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Date:	Sat, 16 Apr 2016 03:23:41 +0300
From:	"Kirill A. Shutemov" <>
To:	Hugh Dickins <>,
	Andrea Arcangeli <>,
	Andrew Morton <>
Cc:	Dave Hansen <>,
	Vlastimil Babka <>,
	Christoph Lameter <>,
	Naoya Horiguchi <>,
	Jerome Marchand <>,
	Yang Shi <>,
	Sasha Levin <>,
	Andres Lagar-Cavilla <>,
	Ning Qu <>,,,,
	"Kirill A. Shutemov" <>
Subject: [PATCHv7 10/29] thp: handle file COW faults

File COW for THP is handled on pte level: just split the pmd.

It's not clear how benefitial would be allocation of huge pages on COW
faults. And it would require some code to make them work.

I think at some point we can consider teaching khugepaged to collapse
pages in COW mappings, but allocating huge on fault is probably

Signed-off-by: Kirill A. Shutemov <>
 mm/memory.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/memory.c b/mm/memory.c
index 23de0567db18..cba29c033702 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3388,6 +3388,11 @@ static int wp_huge_pmd(struct fault_env *fe, pmd_t orig_pmd)
 	if (fe->vma->vm_ops->pmd_fault)
 		return fe->vma->vm_ops->pmd_fault(fe->vma, fe->address, fe->pmd,
+	/* COW handled on pte level: split pmd */
+	VM_BUG_ON_VMA(fe->vma->vm_flags & VM_SHARED, fe->vma);
+	split_huge_pmd(fe->vma, fe->pmd, fe->address);

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