linux-kernel - [PATCH 5.15 64/69] page_alloc: fix invalid watermark check on a negative value

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Date:   Mon,  1 Aug 2022 13:47:28 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Jaewon Kim <jaewon31.kim@...sung.com>,
        GyeongHwan Hong <gh21.hong@...sung.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Minchan Kim <minchan@...nel.org>, Baoquan He <bhe@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Yong-Taek Lee <ytk.lee@...sung.com>, stable@...r.kerenl.org,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH 5.15 64/69] page_alloc: fix invalid watermark check on a negative value

From: Jaewon Kim <jaewon31.kim@...sung.com>

commit 9282012fc0aa248b77a69f5eb802b67c5a16bb13 upstream.

There was a report that a task is waiting at the
throttle_direct_reclaim. The pgscan_direct_throttle in vmstat was
increasing.

This is a bug where zone_watermark_fast returns true even when the free
is very low. The commit f27ce0e14088 ("page_alloc: consider highatomic
reserve in watermark fast") changed the watermark fast to consider
highatomic reserve. But it did not handle a negative value case which
can be happened when reserved_highatomic pageblock is bigger than the
actual free.

If watermark is considered as ok for the negative value, allocating
contexts for order-0 will consume all free pages without direct reclaim,
and finally free page may become depleted except highatomic free.

Then allocating contexts may fall into throttle_direct_reclaim. This
symptom may easily happen in a system where wmark min is low and other
reclaimers like kswapd does not make free pages quickly.

Handle the negative case by using MIN.

Link: https://lkml.kernel.org/r/20220725095212.25388-1-jaewon31.kim@samsung.com
Fixes: f27ce0e14088 ("page_alloc: consider highatomic reserve in watermark fast")
Signed-off-by: Jaewon Kim <jaewon31.kim@...sung.com>
Reported-by: GyeongHwan Hong <gh21.hong@...sung.com>
Acked-by: Mel Gorman <mgorman@...hsingularity.net>
Cc: Minchan Kim <minchan@...nel.org>
Cc: Baoquan He <bhe@...hat.com>
Cc: Vlastimil Babka <vbabka@...e.cz>
Cc: Johannes Weiner <hannes@...xchg.org>
Cc: Michal Hocko <mhocko@...nel.org>
Cc: Yong-Taek Lee <ytk.lee@...sung.com>
Cc: <stable@...r.kerenl.org>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 mm/page_alloc.c |   12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3928,11 +3928,15 @@ static inline bool zone_watermark_fast(s
 	 * need to be calculated.
 	 */
 	if (!order) {
-		long fast_free;
+		long usable_free;
+		long reserved;
 
-		fast_free = free_pages;
-		fast_free -= __zone_watermark_unusable_free(z, 0, alloc_flags);
-		if (fast_free > mark + z->lowmem_reserve[highest_zoneidx])
+		usable_free = free_pages;
+		reserved = __zone_watermark_unusable_free(z, 0, alloc_flags);
+
+		/* reserved may over estimate high-atomic reserves. */
+		usable_free -= min(usable_free, reserved);
+		if (usable_free > mark + z->lowmem_reserve[highest_zoneidx])
 			return true;
 	}