Use a separate memory area for the header. This patch fixes the racy corruption of on-disk header. When the snapshot overflows, __invalidate_snapshot is called, which calls snapshot store method drop_snapshot. It goes to persistent_drop_snapshot that calls write_header. write_header constructs the new header in the "area" location. Concurrently with that, existing kcopyd job may finish, call copy_callback and commit_exception method, that goes to persistent_commit_exception. persistent_commit_exception doesn't do locking, relying on the fact that callbacks are single-threaded, but it can race with snapshot invalidation and overwrite the header that is just being written while the snapshot is being invalidated. The result of this race is a corrupted header being written that can lead to a crash on further reactivation (if chunk_size is zero in the corrupted header). See the bug: https://bugzilla.redhat.com/show_bug.cgi?id=461506 Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx> --- drivers/md/dm-snap-persistent.c | 42 ++++++++++++++++++++++++++++++---------- 1 file changed, 32 insertions(+), 10 deletions(-) Index: linux-2.6.31-rc5-devel/drivers/md/dm-snap-persistent.c =================================================================== --- linux-2.6.31-rc5-devel.orig/drivers/md/dm-snap-persistent.c 2009-08-03 17:40:29.000000000 +0200 +++ linux-2.6.31-rc5-devel/drivers/md/dm-snap-persistent.c 2009-08-03 17:43:28.000000000 +0200 @@ -106,6 +106,13 @@ struct pstore { void *zero_area; /* + * An area used for header. The header can be written + * concurrently with metadata (when invalidating the snapshot), + * so it needs a separate buffer. + */ + void *header_area; + + /* * Used to keep track of which metadata area the data in * 'chunk' refers to. */ @@ -148,16 +155,25 @@ static int alloc_area(struct pstore *ps) */ ps->area = vmalloc(len); if (!ps->area) - return r; + goto err0; ps->zero_area = vmalloc(len); - if (!ps->zero_area) { - vfree(ps->area); - return r; - } + if (!ps->zero_area) + goto err1; memset(ps->zero_area, 0, len); + ps->header_area = vmalloc(len); + if (!ps->header_area) + goto err2; + return 0; + +err2: + vfree(ps->zero_area); +err1: + vfree(ps->area); +err0: + return r; } static void free_area(struct pstore *ps) @@ -169,6 +185,10 @@ static void free_area(struct pstore *ps) if (ps->zero_area) vfree(ps->zero_area); ps->zero_area = NULL; + + if (ps->header_area) + vfree(ps->header_area); + ps->header_area = NULL; } struct mdata_req { @@ -284,11 +304,11 @@ static int read_header(struct pstore *ps if (r) return r; - r = chunk_io(ps, ps->area, 0, READ, 1); + r = chunk_io(ps, ps->header_area, 0, READ, 1); if (r) goto bad; - dh = (struct disk_header *) ps->area; + dh = (struct disk_header *) ps->header_area; if (le32_to_cpu(dh->magic) == 0) { *new_snapshot = 1; @@ -338,15 +358,15 @@ static int write_header(struct pstore *p { struct disk_header *dh; - memset(ps->area, 0, ps->store->chunk_size << SECTOR_SHIFT); + memset(ps->header_area, 0, ps->store->chunk_size << SECTOR_SHIFT); - dh = (struct disk_header *) ps->area; + dh = (struct disk_header *) ps->header_area; dh->magic = cpu_to_le32(SNAP_MAGIC); dh->valid = cpu_to_le32(ps->valid); dh->version = cpu_to_le32(ps->version); dh->chunk_size = cpu_to_le32(ps->store->chunk_size); - return chunk_io(ps, ps->area, 0, WRITE, 1); + return chunk_io(ps, ps->header_area, 0, WRITE, 1); } /* @@ -666,6 +686,8 @@ static int persistent_ctr(struct dm_exce ps->valid = 1; ps->version = SNAPSHOT_DISK_VERSION; ps->area = NULL; + ps->zero_area = NULL; + ps->header_area = NULL; ps->next_free = 2; /* skipping the header and first area */ ps->current_committed = 0; -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
