[PATCH 3/3] mm, oom: protect !costly allocations some more

[Michal Hocko]

From: Michal Hocko <mhocko@xxxxxxxx>

should_reclaim_retry will give up retries for higher order allocations
if none of the eligible zones has any requested or higher order pages
available even if we pass the watermak check for order-0. This is done
because there is no guarantee that the reclaimable and currently free
pages will form the required order.

This can, however, lead to situations were the high-order request (e.g.
order-2 required for the stack allocation during fork) will trigger
OOM too early - e.g. after the first reclaim/compaction round. Such a
system would have to be highly fragmented and there is no guarantee
further reclaim/compaction attempts would help but at least make sure
that the compaction was active before we go OOM and keep retrying even
if should_reclaim_retry tells us to oom if the last compaction round
was either inactive (deferred, skipped or bailed out early due to
contention) or it told us to continue.

Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
---
 include/linux/compaction.h |  5 +++++
 mm/page_alloc.c            | 53 ++++++++++++++++++++++++++++++++--------------
 2 files changed, 42 insertions(+), 16 deletions(-)

diff --git a/include/linux/compaction.h b/include/linux/compaction.h
index b167801187e7..49e04326dcb8 100644
--- a/include/linux/compaction.h
+++ b/include/linux/compaction.h
@@ -14,6 +14,11 @@ enum compact_result {
 	/* compaction should continue to another pageblock */
 	COMPACT_CONTINUE,
 	/*
+	 * whoever is calling compaction should retry because it was either
+	 * not active or it tells us there is more work to be done.
+	 */
+	COMPACT_SHOULD_RETRY = COMPACT_CONTINUE,
+	/*
 	 * direct compaction partially compacted a zone and there are suitable
 	 * pages
 	 */
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 4acc0aa1aee0..041aeb1dc3b4 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2819,28 +2819,20 @@ static struct page *
 __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
 		int alloc_flags, const struct alloc_context *ac,
 		enum migrate_mode mode, int *contended_compaction,
-		bool *deferred_compaction)
+		enum compact_result *compact_result)
 {
-	enum compact_result compact_result;
 	struct page *page;
 
 	if (!order)
 		return NULL;
 
 	current->flags |= PF_MEMALLOC;
-	compact_result = try_to_compact_pages(gfp_mask, order, alloc_flags, ac,
+	*compact_result = try_to_compact_pages(gfp_mask, order, alloc_flags, ac,
 						mode, contended_compaction);
 	current->flags &= ~PF_MEMALLOC;
 
-	switch (compact_result) {
-	case COMPACT_DEFERRED:
-		*deferred_compaction = true;
-		/* fall-through */
-	case COMPACT_SKIPPED:
+	if (*compact_result <= COMPACT_SKIPPED)
 		return NULL;
-	default:
-		break;
-	}
 
 	/*
 	 * At least in one zone compaction wasn't deferred or skipped, so let's
@@ -2870,15 +2862,41 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
 
 	return NULL;
 }
+
+static inline bool
+should_compact_retry(unsigned int order, enum compact_result compact_result,
+		     int contended_compaction)
+{
+	/*
+	 * !costly allocations are really important and we have to make sure
+	 * the compaction wasn't deferred or didn't bail out early due to locks
+	 * contention before we go OOM.
+	 */
+	if (order && order <= PAGE_ALLOC_COSTLY_ORDER) {
+		if (compact_result <= COMPACT_SHOULD_RETRY)
+			return true;
+		if (contended_compaction > COMPACT_CONTENDED_NONE)
+			return true;
+	}
+
+	return false;
+}
 #else
 static inline struct page *
 __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
 		int alloc_flags, const struct alloc_context *ac,
 		enum migrate_mode mode, int *contended_compaction,
-		bool *deferred_compaction)
+		enum compact_result *compact_result)
 {
 	return NULL;
 }
+
+static inline bool
+should_compact_retry(unsigned int order, enum compact_result compact_result,
+		     int contended_compaction)
+{
+	return false;
+}
 #endif /* CONFIG_COMPACTION */
 
 /* Perform direct synchronous page reclaim */
@@ -3118,7 +3136,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 	int alloc_flags;
 	unsigned long did_some_progress;
 	enum migrate_mode migration_mode = MIGRATE_ASYNC;
-	bool deferred_compaction = false;
+	enum compact_result compact_result;
 	int contended_compaction = COMPACT_CONTENDED_NONE;
 	int no_progress_loops = 0;
 
@@ -3227,7 +3245,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 	page = __alloc_pages_direct_compact(gfp_mask, order, alloc_flags, ac,
 					migration_mode,
 					&contended_compaction,
-					&deferred_compaction);
+					&compact_result);
 	if (page)
 		goto got_pg;
 
@@ -3240,7 +3258,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 		 * to heavily disrupt the system, so we fail the allocation
 		 * instead of entering direct reclaim.
 		 */
-		if (deferred_compaction)
+		if (compact_result == COMPACT_DEFERRED)
 			goto nopage;
 
 		/*
@@ -3294,6 +3312,9 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 				 did_some_progress > 0, no_progress_loops))
 		goto retry;
 
+	if (should_compact_retry(order, compact_result, contended_compaction))
+		goto retry;
+
 	/* Reclaim has failed us, start killing things */
 	page = __alloc_pages_may_oom(gfp_mask, order, ac, &did_some_progress);
 	if (page)
@@ -3314,7 +3335,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 	page = __alloc_pages_direct_compact(gfp_mask, order, alloc_flags,
 					    ac, migration_mode,
 					    &contended_compaction,
-					    &deferred_compaction);
+					    &compact_result);
 	if (page)
 		goto got_pg;
 nopage:
-- 
2.7.0