Re: [PATCH] xfs: introduce protection for drop nlink

[<cheng.lin130@zte.com.cn>]

> On Thu, Aug 24, 2023 at 03:43:52PM +0800, cheng.lin130@zte.com.cn wrote:
>> From: Cheng Lin <cheng.lin130@zte.com.cn>
>> An dir nlinks overflow which down form 0 to 0xffffffff, cause the
>> directory to become unusable until the next xfs_repair run.
> Hmmm.  How does this ever happen?
> IMO, if it does happen, we need to fix whatever bug that causes it
> to happen, not issue a warning and do nothing about the fact we
> just hit a corrupt inode state...
Yes, I'm very agree with your opinion. But I don't know how it happened,
and how to reproduce it.
>> Introduce protection for drop nlink to reduce the impact of this.
>> And produce a warning for directory nlink error during remove.
>>
>> Signed-off-by: Cheng Lin <cheng.lin130@zte.com.cn>
>> ---
>>  fs/xfs/xfs_inode.c | 16 +++++++++++++++-
>>  1 file changed, 15 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
>> index 9e62cc5..536dbe4 100644
>> --- a/fs/xfs/xfs_inode.c
>> +++ b/fs/xfs/xfs_inode.c
>> @@ -919,6 +919,15 @@ STATIC int xfs_iunlink_remove(struct xfs_trans *tp, struct xfs_perag *pag,
>>      xfs_trans_t *tp,
>>      xfs_inode_t *ip)
>>  {
>> +    xfs_mount_t     *mp;
>> +
>> +    if (VFS_I(ip)->i_nlink == 0) {
>> +        mp = ip->i_mount;
>> +        xfs_warn(mp, "%s: Deleting inode %llu with no links.",
>> +             __func__, ip->i_ino);
>> +        return 0;
>> +    }
> This is obviously incorrect - whiteout inodes (RENAME_WHITEOUT) have an
> i_nlink of zero when they are removed from the unlinked list. As do
> O_TMPFILE inodes - when they are linked into the filesystem, we
> explicitly check for i_nlink being zero before calling
> xfs_iunlink_remove().
I am not familiar with the above process. You means there is such a
scenario, even if it is (i_nlink==0), it still needs to run drop_nlink()
in xfs_droplink()? But this will cause i_nlink to underflow to 0xffffffff.
>> +
>>      xfs_trans_ichgtime(tp, ip, XFS_ICHGTIME_CHG);
>>
>>      drop_nlink(VFS_I(ip));
> Wait a second - this code doesn't match an upstream kernel. What
> kernel did you make this patch against?
It's kernel mainline linux-6.5-rc7

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