linux-kernel - [PATCH] mm, thp: move invariant bug check out of loop in __split_huge_page

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-Id: <1402947348-60655-1-git-send-email-Waiman.Long@hp.com>
Date:	Mon, 16 Jun 2014 15:35:48 -0400
From:	Waiman Long <Waiman.Long@...com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Scott J Norton <scott.norton@...com>,
	Waiman Long <Waiman.Long@...com>
Subject: [PATCH] mm, thp: move invariant bug check out of loop in __split_huge_page_map

In the __split_huge_page_map() function, 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 move the invariant bug check out of the loop so that it
will be done only once.

Signed-off-by: Waiman Long <Waiman.Long@...com>
---
 mm/huge_memory.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index b4b1feb..b8bb16c 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1744,6 +1744,8 @@ 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) {
@@ -1753,8 +1755,6 @@ 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))
-- 
1.7.1

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/