The patch titled
hugetlb: correct page count for surplus huge pages
has been added to the -mm tree. Its filename is
hugetlb-correct-page-count-for-surplus-huge-pages.patch
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The current -mm tree may be found at http://userweb.kernel.org/~akpm/mmotm/
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Subject: hugetlb: correct page count for surplus huge pages
From: Adam Litke <agl@xxxxxxxxxx>
Free pages in the hugetlb pool are free and as such have a reference count of
zero. Regular allocations into the pool from the buddy are "freed" into the
pool which results in their page_count dropping to zero. However, surplus
pages can be directly utilized by the caller without first being freed to the
pool. Therefore, a call to put_page_testzero() is in order so that such a
page will be handed to the caller with a correct count.
This has not affected end users because the bad page count is reset before the
page is handed off. However, under CONFIG_DEBUG_VM this triggers a BUG when
the page count is validated.
Thanks go to Mel for first spotting this issue and providing an initial fix.
Signed-off-by: Adam Litke <agl@xxxxxxxxxx>
Cc: Mel Gorman <mel@xxxxxxxxx>
Cc: Dave Hansen <haveblue@xxxxxxxxxx>
Cc: William Lee Irwin III <wli@xxxxxxxxxxxxxx>
Cc: Andy Whitcroft <apw@xxxxxxxxxxxx>
Cc: Mel Gorman <mel@xxxxxxxxx>
Cc: David Gibson <david@xxxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/hugetlb.c | 13 ++++++++++---
1 file changed, 10 insertions(+), 3 deletions(-)
diff -puN mm/hugetlb.c~hugetlb-correct-page-count-for-surplus-huge-pages mm/hugetlb.c
--- a/mm/hugetlb.c~hugetlb-correct-page-count-for-surplus-huge-pages
+++ a/mm/hugetlb.c
@@ -268,6 +268,12 @@ static struct page *alloc_buddy_huge_pag
spin_lock(&hugetlb_lock);
if (page) {
+ /*
+ * This page is now managed by the hugetlb allocator and has
+ * no users -- drop the buddy allocator's reference.
+ */
+ int page_count = put_page_testzero(page);
+ BUG_ON(page_count != 0);
nid = page_to_nid(page);
set_compound_page_dtor(page, free_huge_page);
/*
@@ -346,13 +352,14 @@ free:
enqueue_huge_page(page);
else {
/*
- * Decrement the refcount and free the page using its
- * destructor. This must be done with hugetlb_lock
+ * The page has a reference count of zero already, so
+ * call free_huge_page directly instead of using
+ * put_page. This must be done with hugetlb_lock
* unlocked which is safe because free_huge_page takes
* hugetlb_lock before deciding how to free the page.
*/
spin_unlock(&hugetlb_lock);
- put_page(page);
+ free_huge_page(page);
spin_lock(&hugetlb_lock);
}
}
_
Patches currently in -mm which might be from agl@xxxxxxxxxx are
hugetlb-correct-page-count-for-surplus-huge-pages.patch
hugetlb-close-a-difficult-to-trigger-reservation-race.patch
hugetlb-decrease-hugetlb_lock-cycling-in-gather_surplus_huge_pages.patch
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