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Subject[PATCH] mm/page_alloc: Fix race conditions on getting migratetype in buffered_rmqueue
Date
From: Hui Zhu <zhuhui@xiaomi.com>

To test the patch [1], I use KGTP and a script [2] to show NR_FREE_CMA_PAGES
and gross of cma_nr_free. The values are always not same.
I check the code of pages alloc and free and found that race conditions
on getting migratetype in buffered_rmqueue.
Then I add move the code of getting migratetype inside the zone->lock
protection part.

Because this issue will affect system even if the Linux kernel does't
have [1]. So I post this patch separately.

This patchset is based on fc7f0dd381720ea5ee5818645f7d0e9dece41cb0.

[1] https://lkml.org/lkml/2015/1/18/28
[2] https://github.com/teawater/kgtp/blob/dev/add-ons/cma_free.py

Signed-off-by: Hui Zhu <zhuhui@xiaomi.com>
---
mm/page_alloc.c | 11 +++++++----
1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 7633c50..f3d6922 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1694,11 +1694,12 @@ again:
}
spin_lock_irqsave(&zone->lock, flags);
page = __rmqueue(zone, order, migratetype);
+ if (page)
+ migratetype = get_pageblock_migratetype(page);
+ else
+ goto failed_unlock;
spin_unlock(&zone->lock);
- if (!page)
- goto failed;
- __mod_zone_freepage_state(zone, -(1 << order),
- get_freepage_migratetype(page));
+ __mod_zone_freepage_state(zone, -(1 << order), migratetype);
}

__mod_zone_page_state(zone, NR_ALLOC_BATCH, -(1 << order));
@@ -1715,6 +1716,8 @@ again:
goto again;
return page;

+failed_unlock:
+ spin_unlock(&zone->lock);
failed:
local_irq_restore(flags);
return NULL;
--
1.9.3


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