Re: [PATCH 14/24] xfs: remove IO submission from xfs_reclaim_inode()

Darrick J. Wong
2020-05-22T23:08:49Z

On Fri, May 22, 2020 at 01:50:19PM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> We no longer need to issue IO from shrinker based inode reclaim to
> prevent spurious OOM killer invocation. This leaves only the global
> filesystem management operations such as unmount needing to
> writeback dirty inodes and reclaim them.
>
> Instead of using the reclaim pass to write dirty inodes before
> reclaiming them, use the AIL to push all the dirty inodes before we
> try to reclaim them. This allows us to remove all the conditional
> SYNC_WAIT locking and the writeback code from xfs_reclaim_inode()
> and greatly simplify the checks we need to do to reclaim an inode.
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
> fs/xfs/xfs_icache.c | 116 +++++++++++---------------------------------
> 1 file changed, 29 insertions(+), 87 deletions(-)
>
> diff --git a/fs/xfs/xfs_icache.c b/fs/xfs/xfs_icache.c
> index 0f0f8fcd61b03..ee9bc82a0dfbe 100644
> --- a/fs/xfs/xfs_icache.c
> +++ b/fs/xfs/xfs_icache.c
> @@ -1130,24 +1130,17 @@ xfs_reclaim_inode_grab(
> * dirty, async => requeue
> * dirty, sync => flush, wait and reclaim
> */

The function comment probably ought to describe what the two return
values mean. true when the inode was freed and false if we need to try
again, right?

> -STATIC int
> +static bool
> xfs_reclaim_inode(
> struct xfs_inode *ip,
> struct xfs_perag *pag,
> int sync_mode)
> {
> - struct xfs_buf *bp = NULL;
> xfs_ino_t ino = ip->i_ino; /* for radix_tree_delete */
> - int error;
>
> -restart:
> - error = 0;
> xfs_ilock(ip, XFS_ILOCK_EXCL);
> - if (!xfs_iflock_nowait(ip)) {
> - if (!(sync_mode & SYNC_WAIT))
> - goto out;
> - xfs_iflock(ip);
> - }
> + if (!xfs_iflock_nowait(ip))
> + goto out;
>
> if (XFS_FORCED_SHUTDOWN(ip->i_mount)) {
> xfs_iunpin_wait(ip);
> @@ -1155,51 +1148,19 @@ xfs_reclaim_inode(
> xfs_iflush_abort(ip);
> goto reclaim;
> }
> - if (xfs_ipincount(ip)) {
> - if (!(sync_mode & SYNC_WAIT))
> - goto out_ifunlock;
> - xfs_iunpin_wait(ip);
> - }
> + if (xfs_ipincount(ip))
> + goto out_ifunlock;
> if (xfs_iflags_test(ip, XFS_ISTALE) || xfs_inode_clean(ip)) {
> xfs_ifunlock(ip);
> goto reclaim;
> }
>
> - /*
> - * Never flush out dirty data during non-blocking reclaim, as it would
> - * just contend with AIL pushing trying to do the same job.
> - */
> - if (!(sync_mode & SYNC_WAIT))
> - goto out_ifunlock;
> -
> - /*
> - * Now we have an inode that needs flushing.
> - *
> - * Note that xfs_iflush will never block on the inode buffer lock, as
> - * xfs_ifree_cluster() can lock the inode buffer before it locks the
> - * ip->i_lock, and we are doing the exact opposite here. As a result,
> - * doing a blocking xfs_imap_to_bp() to get the cluster buffer would
> - * result in an ABBA deadlock with xfs_ifree_cluster().
> - *
> - * As xfs_ifree_cluser() must gather all inodes that are active in the
> - * cache to mark them stale, if we hit this case we don't actually want
> - * to do IO here - we want the inode marked stale so we can simply
> - * reclaim it. Hence if we get an EAGAIN error here, just unlock the
> - * inode, back off and try again. Hopefully the next pass through will
> - * see the stale flag set on the inode.
> - */
> - error = xfs_iflush(ip, &bp);
> - if (error == -EAGAIN) {
> - xfs_iunlock(ip, XFS_ILOCK_EXCL);
> - /* backoff longer than in xfs_ifree_cluster */
> - delay(2);
> - goto restart;
> - }
> -
> - if (!error) {
> - error = xfs_bwrite(bp);
> - xfs_buf_relse(bp);
> - }
> +out_ifunlock:
> + xfs_ifunlock(ip);
> +out:
> + xfs_iunlock(ip, XFS_ILOCK_EXCL);
> + xfs_iflags_clear(ip, XFS_IRECLAIM);
> + return false;
>
> reclaim:
> ASSERT(!xfs_isiflocked(ip));
> @@ -1249,21 +1210,7 @@ xfs_reclaim_inode(
> xfs_iunlock(ip, XFS_ILOCK_EXCL);
>
> __xfs_inode_free(ip);
> - return error;
> -
> -out_ifunlock:
> - xfs_ifunlock(ip);
> -out:
> - xfs_iflags_clear(ip, XFS_IRECLAIM);
> - xfs_iunlock(ip, XFS_ILOCK_EXCL);
> - /*
> - * We could return -EAGAIN here to make reclaim rescan the inode tree in
> - * a short while. However, this just burns CPU time scanning the tree
> - * waiting for IO to complete and the reclaim work never goes back to
> - * the idle state. Instead, return 0 to let the next scheduled
> - * background reclaim attempt to reclaim the inode again.
> - */
> - return 0;
> + return true;
> }
>
> /*
> @@ -1272,20 +1219,17 @@ xfs_reclaim_inode(
> * then a shut down during filesystem unmount reclaim walk leak all the
> * unreclaimed inodes.
> */
> -STATIC int
> +static int

