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

[Jaewon Kim]

>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@xxxxxxxxxxx>
>> Signed-off-by: Jaewon Kim <jaewon31.kim@xxxxxxxxxxx>
>
>Add
>
>Fixes: f27ce0e14088 ("page_alloc: consider highatomic reserve in watermark fast")

I will add the Fixes.

>
>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@xxxxxxxxxxxxxxxxxxx>

Thank you for your Ack
Yes leaving comment will be helpful. Actually let me take your patch.
I think this but is obvious and fix is sipmle, I can resubmit right away.

>
>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.

The min in __zone_watermark_ok is positive because mark is always unsigned.
But I think free_pages in __zone_watermark_ok can go negative because of the
same reason.

>
>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