page_alloc: fix invalid watermark check on a negative value

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/[email protected]
Fixes: f27ce0e14088 ("page_alloc: consider highatomic reserve in watermark fast")
Signed-off-by: Jaewon Kim <[email protected]>
Reported-by: GyeongHwan Hong <[email protected]>
Acked-by: Mel Gorman <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Baoquan He <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Yong-Taek Lee <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index e008a3df0485c79ac24ef31b4407dd5921a9649a..b5b14b78c4fd4844cb071022570fd4b7e6959eb2 100644 (file)
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3968,11 +3968,15 @@ 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;
        }