block/copy-before-write: add on-cbw-error open parameter

Currently, behavior on copy-before-write operation failure is simple:
report error to the guest.

Let's implement alternative behavior: break the whole copy-before-write
process (and corresponding backup job or NBD client) but keep guest
working. It's needed if we consider guest stability as more important.

The realisation is simple: on copy-before-write failure we set
s->snapshot_ret and continue guest operations. s->snapshot_ret being
set will lead to all further snapshot API requests. Note that all
in-flight snapshot-API requests may still success: we do wait for them
on BREAK_SNAPSHOT-failure path in cbw_do_copy_before_write().

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@openvz.org>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>

1 files changed, 30 insertions, 2 deletions

diff --git a/block/copy-before-write.c b/block/copy-before-write.c
index e29c46cd7a..c8a11a09d2 100644
--- a/block/copy-before-write.c
+++ b/block/copy-before-write.c
@@ -41,6 +41,7 @@
 typedef struct BDRVCopyBeforeWriteState {
     BlockCopyState *bcs;
     BdrvChild *target;
+    OnCbwError on_cbw_error;
 
     /*
      * @lock: protects access to @access_bitmap, @done_bitmap and
@@ -65,6 +66,14 @@ typedef struct BDRVCopyBeforeWriteState {
      * node. These areas must not be rewritten by guest.
      */
     BlockReqList frozen_read_reqs;
+
+    /*
+     * @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.
+     */
+    int snapshot_error;
 } BDRVCopyBeforeWriteState;
 
 static coroutine_fn int cbw_co_preadv(
@@ -95,16 +104,27 @@ static coroutine_fn int cbw_do_copy_before_write(BlockDriverState *bs,
         return 0;
     }
 
+    if (s->snapshot_error) {
+        return 0;
+    }
+
     off = QEMU_ALIGN_DOWN(offset, cluster_size);
     end = QEMU_ALIGN_UP(offset + bytes, cluster_size);
 
     ret = block_copy(s->bcs, off, end - off, true);
-    if (ret < 0) {
+    if (ret < 0 && s->on_cbw_error == ON_CBW_ERROR_BREAK_GUEST_WRITE) {
         return ret;
     }
 
     WITH_QEMU_LOCK_GUARD(&s->lock) {
-        bdrv_set_dirty_bitmap(s->done_bitmap, off, end - off);
+        if (ret < 0) {
+            assert(s->on_cbw_error == ON_CBW_ERROR_BREAK_SNAPSHOT);
+            if (!s->snapshot_error) {
+                s->snapshot_error = ret;
+            }
+        } else {
+            bdrv_set_dirty_bitmap(s->done_bitmap, off, end - off);
+        }
         reqlist_wait_all(&s->frozen_read_reqs, off, end - off, &s->lock);
     }
 
@@ -176,6 +196,11 @@ static BlockReq *cbw_snapshot_read_lock(BlockDriverState *bs,
 
     QEMU_LOCK_GUARD(&s->lock);
 
+    if (s->snapshot_error) {
+        g_free(req);
+        return NULL;
+    }
+
     if (bdrv_dirty_bitmap_next_zero(s->access_bitmap, offset, bytes) != -1) {
         g_free(req);
         return NULL;
@@ -351,6 +376,7 @@ static BlockdevOptions *cbw_parse_options(QDict *options, Error **errp)
      * object for original options.
      */
     qdict_extract_subqdict(options, NULL, "bitmap");
+    qdict_del(options, "on-cbw-error");
 
 out:
     visit_free(v);
@@ -395,6 +421,8 @@ static int cbw_open(BlockDriverState *bs, QDict *options, int flags,
             return -EINVAL;
         }
     }
+    s->on_cbw_error = opts->has_on_cbw_error ? opts->on_cbw_error :
+            ON_CBW_ERROR_BREAK_GUEST_WRITE;
 
     bs->total_sectors = bs->file->bs->total_sectors;
     bs->supported_write_flags = BDRV_REQ_WRITE_UNCHANGED |