From: Waiman Long Date: Thu, 26 Jun 2014 00:42:35 +0000 (+1000) Subject: mm, thp: move invariant bug check out of loop in __split_huge_page_map X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=192d7878bbc2fa6b227853a996f2c18936209ca9;p=karo-tx-linux.git 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 Acked-by: Kirill A. Shutemov Cc: Andrea Arcangeli Cc: Mel Gorman Cc: Rik van Riel Cc: Scott J Norton Signed-off-by: Andrew Morton --- diff --git a/mm/huge_memory.c b/mm/huge_memory.c index c491854d9b3d..7a834d13413b 100644 --- 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))