Re: [PATCH 09/11] mm, compaction: Abstract compaction feedback to helpers

[Michal Hocko <mhocko@kernel.org>]

Hi Andrew,
Vlastimil has pointed out[1] that using compaction_withdrawn() for THP
allocations has some non-trivial consequences. While I still think that
the check is OK it is true we shouldn't sneak in a potential behavior
change into something that basically provides an API. So can you fold
the following partial revert into the original patch please?

[1] http://lkml.kernel.org/r/570BB719.2030007@suse.cz

---
>From 71ddeee4238e33d67ef07883e73f946a7cc40e73 Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@suse.com>
Date: Mon, 11 Apr 2016 17:38:22 +0200
Subject: [PATCH] ction-abstract-compaction-feedback-to-helpers-fix

Preserve the original thp back off checks to not introduce any
functional changes as per Vlastimil.

Signed-off-by: Michal Hocko <mhocko@suse.com>
---
 mm/page_alloc.c | 25 +++++++++++++++++++------
 1 file changed, 19 insertions(+), 6 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index c05de84c8157..c37e6d1ad643 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3362,12 +3362,25 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 	if (page)
 		goto got_pg;
 
-	/*
-	 * Checks for THP-specific high-order allocations and back off
-	 * if the the compaction backed off
-	 */
-	if (is_thp_gfp_mask(gfp_mask) && compaction_withdrawn(compact_result))
-		goto nopage;
+	/* Checks for THP-specific high-order allocations */
+	if (is_thp_gfp_mask(gfp_mask)) {
+		/*
+		 * If compaction is deferred for high-order allocations, it is
+		 * because sync compaction recently failed. If this is the case
+		 * and the caller requested a THP allocation, we do not want
+		 * to heavily disrupt the system, so we fail the allocation
+		 * instead of entering direct reclaim.
+		 */
+		if (compact_result == COMPACT_DEFERRED)
+			goto nopage;
+
+		/*
+		 * Compaction is contended so rather back off than cause
+		 * excessive stalls.
+		 */
+		if(compact_result == COMPACT_CONTENDED)
+			goto nopage;
+	}
 
 	/*
 	 * It can become very expensive to allocate transparent hugepages at
-- 
2.8.0.rc3

-- 
Michal Hocko
SUSE Labs