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

[Filipe Manana <fdmanana@kernel.org>]

On Thu, Nov 14, 2019 at 3:18 PM David Sterba <dsterba@suse.cz> wrote:
>
> 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>
>
> Added to misc-next, thanks.

Actually, could you hold on a bit for this one?
There's nothing wrong with it, I'm simply fixing other cases and
realized I can fix them all in a single patch.
If I take too long or end up not being able to fix all as I'm
expecting, I'll let you know, otherwise I'll send a very different v2
tomorrow.

Thanks.