that code is not the root cause and just confusing, so update it.
+         */
+        int snapshot_error = bdrv_cbw_snapshot_error(di->fleecing.cbw);
+        if (di->completed_ret == -EACCES && snapshot_error) {
+            di->completed_ret = snapshot_error;
+        }
         bdrv_cbw_drop(di->fleecing.cbw);
         di->fleecing.cbw = NULL;
     }
-- 
2.39.2