linux-kernel - [PATCH 07/15] mm: numa: Do not clear PMD during PTE update scan

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Date:	Tue,  3 Dec 2013 08:51:54 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Alex Thorlton <athorlton@....com>
Cc:	Rik van Riel <riel@...hat.com>, Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>, Mel Gorman <mgorman@...e.de>
Subject: [PATCH 07/15] mm: numa: Do not clear PMD during PTE update scan

If the PMD is flushed then a parallel fault in handle_mm_fault() will
enter the pmd_none and do_huge_pmd_anonymous_page() path where it'll
attempt to insert a huge zero page. This is wasteful so the patch
avoids clearing the PMD when setting pmd_numa.

Signed-off-by: Mel Gorman <mgorman@...e.de>
---
 mm/huge_memory.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 203b5bc..d6c3bf4 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1474,20 +1474,24 @@ int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
 
 	if (__pmd_trans_huge_lock(pmd, vma) == 1) {
 		pmd_t entry;
-		entry = pmdp_get_and_clear(mm, addr, pmd);
+
 		if (!prot_numa) {
+			entry = pmdp_get_and_clear(mm, addr, pmd);
 			entry = pmd_modify(entry, newprot);
 			BUG_ON(pmd_write(entry));
+			set_pmd_at(mm, addr, pmd, entry);
 		} else {
 			struct page *page = pmd_page(*pmd);
+			entry = *pmd;
 
 			/* only check non-shared pages */
 			if (page_mapcount(page) == 1 &&
 			    !pmd_numa(*pmd)) {
 				entry = pmd_mknuma(entry);
+				set_pmd_at(mm, addr, pmd, entry);
 			}
 		}
-		set_pmd_at(mm, addr, pmd, entry);
+
 		spin_unlock(&vma->vm_mm->page_table_lock);
 		ret = 1;
 	}
-- 
1.8.4

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