Re: [PATCH] mm: page_alloc: unreserve highatomic page blocks before oom

[Michal Hocko]

On Mon 30-10-23 18:09:50, Charan Teja Kalla wrote:
> __alloc_pages_direct_reclaim() is called from slowpath allocation where
> high atomic reserves can be unreserved after there is a progress in
> reclaim and yet no suitable page is found. Later should_reclaim_retry()
> gets called from slow path allocation to decide if the reclaim needs to
> be retried before OOM kill path is taken.
> 
> should_reclaim_retry() checks the available(reclaimable + free pages)
> memory against the min wmark levels of a zone and returns:
> a)  true, if it is above the min wmark so that slow path allocation will
> do the reclaim retries.
> b) false, thus slowpath allocation takes oom kill path.
> 
> should_reclaim_retry() can also unreserves the high atomic reserves
> **but only after all the reclaim retries are exhausted.**
> 
> In a case where there are almost none reclaimable memory and free pages
> contains mostly the high atomic reserves but allocation context can't
> use these high atomic reserves, makes the available memory below min
> wmark levels hence false is returned from should_reclaim_retry() leading
> the allocation request to take OOM kill path. This is an early oom kill
> because high atomic reserves are holding lot of free memory and 
> unreserving of them is not attempted.

OK, I see. So we do not release those reserved pages because OOM hits
too early. 

> (early)OOM is encountered on a machine in the below state(excerpt from
> the oom kill logs):
> [  295.998653] Normal free:7728kB boost:0kB min:804kB low:1004kB
> high:1204kB reserved_highatomic:8192KB active_anon:4kB inactive_anon:0kB
> active_file:24kB inactive_file:24kB unevictable:1220kB writepending:0kB
> present:70732kB managed:49224kB mlocked:0kB bounce:0kB free_pcp:688kB
> local_pcp:492kB free_cma:0kB
> [  295.998656] lowmem_reserve[]: 0 32
> [  295.998659] Normal: 508*4kB (UMEH) 241*8kB (UMEH) 143*16kB (UMEH)
> 33*32kB (UH) 7*64kB (UH) 0*128kB 0*256kB 0*512kB 0*1024kB 0*2048kB
> 0*4096kB = 7752kB

OK, this is quite interesting as well. The system is really tiny and 8MB
of reserved memory is indeed really high. How come those reservations
have grown that high?
> 
> Per above log, the free memory of ~7MB exist in the high atomic
> reserves is not freed up before falling back to oom kill path.
> 
> This fix includes unreserving these atomic reserves in the OOM path
> before going for a kill. The side effect of unreserving in oom kill path
> is that these free pages are checked against the high wmark. If
> unreserved from should_reclaim_retry()/__alloc_pages_direct_reclaim(),
> they are checked against the min wmark levels.

I do not like the fix much TBH. I think the logic should live in
should_reclaim_retry. One way to approach it is to unreserve at the end
of the function, something like this:
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 95546f376302..d04e14adf2c5 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3813,10 +3813,8 @@ should_reclaim_retry(gfp_t gfp_mask, unsigned order,
 	 * Make sure we converge to OOM if we cannot make any progress
 	 * several times in the row.
 	 */
-	if (*no_progress_loops > MAX_RECLAIM_RETRIES) {
-		/* Before OOM, exhaust highatomic_reserve */
-		return unreserve_highatomic_pageblock(ac, true);
-	}
+	if (*no_progress_loops > MAX_RECLAIM_RETRIES)
+		goto out;
 
 	/*
 	 * Keep reclaiming pages while there is a chance this will lead
@@ -3859,6 +3857,12 @@ should_reclaim_retry(gfp_t gfp_mask, unsigned order,
 		schedule_timeout_uninterruptible(1);
 	else
 		cond_resched();
+
+out:
+	/* Before OOM, exhaust highatomic_reserve */
+	if (!ret)
+		return unreserve_highatomic_pageblock(ac, true);
+
 	return ret;
 }
 
-- 
Michal Hocko
SUSE Labs