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

Brian Foster
2021-01-25T18:16:23Z

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;
>