Re: [PATCH] Btrfs: send, fix emission of invalid clone operations within the same file

[Filipe Manana <fdmanana@kernel.org>]

On Tue, Jan 28, 2020 at 4:22 PM David Sterba <dsterba@suse.cz> wrote:
>
> On Fri, Jan 24, 2020 at 11:52:04AM +0000, fdmanana@kernel.org wrote:
> > From: Filipe Manana <fdmanana@suse.com>
> >
> > When doing an incremental send and a file has extents shared with itself
> > at different file offsets, it's possible for send to emit clone operations
> > that will fail at the destination because the source range goes beyond the
> > file's current size. This happens when the file size has increased in the
> > send snapshot, there is a hole between the shared extents and both shared
> > extents are at file offsets which are greater the file's size in the
> > parent snapshot.
> >
> > Example:
> >
> >   $ mkfs.btrfs -f /dev/sdb
> >   $ mount /dev/sdb /mnt/sdb
> >
> >   $ xfs_io -f -c "pwrite -S 0xf1 0 64K" /mnt/sdb/foobar
> >   $ btrfs subvolume snapshot -r /mnt/sdb /mnt/sdb/base
> >   $ btrfs send -f /tmp/1.snap /mnt/sdb/base
> >
> >   # Create a 320K extent at file offset 512K.
> >   $ xfs_io -c "pwrite -S 0xab 512K 64K" /mnt/sdb/foobar
> >   $ xfs_io -c "pwrite -S 0xcd 576K 64K" /mnt/sdb/foobar
> >   $ xfs_io -c "pwrite -S 0xef 640K 64K" /mnt/sdb/foobar
> >   $ xfs_io -c "pwrite -S 0x64 704K 64K" /mnt/sdb/foobar
> >   $ xfs_io -c "pwrite -S 0x73 768K 64K" /mnt/sdb/foobar
> >
> >   # Clone part of that 320K extent into a lower file offset (192K).
> >   # This file offset is greater than the file's size in the parent
> >   # snapshot (64K). Also the clone range is a bit behind the offset of
> >   # the 320K extent so that we leave a hole between the shared extents.
> >   $ xfs_io -c "reflink /mnt/sdb/foobar 448K 192K 192K" /mnt/sdb/foobar
> >
> >   $ btrfs subvolume snapshot -r /mnt/sdb /mnt/sdb/incr
> >   $ btrfs send -p /mnt/sdb/base -f /tmp/2.snap /mnt/sdb/incr
> >
> >   $ mkfs.btrfs -f /dev/sdc
> >   $ mount /dev/sdc /mnt/sdc
> >
> >   $ btrfs receive -f /tmp/1.snap /mnt/sdc
> >   $ btrfs receive -f /tmp/2.snap /mnt/sdc
> >   ERROR: failed to clone extents to foobar: Invalid argument
> >
> > The problem is that after processing the extent at file offset 192K, send
> > does not issue a write operation full of zeroes for the hole between that
> > extent and the extent starting at file offset 520K (hole range from 384K
> > to 512K), this is because the hole is at an offset larger the size of the
> > file in the parent snapshot (384K > 64K). As a consequence the field
> > 'cur_inode_next_write_offset' of the send context remains with a value of
> > 384K when we start to process the extent at file offset 512K, which is the
> > value set after processing the extent at offset 192K.
> >
> > This screws up the lookup of possible extents to clone because due to an
> > incorrect value of 'cur_inode_next_write_offset' we can now consider
> > extents for cloning, in the same inode, that lie beyond the current size
> > of the file in the receiver of the send stream. Also, when checking if
> > an extent in the same file can be used for cloning, we must also check
> > that not only its start offset doesn't start at or beyond the current eof
> > of the file in the receiver but that the source range doesn't go beyond
> > current eof, that is we must check offset + length does not cross the
> > current eof, as that makes clone operations fail with -EINVAL.
> >
> > So fix this by updating 'cur_inode_next_write_offset' whenever we start
> > processing an extent and checking an extent's offset and length when
> > considering it for cloning operations.
> >
> > A test case for fstests follows soon.
> >
> > Fixes: 11f2069c113e02 ("Btrfs: send, allow clone operations within the same file")
> > Signed-off-by: Filipe Manana <fdmanana@suse.com>
>
> Added to misc-next, with the tested-by tag, thanks.

Hold on a bit, one hunk of the patch besides not being necessary, it
causes problems with fallocated extents beyond i_size and trailing
holes.
I'll run some more tests to confirm the hunk is not needed and see if
Craig can test it too in the meanwhile.

Thanks.