3.16.78-rc1 review patch. If anyone has any objections, please let me know.
------------------
From: Hans van Kranenburg hans.van.kranenburg@mendix.com
commit baf92114c7e6dd6124aa3d506e4bc4b694da3bc3 upstream.
Commit 92e222df7b "btrfs: alloc_chunk: fix DUP stripe size handling" fixed calculating the stripe_size for a new DUP chunk.
However, the same calculation reappears a bit later, and that one was not changed yet. The resulting bug that is exposed is that the newly allocated device extents ('stripes') can have a few MiB overlap with the next thing stored after them, which is another device extent or the end of the disk.
The scenario in which this can happen is: * The block device for the filesystem is less than 10GiB in size. * The amount of contiguous free unallocated disk space chosen to use for chunk allocation is 20% of the total device size, or a few MiB more or less.
An example: - The filesystem device is 7880MiB (max_chunk_size gets set to 788MiB) - There's 1578MiB unallocated raw disk space left in one contiguous piece.
In this case stripe_size is first calculated as 789MiB, (half of 1578MiB).
Since 789MiB (stripe_size * data_stripes) > 788MiB (max_chunk_size), we enter the if block. Now stripe_size value is immediately overwritten while calculating an adjusted value based on max_chunk_size, which ends up as 788MiB.
Next, the value is rounded up to a 16MiB boundary, 800MiB, which is actually more than the value we had before. However, the last comparison fails to detect this, because it's comparing the value with the total amount of free space, which is about twice the size of stripe_size.
In the example above, this means that the resulting raw disk space being allocated is 1600MiB, while only a gap of 1578MiB has been found. The second device extent object for this DUP chunk will overlap for 22MiB with whatever comes next.
The underlying problem here is that the stripe_size is reused all the time for different things. So, when entering the code in the if block, stripe_size is immediately overwritten with something else. If later we decide we want to have the previous value back, then the logic to compute it was copy pasted in again.
With this change, the value in stripe_size is not unnecessarily destroyed, so the duplicated calculation is not needed any more.
Signed-off-by: Hans van Kranenburg hans.van.kranenburg@mendix.com Signed-off-by: David Sterba dsterba@suse.com Signed-off-by: Ben Hutchings ben@decadent.org.uk --- fs/btrfs/volumes.c | 16 +++++++--------- 1 file changed, 7 insertions(+), 9 deletions(-)
--- a/fs/btrfs/volumes.c +++ b/fs/btrfs/volumes.c @@ -4271,19 +4271,17 @@ static int __btrfs_alloc_chunk(struct bt /* * Use the number of data stripes to figure out how big this chunk * is really going to be in terms of logical address space, - * and compare that answer with the max chunk size + * and compare that answer with the max chunk size. If it's higher, + * we try to reduce stripe_size. */ if (stripe_size * data_stripes > max_chunk_size) { - stripe_size = div_u64(max_chunk_size, data_stripes); - - /* bump the answer up to a 16MB boundary */ - stripe_size = round_up(stripe_size, SZ_16M); - /* - * But don't go higher than the limits we found while searching - * for free extents + * Reduce stripe_size, round it up to a 16MB boundary again and + * then use it, unless it ends up being even bigger than the + * previous value we had already. */ - stripe_size = min(devices_info[ndevs - 1].max_avail, + stripe_size = min(round_up(div_u64(max_chunk_size, + data_stripes), SZ_16M), stripe_size); }