[PATCH] mm, oom: refactor oom detection

[Michal Hocko]

__alloc_pages_slowpath has traditionally relied on the direct reclaim
and did_some_progress as an indicator that it makes sense to retry
allocation rather than declaring OOM. shrink_zones had to rely on
zone_reclaimable if shrink_zone didn't make any progress to prevent
from pre mature OOM killer invocation - the LRU might be full of dirty
or writeback pages and direct reclaim cannot clean those up.
zone_reclaimable will allow to rescan the reclaimable lists several
times and restart if a page is freed. This is really subtle behavior
and it might lead to a livelock when a single freed page keeps allocator
looping but the current task will not be able to allocate that single
page. OOM killer would be more appropriate than looping without any
progress for unbounded amount of time.

This patch changes OOM detection logic and pulls it out from shrink_zone
which is too low to be appropriate for any high level decisions such as OOM
which is per zonelist property. It is __alloc_pages_slowpath which knows
how many attempts have been done and what was the progress so far
therefore it is more appropriate to implement this logic.

The new heuristic tries to be more deterministic and easier to
follow. Retrying makes sense only if the currently reclaimable memory
(pages on reclaimable LRUs + writeback pages) + free pages would allow
the current allocation request to succeed (as per __zone_watermark_ok)
at least for one zone.
This alone wouldn't be sufficient because the writeback might get stuck
and reclaimable pages might be pinned for a really long time or even
depend on the current allocation context. Therefore there is a feedback
mechanism implemented which reduces the reclaim target after each
reclaim round without any progress. This means that we should eventually
converge to only NR_FREE_PAGES and fail on the wmark check and proceed
to OOM. The nice aspect of this approach is that it works independently
on the allocation order. The backoff is simple and linear with 1/16 of
the reclaimable pages for each round without any progress. We are
optimistic and reset counter for successful reclaim rounds.

TODO: what are we going to do with __GFP_REPEAT? We should try harder
but how much harder? Do we even need it? Opportunistic high order
allocations can use __GFP_NORETRY...

Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
---
 include/linux/swap.h |  1 +
 mm/page_alloc.c      | 66 +++++++++++++++++++++++++++++++++++++++++++++-------
 mm/vmscan.c          | 10 +-------
 3 files changed, 59 insertions(+), 18 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 9c7c4b418498..8298e1dc20f9 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -317,6 +317,7 @@ extern void lru_cache_add_active_or_unevictable(struct page *page,
 						struct vm_area_struct *vma);
 
 /* linux/mm/vmscan.c */
+extern unsigned long zone_reclaimable_pages(struct zone *zone);
 extern unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
 					gfp_t gfp_mask, nodemask_t *mask);
 extern int __isolate_lru_page(struct page *page, isolate_mode_t mode);
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index c73913648357..ae927c762917 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2972,6 +2972,13 @@ static inline bool is_thp_gfp_mask(gfp_t gfp_mask)
 	return (gfp_mask & (GFP_TRANSHUGE | __GFP_KSWAPD_RECLAIM)) == GFP_TRANSHUGE;
 }
 
+/*
+ * Number of backoff steps for potentially reclaimable pages if the direct reclaim
+ * cannot make any progress. Each step will reduce 1/MAX_STALL_BACKOFF of the
+ * reclaimable memory.
+ */
+#define MAX_STALL_BACKOFF 16
+
 static inline struct page *
 __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 						struct alloc_context *ac)
@@ -2979,11 +2986,13 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 	bool can_direct_reclaim = gfp_mask & __GFP_DIRECT_RECLAIM;
 	struct page *page = NULL;
 	int alloc_flags;
-	unsigned long pages_reclaimed = 0;
 	unsigned long did_some_progress;
 	enum migrate_mode migration_mode = MIGRATE_ASYNC;
 	bool deferred_compaction = false;
 	int contended_compaction = COMPACT_CONTENDED_NONE;
+	struct zone *zone;
+	struct zoneref *z;
+	int stall_backoff = 0;
 
 	/*
 	 * In the slowpath, we sanity check order to avoid ever trying to
@@ -3135,23 +3144,62 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 	if (gfp_mask & __GFP_NORETRY)
 		goto noretry;
 
