On 05/23/2014 04:48 AM, Shawn Guo wrote:

On 23 May 2014 07:49, Kevin Hilman [off-list ref] wrote:

quoted

On Fri, May 16, 2014 at 2:47 AM, Vlastimil Babka [off-list ref] wrote:

quoted

Compaction uses compact_checklock_irqsave() function to periodically check for
lock contention and need_resched() to either abort async compaction, or to
free the lock, schedule and retake the lock. When aborting, cc->contended is
set to signal the contended state to the caller. Two problems have been
identified in this mechanism.

This patch (or later version) has hit next-20140522 (in the form
commit 645ceea9331bfd851bc21eea456dda27862a10f4) and according to my
bisect, appears to be the culprit of several boot failures on ARM
platforms.

On i.MX6 where CMA is enabled, the commit causes the drivers calling
dma_alloc_coherent() fail to probe.  Tracing it a little bit, it seems
dma_alloc_from_contiguous() always return page as NULL after this
commit.

Shawn

Really sorry, guys :/

-----8<-----
From: Vlastimil Babka <redacted>
Date: Fri, 23 May 2014 10:18:56 +0200
Subject: mm-compaction-properly-signal-and-act-upon-lock-and-need_sched-contention-fix2

Step 1: Change function name and comment between v1 and v2 so that the return
        value signals the opposite thing.
Step 2: Change the call sites to reflect the opposite return value.
Step 3: ???
Step 4: Make a complete fool of yourself.

Signed-off-by: Vlastimil Babka <redacted>
---
 mm/compaction.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index a525cd4..5175019 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c

@@ -237,13 +237,13 @@ static inline bool compact_should_abort(struct compact_control *cc)
 	if (need_resched()) {
 		if (cc->mode == MIGRATE_ASYNC) {
 			cc->contended = true;
-			return false;
+			return true;
 		}
 
 		cond_resched();
 	}
 
-	return true;
+	return false;
 }
 
 /* Returns true if the page is within a block suitable for migration to */

-- 
1.8.4.5