From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx204.postini.com [74.125.245.204]) by kanga.kvack.org (Postfix) with SMTP id D83AB6B004F for ; Fri, 13 Jan 2012 10:13:21 -0500 (EST) From: Will Deacon Subject: [RFC PATCH] proc: clear_refs: do not clear reserved pages Date: Fri, 13 Jan 2012 15:13:07 +0000 Message-Id: <1326467587-22218-1-git-send-email-will.deacon@arm.com> Sender: owner-linux-mm@kvack.org List-ID: To: linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org Cc: moussaba@micron.com, Will Deacon , David Rientjes , Andrew Morton , Nicolas Pitre /proc/pid/clear_refs is used to clear the Referenced and YOUNG bits for pages and corresponding page table entries of the task with PID pid, which includes any special mappings inserted into the page tables in order to provide things like vDSOs and user helper functions. On ARM this causes a problem because the vectors page is mapped as a global mapping and since ec706dab ("ARM: add a vma entry for the user accessible vector page"), a VMA is also inserted into each task for this page to aid unwinding through signals and syscall restarts. