[PATCH 3.5 25/64] dm snapshot: fix data corruption

From: Luis Henriques
Date: Mon Oct 28 2013 - 11:02:16 EST -stable review patch. If anyone has any objections, please let me know.


From: Mikulas Patocka <mpatocka@xxxxxxxxxx>

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: Mikulas Patocka <mpatocka@xxxxxxxxxx>
Cc: Mike Snitzer <snitzer@xxxxxxxxxx>
Signed-off-by: Alasdair G Kergon <agk@xxxxxxxxxx>
Signed-off-by: Luis Henriques <luis.henriques@xxxxxxxxxxxxx>
drivers/md/dm-snap-persistent.c | 18 ++++++++++++------
1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/drivers/md/dm-snap-persistent.c b/drivers/md/dm-snap-persistent.c
index 4caa8e6..2d2b1b7 100644
--- a/drivers/md/dm-snap-persistent.c
+++ b/drivers/md/dm-snap-persistent.c
@@ -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)
+ ps->next_free++;
* 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,


+ skip_metadata(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)
- ps->next_free++;
+ ps->next_free++;
+ skip_metadata(ps);

return 0;

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