linux-kernel - [PATCHv6 10/30] thp: handle file COW faults

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Message-Id: <1459983080-106718-11-git-send-email-kirill.shutemov@linux.intel.com>
Date:	Thu,  7 Apr 2016 01:51:00 +0300
From:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:	Hugh Dickins <hughd@...gle.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Dave Hansen <dave.hansen@...el.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Christoph Lameter <cl@...two.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Jerome Marchand <jmarchan@...hat.com>,
	Yang Shi <yang.shi@...aro.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	Andres Lagar-Cavilla <andreslc@...gle.com>,
	Ning Qu <quning@...il.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: [PATCHv6 10/30] 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
overkill.

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

diff --git a/mm/memory.c b/mm/memory.c
index c41804c0d524..e5c6b4387da3 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3395,6 +3395,11 @@ 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,
 				fe->flags);
+
+	/* 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);
+
 	return VM_FAULT_FALLBACK;
 }
 
-- 
2.8.0.rc3