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Cc: dm-devel@redhat.com, Alasdair G Kergon <agk@redhat.com>
Subject: [dm-devel] Re: clustered snapshots
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diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
index bcfbe85..b271f56 100644
--- a/drivers/md/dm-snap.c
+++ b/drivers/md/dm-snap.c
@@ -790,17 +790,25 @@ static void merge_callback(int read_err, unsigned long write_err, void *context)
         * so that dm_consecutive_chunk_count_dec() accounting works
         */
        for (i = s->merge_write_interlock_n - 1; i >= 0; i--) {
-               e = lookup_exception(&s->complete,
-                                    s->merge_write_interlock + i);
+               chunk_t old_chunk = s->merge_write_interlock + i;
+               e = lookup_exception(&s->complete, old_chunk);
                if (!e) {
                        DMERR("exception for block %llu is on "
                              "disk but not in memory",
-                             (unsigned long long)
-                             s->merge_write_interlock + i);
+                             (unsigned long long)old_chunk);
                        up_write(&s->lock);
                        goto shut;
                }
                if (dm_consecutive_chunk_count(e)) {
+                       if (old_chunk == e->old_chunk) {
+                               e->old_chunk++;
+                               e->new_chunk++;
+                       } else if (old_chunk != e->old_chunk +
+                                  dm_consecutive_chunk_count(e)) {
+                               DMERR("merge from the middle of a chunk range");
+                               up_write(&s->lock);
+                               goto shut;
+                       }
                        dm_consecutive_chunk_count_dec(e);
                } else {
                        remove_exception(e);