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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: Cody P Schafer <cody@linux.vnet.ibm.com>
Reviewed-by: Gilad 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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Cody P Schafer 2013-07-03 15:01:31 -07:00 committed by Linus Torvalds
parent c8e251fadc
commit 8d7a8fa97a
1 changed files with 32 additions and 9 deletions
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41
mm/page_alloc.c
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@ -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;
unsigned long batch = max(1UL, high / 4);
if ((high / 4) > (PAGE_SHIFT * 8))
batch = PAGE_SHIFT * 8;
pcp = &p->pcp;
pcp->high = high;
pcp->batch = max(1UL, high/4);
if ((high/4) > (PAGE_SHIFT * 8))
pcp->batch = PAGE_SHIFT * 8;
pageset_update(&p->pcp, high, batch);
}
static void __meminit setup_zone_pageset(struct zone *zone)