The patch titled
Subject: mm, thp: move invariant bug check out of loop in __split_huge_page_map
has been added to the -mm tree. Its filename is
mm-thp-move-invariant-bug-check-out-of-loop-in-__split_huge_page_map.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/mm-thp-move-invariant-bug-check-out-of-loop-in-__split_huge_page_map.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/mm-thp-move-invariant-bug-check-out-of-loop-in-__split_huge_page_map.patch
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From: Waiman Long <Waiman.Long@xxxxxx>
Subject: mm, thp: move invariant bug check out of loop in __split_huge_page_map
In __split_huge_page_map(), the check for page_mapcount(page) is invariant
within the for loop. Because of the fact that the macro is implemented
using atomic_read(), the redundant check cannot be optimized away by the
compiler leading to unnecessary read to the page structure.
This patch moves the invariant bug check out of the loop so that it will
be done only once. On a 3.16-rc1 based kernel, the execution time of a
microbenchmark that broke up 1000 transparent huge pages using munmap()
had an execution time of 38,245us and 38,548us with and without the patch
respectively. The performance gain is about 1%.
Signed-off-by: Waiman Long <Waiman.Long@xxxxxx>
Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Scott J Norton <scott.norton@xxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/huge_memory.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff -puN mm/huge_memory.c~mm-thp-move-invariant-bug-check-out-of-loop-in-__split_huge_page_map mm/huge_memory.c
--- a/mm/huge_memory.c~mm-thp-move-invariant-bug-check-out-of-loop-in-__split_huge_page_map
+++ a/mm/huge_memory.c
@@ -1775,6 +1775,8 @@ static int __split_huge_page_map(struct
if (pmd) {
pgtable = pgtable_trans_huge_withdraw(mm, pmd);
pmd_populate(mm, &_pmd, pgtable);
+ if (pmd_write(*pmd))
+ BUG_ON(page_mapcount(page) != 1);
haddr = address;
for (i = 0; i < HPAGE_PMD_NR; i++, haddr += PAGE_SIZE) {
@@ -1784,8 +1786,6 @@ static int __split_huge_page_map(struct
entry = maybe_mkwrite(pte_mkdirty(entry), vma);
if (!pmd_write(*pmd))
entry = pte_wrprotect(entry);
- else
- BUG_ON(page_mapcount(page) != 1);
if (!pmd_young(*pmd))
entry = pte_mkold(entry);
if (pmd_numa(*pmd))
_
Patches currently in -mm which might be from Waiman.Long@xxxxxx are
mm-thp-move-invariant-bug-check-out-of-loop-in-__split_huge_page_map.patch
mm-thp-replace-smp_mb-after-atomic_add-by-smp_mb__after_atomic.patch
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