-	/* Keep reclaiming pages as long as there is reasonable progress */
-	pages_reclaimed += did_some_progress;
-	if ((did_some_progress && order <= PAGE_ALLOC_COSTLY_ORDER) ||
-	    ((gfp_mask & __GFP_REPEAT) && pages_reclaimed < (1 << order))) {
-		/* Wait for some write requests to complete then retry */
-		wait_iff_congested(ac->preferred_zone, BLK_RW_ASYNC, HZ/50);
-		goto retry;
+	/*
+	 * Be optimistic and consider all pages on reclaimable LRUs + those
+	 * currently on writeback as usable but make sure we converge to
+	 * OOM if we cannot make any progress after multiple consecutive
+	 * attempts.
+	 */
+	if (did_some_progress)
+		stall_backoff = 0;
+	else
+		stall_backoff = min(stall_backoff+1, MAX_STALL_BACKOFF);
+
+	/*
+	 * Keep reclaiming pages while there is a chance this will lead somewhere.
+	 * If none of the target zones can satisfy our allocation request even
+	 * if all reclaimable pages are considered then we are screwed and have
+	 * to go OOM.
+	 */
+	for_each_zone_zonelist_nodemask(zone, z, ac->zonelist, ac->high_zoneidx, ac->nodemask) {
+		unsigned long free = zone_page_state(zone, NR_FREE_PAGES);
+		unsigned long writeback = zone_page_state(zone, NR_WRITEBACK);
+		unsigned long reclaimable;
+		unsigned long target;
+
+		reclaimable = zone_reclaimable_pages(zone) +
+			      zone_page_state(zone, NR_ISOLATED_FILE) +
+			      zone_page_state(zone, NR_ISOLATED_ANON);
+		target = reclaimable + writeback;
+		target -= stall_backoff * (1 + target/MAX_STALL_BACKOFF);
+		target += free;
+
+		/*
+		 * Would the allocation succeed if we reclaimed the whole target?
+		 * We might over-account here because some pages under writeback
+		 * might be on the LRU as well but that shouldn't confuse us too
+		 * much.
+		 */
+		if (__zone_watermark_ok(zone, order, min_wmark_pages(zone),
+				ac->high_zoneidx, alloc_flags, target)) {
+			/* Wait for some write requests to complete then retry */
+			wait_iff_congested(zone, BLK_RW_ASYNC, HZ/50);
+			goto retry;
+		}
 	}
 
+	/* TODO what about GFP_REPEAT */
+
 	/* Reclaim has failed us, start killing things */
 	page = __alloc_pages_may_oom(gfp_mask, order, ac, &did_some_progress);
 	if (page)
 		goto got_pg;
 
 	/* Retry as long as the OOM killer is making progress */
-	if (did_some_progress)
+	if (did_some_progress) {
+		stall_backoff = 0;
 		goto retry;
+	}
 
 noretry:
 	/*
diff --git a/mm/vmscan.c b/mm/vmscan.c
index c88d74ad9304..bc14217acd47 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -193,7 +193,7 @@ static bool sane_reclaim(struct scan_control *sc)
 }
 #endif
 
-static unsigned long zone_reclaimable_pages(struct zone *zone)
+unsigned long zone_reclaimable_pages(struct zone *zone)
 {
 	unsigned long nr;
 
@@ -2639,10 +2639,6 @@ static bool shrink_zones(struct zonelist *zonelist, struct scan_control *sc)
 
 		if (shrink_zone(zone, sc, zone_idx(zone) == classzone_idx))
 			reclaimable = true;
-
-		if (global_reclaim(sc) &&
-		    !reclaimable && zone_reclaimable(zone))
-			reclaimable = true;
 	}
 
 	/*
@@ -2734,10 +2730,6 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
 		goto retry;
 	}
 
-	/* Any of the zones still reclaimable?  Don't OOM. */
-	if (zones_reclaimable)
-		return 1;
-
 	return 0;
 }
 
-- 
2.6.1

-- 
Michal Hocko
SUSE Labs
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