Re: [PATCH 07/11] xfs: superblock scrub should use uncached buffers

[Brian Foster <bfoster@redhat.com>]

On Tue, Apr 17, 2018 at 07:40:20PM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@oracle.com>
> 
> We've never cached buffers when reading or writing superblocks, so we
> need to change scrub to do likewise or risk screwing up the uncached sb
> buffer usage everywhere else.
> 
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> ---

Hmm, so this goes back to this[1] thread, right? IIRC, we wanted to end
up with an lru-bypassing uncached buffer lookup mechanism to provide
uncached behavior for resource-saving purposes but without introducing
serialization issues between multiple users of uncached buffers.

On a quick look back, growfs currently uses cached buffers for secondary
superblocks and the associated patch was looking to change that to
something like the above. Don't we have the same requirement here (since
growfs currently still uses cached buffers)?

Brian

[1] https://marc.info/?l=linux-xfs&m=151746733326282&w=2

>  fs/xfs/scrub/agheader.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> 
> diff --git a/fs/xfs/scrub/agheader.c b/fs/xfs/scrub/agheader.c
> index 018aabbd..aacbc3f 100644
> --- a/fs/xfs/scrub/agheader.c
> +++ b/fs/xfs/scrub/agheader.c
> @@ -157,7 +157,7 @@ xfs_scrub_superblock(
>  	if (agno == 0)
>  		return 0;
>  
> -	error = xfs_trans_read_buf(mp, sc->tp, mp->m_ddev_targp,
> +	error = xfs_buf_read_uncached(mp->m_ddev_targp,
>  		  XFS_AGB_TO_DADDR(mp, agno, XFS_SB_BLOCK(mp)),
>  		  XFS_FSS_TO_BB(mp, 1), 0, &bp, &xfs_sb_buf_ops);
>  	/*
> @@ -421,6 +421,7 @@ xfs_scrub_superblock(
>  		xfs_scrub_block_set_corrupt(sc, bp);
>  
>  	xfs_scrub_superblock_xref(sc, bp);
> +	xfs_buf_relse(bp);
>  
>  	return error;
>  }
> 
> --
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