Re: [PATCH v4 1/1] mm: vmscan: Reduce throttling due to a failure to make progress

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 31 Dec 2021 11:21:14 -0800 Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Fri, Dec 31, 2021 at 11:14 AM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > On Fri, Dec 31, 2021 at 6:24 AM Thorsten Leemhuis
> > <regressions@leemhuis.info> wrote:
> > >
> > > If we get it into rc8 (which is still possible, even if a bit hard due
> > > to the new year festivities), it will get at least one week of testing.
> >
> > I took it with Hugh's ack from his reply to this, so it should be in rc8.
> 
> Pushed out as 1b4e3f26f9f7 ("mm: vmscan: Reduce throttling due to a
> failure to make progress")

Needs this fixup, which I shall tweak a bit then send formally
in a few minutes.


From: Mel Gorman <mgorman@techsingularity.net>
Subject: mm: vmscan: reduce throttling due to a failure to make progress -fix

Hugh Dickins reported the following

	My tmpfs swapping load (tweaked to use huge pages more heavily
	than in real life) is far from being a realistic load: but it was
	notably slowed down by your throttling mods in 5.16-rc, and this
	patch makes it well again - thanks.

	But: it very quickly hit NULL pointer until I changed that last
	line to

        if (first_pgdat)
                consider_reclaim_throttle(first_pgdat, sc);

The likely issue is that huge pages are a major component of the test
workload. When this is the case, first_pgdat may never get set if
compaction is ready to continue due to this check

        if (IS_ENABLED(CONFIG_COMPACTION) &&
            sc->order > PAGE_ALLOC_COSTLY_ORDER &&
            compaction_ready(zone, sc)) {
                sc->compaction_ready = true;
                continue;
        }

If this was true for every zone in the zonelist, first_pgdat would never
get set resulting in a NULL pointer exception.

This is a fix to the mmotm patch
mm-vmscan-reduce-throttling-due-to-a-failure-to-make-progress.patch

Link: https://lkml.kernel.org/r/20211209095453.GM3366@techsingularity.net
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Reported-by: Hugh Dickins <hughd@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Rik van Riel <riel@surriel.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Darrick J. Wong <djwong@kernel.org>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/vmscan.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/mm/vmscan.c~mm-vmscan-reduce-throttling-due-to-a-failure-to-make-progress-fix
+++ a/mm/vmscan.c
@@ -3530,7 +3530,8 @@ static void shrink_zones(struct zonelist
 		shrink_node(zone->zone_pgdat, sc);
 	}
 
-	consider_reclaim_throttle(first_pgdat, sc);
+	if (first_pgdat)
+		consider_reclaim_throttle(first_pgdat, sc);
 
 	/*
 	 * Restore to original mask to avoid the impact on the caller if we
_