Commit 2d99b6dd authored by Mikulas Patocka's avatar Mikulas Patocka Committed by Greg Kroah-Hartman

dm snapshot: fix data corruption

commit e9c6a182649f4259db704ae15a91ac820e63b0ca upstream.

This patch fixes a particular type of data corruption that has been
encountered when loading a snapshot's metadata from disk.

When we allocate a new chunk in persistent_prepare, we increment
ps->next_free and we make sure that it doesn't point to a metadata area
by further incrementing it if necessary.

When we load metadata from disk on device activation, ps->next_free is
positioned after the last used data chunk. However, if this last used
data chunk is followed by a metadata area, ps->next_free is positioned
erroneously to the metadata area. A newly-allocated chunk is placed at
the same location as the metadata area, resulting in data or metadata

This patch changes the code so that ps->next_free skips the metadata
area when metadata are loaded in function read_exceptions.

The patch also moves a piece of code from persistent_prepare_exception
to a separate function skip_metadata to avoid code duplication.

Signed-off-by: default avatarMikulas Patocka <>
Cc: Mike Snitzer <>
Signed-off-by: default avatarAlasdair G Kergon <>
Signed-off-by: default avatarGreg Kroah-Hartman <>
parent 5e267213
......@@ -269,6 +269,14 @@ static chunk_t area_location(struct pstore *ps, chunk_t area)
return NUM_SNAPSHOT_HDR_CHUNKS + ((ps->exceptions_per_area + 1) * area);
static void skip_metadata(struct pstore *ps)
uint32_t stride = ps->exceptions_per_area + 1;
chunk_t next_free = ps->next_free;
if (sector_div(next_free, stride) == NUM_SNAPSHOT_HDR_CHUNKS)
* Read or write a metadata area. Remembering to skip the first
* chunk which holds the header.
......@@ -502,6 +510,8 @@ static int read_exceptions(struct pstore *ps,
return 0;
......@@ -616,8 +626,6 @@ static int persistent_prepare_exception(struct dm_exception_store *store,
struct dm_exception *e)
struct pstore *ps = get_info(store);
uint32_t stride;
chunk_t next_free;
sector_t size = get_dev_size(dm_snap_cow(store->snap)->bdev);
/* Is there enough room ? */
......@@ -630,10 +638,8 @@ static int persistent_prepare_exception(struct dm_exception_store *store,
* Move onto the next free pending, making sure to take
* into account the location of the metadata chunks.
stride = (ps->exceptions_per_area + 1);
next_free = ++ps->next_free;
if (sector_div(next_free, stride) == 1)
return 0;
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