Re: [PATCH 2/2] xfs: fix handling of RCU freed inodes from other AGs in xrep_iunlink_mark_incore

Dave Chinner
2024-08-14T22:04:07Z

On Mon, Aug 12, 2024 at 07:23:01AM +0200, Christoph Hellwig wrote:
> When xrep_iunlink_mark_incore skips an inode because it was RCU freed
> from another AG, the slot for the inode in the batch array needs to be
> zeroed. Also un-duplicate the check and remove the need for the
> xrep_iunlink_igrab helper.
>
> Fixes: ab97f4b1c030 ("xfs: repair AGI unlinked inode bucket lists")
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
> fs/xfs/scrub/agheader_repair.c | 28 +++++++---------------------
> 1 file changed, 7 insertions(+), 21 deletions(-)
>
> diff --git a/fs/xfs/scrub/agheader_repair.c b/fs/xfs/scrub/agheader_repair.c
> index 2f98d90d7fd66d..558bc86b1b83c3 100644
> --- a/fs/xfs/scrub/agheader_repair.c
> +++ b/fs/xfs/scrub/agheader_repair.c
> @@ -1108,23 +1108,6 @@ xrep_iunlink_walk_ondisk_bucket(
> return 0;
> }
>
> -/* Decide if this is an unlinked inode in this AG. */
> -STATIC bool
> -xrep_iunlink_igrab(
> - struct xfs_perag *pag,
> - struct xfs_inode *ip)
> -{
> - struct xfs_mount *mp = pag->pag_mount;
> -
> - if (XFS_INO_TO_AGNO(mp, ip->i_ino) != pag->pag_agno)
> - return false;
> -
> - if (!xfs_inode_on_unlinked_list(ip))
> - return false;
> -
> - return true;
> -}

This code is wrong. It does not explicitly check for RCU freed
inodes (i.e. ip->i_ino = 0 or XFS_IRECLAIM being set) and so will
never detect stale RCU freed inodes in AG 0.

It is probably working by chance to avoid stale freed inodes
because ip->i_prev_unlinked will be 0 for such inodes.

*However*, this code does not have the necessary memory barriers to
guarantee it catches the ip->i_ino or ip->i_prev_unlinked writes
prior to freeing. The ip->i_ino check needs to be done under the
ip->i_flags_lock as it is the unlock->lock memory barrier that
the inode cache RCU lookup algorithms rely on for correct detection
for RCU freed inodes.

> -
> /*
> * Mark the given inode in the lookup batch in our unlinked inode bitmap, and
> * remember if this inode is the start of the unlinked chain.
> @@ -1196,9 +1179,6 @@ xrep_iunlink_mark_incore(
> for (i = 0; i < nr_found; i++) {
> struct xfs_inode *ip = ragi->lookup_batch[i];
>
> - if (done || !xrep_iunlink_igrab(pag, ip))
> - ragi->lookup_batch[i] = NULL;
> -
> /*
> * Update the index for the next lookup. Catch
> * overflows into the next AG range which can occur if
> @@ -1211,8 +1191,14 @@ xrep_iunlink_mark_incore(
> * us to see this inode, so another lookup from the
> * same index will not find it again.
> */
> - if (XFS_INO_TO_AGNO(mp, ip->i_ino) != pag->pag_agno)
> + if (XFS_INO_TO_AGNO(mp, ip->i_ino) != pag->pag_agno) {
> + ragi->lookup_batch[i] = NULL;
> continue;
> + }
> +
> + if (done || !xfs_inode_on_unlinked_list(ip))
> + ragi->lookup_batch[i] = NULL;

Same with this new code - it's not explicitly checking for RCU freed
inodes and doesn't have the correct memory barriers.

Hence I think the fixes for this code are:

1. change xrep_iunlink_igrab() to use the same RCU freed inode
checks as xfs_blockgc_igrab(); and
2. remove the (XFS_INO_TO_AGNO(mp, ip->i_ino) != pag->pag_agno)
check altogether.

-Dave.
--
Dave Chinner
david@fromorbit.com