On 06/22/2015 01:14 PM, Kirill A. Shutemov wrote:
On Thu, Jun 11, 2015 at 11:49:48AM +0200, Vlastimil Babka wrote:
On 06/03/2015 07:06 PM, Kirill A. Shutemov wrote:
The order of actions here means that between TestSetPageDoubleMap() and the
atomic incs, anyone calling page_mapcount() on one of the pages not
processed by the for loop yet, will see a value lower by 1 from what he
should see. I wonder if that can cause any trouble somewhere, especially if
there's only one other compound mapping and page_mapcount() will return 0
instead of 1?
Good catch. Thanks.
What about this?
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 0f1f5731a893..cd0e6addb662 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2636,15 +2636,25 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
for (i = 0; i < HPAGE_PMD_NR; i++)
atomic_dec(&page[i]._mapcount);
}
- } else if (!TestSetPageDoubleMap(page)) {
+ } else if (!PageDoubleMap(page)) {
/*
* The first PMD split for the compound page and we still
* have other PMD mapping of the page: bump _mapcount in
* every small page.
+ *
* This reference will go away with last compound_mapcount.
+ *
+ * Note, we need to increment mapcounts before setting
+ * PG_double_map to avoid false-negative page_mapped().
*/
for (i = 0; i < HPAGE_PMD_NR; i++)
atomic_inc(&page[i]._mapcount);
+
+ if (TestSetPageDoubleMap(page)) {
+ /* Race with another __split_huge_pmd() for the page */
+ for (i = 0; i < HPAGE_PMD_NR; i++)
+ atomic_dec(&page[i]._mapcount);
+ }
}
Yeah that should work.
smp_wmb(); /* make pte visible before pmd */
Conversely, when clearing PageDoubleMap() above (or in one of those rmap
functions IIRC), one could see mapcount inflated by one. But I guess that's
less dangerous.
I think it's safe.
OK.
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