Re: [PATCH] page_alloc: fix invalid watemark check on a negative value

[Mel Gorman <mgorman@techsingularity.net>]

On Mon, Jul 25, 2022 at 10:28:43AM +0900, Jaewon Kim wrote:
> 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.
> 
> To handle the negative value, get the value as long type like
> __zone_watermark_ok does.
> 
> Reported-by: GyeongHwan Hong <gh21.hong@samsung.com>
> Signed-off-by: Jaewon Kim <jaewon31.kim@samsung.com>

Add

Fixes: f27ce0e14088 ("page_alloc: consider highatomic reserve in watermark fast")

The fix is fine as-is but it's not immediately obvious why this
can wrap negative as it depends on an implementation detail of
__zone_watermark_unusable_free.  The variable copy just to change the sign
could get accidentally "fixed" later as a micro-optimisation (same if the
type of mark was changed) so maybe leave a comment like

                /* unusable may over-estimate high-atomic reserves */

Otherwise

Acked-by: Mel Gorman <mgorman@techsingularity.net>

The problem could also be made explicit with something like below. I know
you are copying the logic of __zone_watermark_ok but I don't think min
can go negative there.

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 934d1b5a5449..f8f50a2aa43e 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4048,11 +4048,15 @@ static inline bool zone_watermark_fast(struct zone *z, unsigned int order,
 	 * 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;
 	}
 
-- 
Mel Gorman
SUSE Labs