Re: [PATCH 02/11] xfs: don't stall cowblocks scan if we can't take locks

From: Brian Foster <bfoster@redhat.com>
To: "Darrick J. Wong" <djwong@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>,
	linux-xfs@vger.kernel.org, hch@infradead.org,
	david@fromorbit.com
Subject: Re: [PATCH 02/11] xfs: don't stall cowblocks scan if we can't take locks
Date: Mon, 25 Jan 2021 13:14:06 -0500	[thread overview]
Message-ID: <20210125181406.GH2047559@bfoster> (raw)
In-Reply-To: <161142793080.2171939.11486862758521454210.stgit@magnolia>

On Sat, Jan 23, 2021 at 10:52:10AM -0800, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@kernel.org>
> 
> Don't stall the cowblocks scan on a locked inode if we possibly can.
> We'd much rather the background scanner keep moving.
> 
> Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/xfs/xfs_icache.c |   21 ++++++++++++++++++---
>  1 file changed, 18 insertions(+), 3 deletions(-)
> 
> 
> diff --git a/fs/xfs/xfs_icache.c b/fs/xfs/xfs_icache.c
> index c71eb15e3835..89f9e692fde7 100644
> --- a/fs/xfs/xfs_icache.c
> +++ b/fs/xfs/xfs_icache.c
> @@ -1605,17 +1605,31 @@ xfs_inode_free_cowblocks(
>  	void			*args)
>  {
>  	struct xfs_eofblocks	*eofb = args;
> +	bool			wait;
>  	int			ret = 0;
>  
> +	wait = eofb && (eofb->eof_flags & XFS_EOF_FLAGS_SYNC);
> +
>  	if (!xfs_prep_free_cowblocks(ip))
>  		return 0;
>  
>  	if (!xfs_inode_matches_eofb(ip, eofb))
>  		return 0;
>  
> -	/* Free the CoW blocks */
> -	xfs_ilock(ip, XFS_IOLOCK_EXCL);
> -	xfs_ilock(ip, XFS_MMAPLOCK_EXCL);
> +	/*
> +	 * If the caller is waiting, return -EAGAIN to keep the background
> +	 * scanner moving and revisit the inode in a subsequent pass.
> +	 */
> +	if (!xfs_ilock_nowait(ip, XFS_IOLOCK_EXCL)) {
> +		if (wait)
> +			return -EAGAIN;
> +		return 0;
> +	}
> +	if (!xfs_ilock_nowait(ip, XFS_MMAPLOCK_EXCL)) {
> +		if (wait)
> +			ret = -EAGAIN;
> +		goto out_iolock;
> +	}

Hmm.. I'd be a little concerned over this allowing a scan to repeat
indefinitely with a competing workload because a restart doesn't carry
over any state from the previous scan. I suppose the
xfs_prep_free_cowblocks() checks make that slightly less likely on a
given file, but I more wonder about a scenario with a large set of
inodes in a particular AG with a sufficient amount of concurrent
activity. All it takes is one trylock failure per scan to have to start
the whole thing over again... hm?

Brian

>  
>  	/*
>  	 * Check again, nobody else should be able to dirty blocks or change
> @@ -1625,6 +1639,7 @@ xfs_inode_free_cowblocks(
>  		ret = xfs_reflink_cancel_cow_range(ip, 0, NULLFILEOFF, false);
>  
>  	xfs_iunlock(ip, XFS_MMAPLOCK_EXCL);
> +out_iolock:
>  	xfs_iunlock(ip, XFS_IOLOCK_EXCL);
>  
>  	return ret;
> 