The patch titled
Subject: mm/page_alloc: insert memory barriers to allow async update of pcp batch and high
has been added to the -mm tree. Its filename is
mm-page_alloc-insert-memory-barriers-to-allow-async-update-of-pcp-batch-and-high.patch
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------------------------------------------------------
From: Cody P Schafer <cody@xxxxxxxxxxxxxxxxxx>
Subject: 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@xxxxxxxxxxxxxxxxxx>
Reviewed-by: Gilad Ben-Yossef <gilad@xxxxxxxxxxxxx>
Cc: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Pekka Enberg <penberg@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/page_alloc.c | 41 ++++++++++++++++++++++++++++++++---------
1 file changed, 32 insertions(+), 9 deletions(-)
diff -puN mm/page_alloc.c~mm-page_alloc-insert-memory-barriers-to-allow-async-update-of-pcp-batch-and-high mm/page_alloc.c
--- a/mm/page_alloc.c~mm-page_alloc-insert-memory-barriers-to-allow-async-update-of-pcp-batch-and-high
+++ a/mm/page_alloc.c
@@ -4035,12 +4035,37 @@ static int __meminit zone_batchsize(stru
#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
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)
_
Patches currently in -mm which might be from cody@xxxxxxxxxxxxxxxxxx are
mm-page_alloc-factor-out-setting-of-pcp-high-and-pcp-batch.patch
mm-page_alloc-prevent-concurrent-updaters-of-pcp-batch-and-high.patch
mm-page_alloc-insert-memory-barriers-to-allow-async-update-of-pcp-batch-and-high.patch
mm-page_alloc-protect-pcp-batch-accesses-with-access_once.patch
mm-page_alloc-convert-zone_pcp_update-to-rely-on-memory-barriers-instead-of-stop_machine.patch
mm-page_alloc-when-handling-percpu_pagelist_fraction-dont-unneedly-recalulate-high.patch
mm-page_alloc-factor-setup_pageset-into-pageset_init-and-pageset_set_batch.patch
mm-page_alloc-relocate-comment-to-be-directly-above-code-it-refers-to.patch
mm-page_alloc-factor-zone_pageset_init-out-of-setup_zone_pageset.patch
mm-page_alloc-in-zone_pcp_update-uze-zone_pageset_init.patch
mm-page_alloc-rename-setup_pagelist_highmark-to-match-naming-of-pageset_set_batch.patch
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