linux-kernel - [PATCHv4 11/43] thp: handle write-protection faults for file THP

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Date:   Tue, 25 Oct 2016 03:13:10 +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: [PATCHv4 11/43] 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>
Reviewed-by: Jan Kara <jack@...e.cz>
---
 mm/memory.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index e18c57bdc75c..e8c04d1d87b8 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3452,8 +3452,17 @@ 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_MASK,
+				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;
-- 
2.9.3