The function comment /really/ needs to note that the return value
here is number of inodes that were skipped, and not just some negative
error code.

> xfs_reclaim_inodes_ag(
> struct xfs_mount *mp,
> int flags,
> int *nr_to_scan)
> {
> struct xfs_perag *pag;
> - int error = 0;
> - int last_error = 0;
> xfs_agnumber_t ag;
> int trylock = flags & SYNC_TRYLOCK;
> int skipped;
>
> -restart:
> ag = 0;
> skipped = 0;
> while ((pag = xfs_perag_get_tag(mp, ag, XFS_ICI_RECLAIM_TAG))) {
> @@ -1359,9 +1303,8 @@ xfs_reclaim_inodes_ag(
> for (i = 0; i < nr_found; i++) {
> if (!batch[i])
> continue;
> - error = xfs_reclaim_inode(batch[i], pag, flags);
> - if (error && last_error != -EFSCORRUPTED)
> - last_error = error;
> + if (!xfs_reclaim_inode(batch[i], pag, flags))
> + skipped++;
> }
>
> *nr_to_scan -= XFS_LOOKUP_BATCH;
> @@ -1377,19 +1320,7 @@ xfs_reclaim_inodes_ag(
> mutex_unlock(&pag->pag_ici_reclaim_lock);
> xfs_perag_put(pag);
> }
> -
> - /*
> - * if we skipped any AG, and we still have scan count remaining, do
> - * another pass this time using blocking reclaim semantics (i.e
> - * waiting on the reclaim locks and ignoring the reclaim cursors). This
> - * ensure that when we get more reclaimers than AGs we block rather
> - * than spin trying to execute reclaim.
> - */
> - if (skipped && (flags & SYNC_WAIT) && *nr_to_scan > 0) {
> - trylock = 0;
> - goto restart;
> - }
> - return last_error;
> + return skipped;
> }
>
> int
> @@ -1398,8 +1329,18 @@ xfs_reclaim_inodes(
> int mode)
> {
> int nr_to_scan = INT_MAX;
> + int skipped;
>
> - return xfs_reclaim_inodes_ag(mp, mode, &nr_to_scan);
> + skipped = xfs_reclaim_inodes_ag(mp, mode, &nr_to_scan);
> + if (!(mode & SYNC_WAIT))
> + return 0;
> +
> + do {
> + xfs_ail_push_all_sync(mp->m_ail);
> + skipped = xfs_reclaim_inodes_ag(mp, mode, &nr_to_scan);
> + } while (skipped > 0);
> +
> + return 0;

Might as well kill the return value here since none of the callers care.

> }
>
> /*
> @@ -1420,7 +1361,8 @@ xfs_reclaim_inodes_nr(
> xfs_reclaim_work_queue(mp);
> xfs_ail_push_all(mp->m_ail);
>
> - return xfs_reclaim_inodes_ag(mp, SYNC_TRYLOCK, &nr_to_scan);

So the old code was returning negative error codes here? Given that the
only caller is free_cached_objects which adds it to the 'freed' count...
wow.

> + xfs_reclaim_inodes_ag(mp, SYNC_TRYLOCK, &nr_to_scan);
> + return 0;

Why do we return zero freed items here? The VFS asked us to clear
shrink_control->nr_to_scan (passed in here as nr_to_scan) and we're
supposed to report what we did, right?

Or is there some odd subtlety here where we hate the shrinker and that's
why we return zero?

--D

> }
>
> /*
> --
> 2.26.2.761.g0e0b3e54be
>