Re: [PATCH V3 08/10] xfs: Check for extent overflow when moving extent from cow to data fork

Darrick J. Wong
2020-08-31T16:29:16Z

On Thu, Aug 20, 2020 at 11:13:47AM +0530, Chandan Babu R wrote:
> Moving an extent to data fork can cause a sub-interval of an existing
> extent to be unmapped. This will increase extent count by 1. Mapping in
> the new extent can increase the extent count by 1 again i.e.
> | Old extent | New extent | Old extent |
> Hence number of extents increases by 2.
>
> Signed-off-by: Chandan Babu R <chandanrlinux@gmail.com>
> ---
> fs/xfs/libxfs/xfs_inode_fork.h | 9 ++++++++-
> fs/xfs/xfs_reflink.c | 5 +++++
> 2 files changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/fs/xfs/libxfs/xfs_inode_fork.h b/fs/xfs/libxfs/xfs_inode_fork.h
> index d0e49b015b62..850d53162545 100644
> --- a/fs/xfs/libxfs/xfs_inode_fork.h
> +++ b/fs/xfs/libxfs/xfs_inode_fork.h
> @@ -78,7 +78,14 @@ struct xfs_ifork {
> * split into two extents causing extent count to increase by 1.
> */
> #define XFS_IEXT_INSERT_HOLE_CNT (1)
> -
> +/*
> + * Moving an extent to data fork can cause a sub-interval of an existing extent
> + * to be unmapped. This will increase extent count by 1. Mapping in the new
> + * extent can increase the extent count by 1 again i.e.
> + * | Old extent | New extent | Old extent |
> + * Hence number of extents increases by 2.
> + */
> +#define XFS_IEXT_REFLINK_END_COW_CNT (2)
>
> /*
> * Fork handling.
> diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
> index aac83f9d6107..c1d2a741e1af 100644
> --- a/fs/xfs/xfs_reflink.c
> +++ b/fs/xfs/xfs_reflink.c
> @@ -628,6 +628,11 @@ xfs_reflink_end_cow_extent(
> xfs_ilock(ip, XFS_ILOCK_EXCL);
> xfs_trans_ijoin(tp, ip, 0);
>
> + error = xfs_iext_count_may_overflow(ip, XFS_DATA_FORK,
> + XFS_IEXT_REFLINK_END_COW_CNT);
> + if (error)
> + goto out_cancel;

What happens if we fail here? I think for buffered writes this means
that writeback fails and we store an EIO in the address space for
eventual return via fsync()? And for a direct write this means that
EIO gets sent back to the caller, right?

Assuming I understood that correctly, I think this is a reasonable
enough place to check for overflows, and hence

Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

It would be nicer to check this kind of thing at write() time to put all
the EFBIG errors up front, but I don't think you can do that without
tracking extent count "reservations" incore.

--D

> +
> /*
> * In case of racing, overlapping AIO writes no COW extents might be
> * left by the time I/O completes for the loser of the race. In that
> --
> 2.28.0
>