On 06/03/2015 07:06 PM, Kirill A. Shutemov wrote:
Original split_huge_page() combined two operations: splitting PMDs into
tables of PTEs and splitting underlying compound page. This patch
implements split_huge_pmd() which split given PMD without splitting
other PMDs this page mapped with or underlying compound page.
Without tail page refcounting, implementation of split_huge_pmd() is
pretty straight-forward.
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Tested-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
[...]
+
+ if (atomic_add_negative(-1, compound_mapcount_ptr(page))) {
+ /* Last compound_mapcount is gone. */
+ __dec_zone_page_state(page, NR_ANON_TRANSPARENT_HUGEPAGES);
+ if (PageDoubleMap(page)) {
+ /* No need in mapcount reference anymore */
+ ClearPageDoubleMap(page);
+ for (i = 0; i < HPAGE_PMD_NR; i++)
+ atomic_dec(&page[i]._mapcount);
+ }
+ } else if (!TestSetPageDoubleMap(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.
+ */
+ for (i = 0; i < HPAGE_PMD_NR; i++)
+ atomic_inc(&page[i]._mapcount);
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?
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.
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