Re: [PATCH] Btrfs: fix missing hole after hole punching and fsync when using NO_HOLES

[Josef Bacik <josef@toxicpanda.com>]

On Tue, Nov 12, 2019 at 03:13:31PM +0000, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
> 
> 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@suse.com>

This adds an extra search for every FULL_SYNC, can we just catch this case in
the main loop, say we keep track of the last extent we found, and then when we
end up with ret > 1 || a min_key that's past the end of the last extent we saw
we know we had a hole punch?  Thanks,

Josef