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Commit 8d7a8fa9 authored by Cody P Schafer's avatar Cody P Schafer Committed by Linus Torvalds
Browse files

mm/page_alloc: insert memory barriers to allow async update of pcp batch and high 

Introduce pageset_update() to perform a safe transision from one set of
pcp->{batch,high} to a new set using memory barriers.

This ensures that batch is always set to a safe value (1) prior to
updating high, and ensure that high is fully updated before setting the
real value of batch.  It avoids ->batch ever rising above ->high.

Suggested by Gilad Ben-Yossef in these threads:

	https://lkml.org/lkml/2013/4/9/23
	https://lkml.org/lkml/2013/4/10/49



Also reproduces his proposed comment.

Signed-off-by: default avatarCody P Schafer <cody@linux.vnet.ibm.com>
Reviewed-by: default avatarGilad Ben-Yossef <gilad@benyossef.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Pekka Enberg <penberg@kernel.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent c8e251fa

mm/page_alloc.c

+32 −9
Original line number Diff line number Diff line
@@ -4035,12 +4035,37 @@ static int __meminit zone_batchsize(struct zone *zone) 
#endif

}



/*

 * pcp->high and pcp->batch values are related and dependent on one another:

 * ->batch must never be higher then ->high.

 * The following function updates them in a safe manner without read side

 * locking.

 *

 * Any new users of pcp->batch and pcp->high should ensure they can cope with

 * those fields changing asynchronously (acording the the above rule).

 *

 * mutex_is_locked(&pcp_batch_high_lock) required when calling this function

 * outside of boot time (or some other assurance that no concurrent updaters

 * exist).

 */

static void pageset_update(struct per_cpu_pages *pcp, unsigned long high,

		unsigned long batch)

{

       /* start with a fail safe value for batch */

	pcp->batch = 1;

	smp_wmb();



       /* Update high, then batch, in order */

	pcp->high = high;

	smp_wmb();



	pcp->batch = batch;

}



/* a companion to setup_pagelist_highmark() */

static void pageset_set_batch(struct per_cpu_pageset *p, unsigned long batch)

{

	struct per_cpu_pages *pcp = &p->pcp;

	pcp->high = 6 * batch;

	pcp->batch = max(1UL, 1 * batch);

	pageset_update(&p->pcp, 6 * batch, max(1UL, 1 * batch));

}



static void setup_pageset(struct per_cpu_pageset *p, unsigned long batch)
@@ -4064,13 +4089,11 @@ static void setup_pageset(struct per_cpu_pageset *p, unsigned long batch) 
static void setup_pagelist_highmark(struct per_cpu_pageset *p,

				unsigned long high)

{

	struct per_cpu_pages *pcp;



	pcp = &p->pcp;

	pcp->high = high;

	pcp->batch = max(1UL, high/4);

	unsigned long batch = max(1UL, high / 4);

	if ((high / 4) > (PAGE_SHIFT * 8))

		pcp->batch = PAGE_SHIFT * 8;

		batch = PAGE_SHIFT * 8;



	pageset_update(&p->pcp, high, batch);

}



static void __meminit setup_zone_pageset(struct zone *zone)