linux-ext4 - [PATCHv2 12/41] thp: handle write-protection faults for file THP

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Date:	Fri, 12 Aug 2016 21:37:55 +0300
From:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:	"Theodore Ts'o" <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Jan Kara <jack@...e.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	Hugh Dickins <hughd@...gle.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Matthew Wilcox <willy@...radead.org>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-block@...r.kernel.org,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: [PATCHv2 12/41] thp: handle write-protection faults for file THP

For filesystems that wants to be write-notified (has mkwrite), we will
encount write-protection faults for huge PMDs in shared mappings.

The easiest way to handle them is to clear the PMD and let it refault as
wriable.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
---
 mm/memory.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index 4425b6059339..5b7f0ce44a27 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3448,8 +3448,17 @@ static int wp_huge_pmd(struct fault_env *fe, pmd_t orig_pmd)
 		return fe->vma->vm_ops->pmd_fault(fe->vma, fe->address, fe->pmd,
 				fe->flags);
 
+	if (fe->vma->vm_flags & VM_SHARED) {
+		/* Clear PMD */
+		zap_page_range_single(fe->vma, fe->address,
+				HPAGE_PMD_SIZE, NULL);
+		VM_BUG_ON(!pmd_none(*fe->pmd));
+
+		/* Refault to establish writable PMD */
+		return 0;
+	}
+
 	/* 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;
-- 
2.8.1

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