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@hp.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Rik van Riel <riel@redhat.com>
Cc: Scott J Norton <scott.norton@hp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index c491854d9b3d51545b705a9cff308ef3a895defd..7a834d13413bab2b1e7ec48a29a428025a4e71f8 100644 (file)
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1775,6 +1775,8 @@ static int __split_huge_page_map(struct page *page,
        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 page *page,
                        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))