On Tue, Nov 12, 2019 at 03:13:31PM +0000, fdmanana@xxxxxxxxxx wrote:
> From: Filipe Manana <fdmanana@xxxxxxxx>
>
> When using the NO_HOLES feature, if we punch a hole into a file and then
> fsync it, there is a case where a subsequent fsync will miss the fact that
> a hole was punched:
>
> 1) The extent items of the inode span multiple leafs;
>
> 2) The hole covers a range that affects only the extent items of the first
> leaf;
>
> 3) The fsync operation is done in full mode (BTRFS_INODE_NEEDS_FULL_SYNC
> is set in the inode's runtime flags).
>
> That results in the hole not existing after replaying the log tree.
>
> For example, if the fs/subvolume tree has the following layout for a
> particular inode:
>
> Leaf N, generation 10:
>
> [ ... INODE_ITEM INODE_REF EXTENT_ITEM (0 64K) EXTENT_ITEM (64K 128K) ]
>
> Leaf N + 1, generation 10:
>
> [ EXTENT_ITEM (128K 64K) ... ]
>
> If at transaction 11 we punch a hole coverting the range [0, 128K[, we end
> up dropping the two extent items from leaf N, but we don't touch the other
> leaf, so we end up in the following state:
>
> Leaf N, generation 11:
>
> [ ... INODE_ITEM INODE_REF ]
>
> Leaf N + 1, generation 10:
>
> [ EXTENT_ITEM (128K 64K) ... ]
>
> A full fsync after punching the hole will only process leaf N because it
> was modified in the current transaction, but not leaf N + 1, since it was
> not modified in the current transaction (generation 10 and not 11). As
> a result the fsync will not log any holes, because it didn't process any
> leaf with extent items.
>
> So fix this by detecting any leading hole in the file for a full fsync
> when using the NO_HOLES feature if we didn't process any extent items for
> the file.
>
> A test case for fstests follows soon.
>
> Fixes: 16e7549f045d33 ("Btrfs: incompatible format change to remove hole extents")
> Signed-off-by: Filipe Manana <fdmanana@xxxxxxxx>
Added to misc-next, thanks.
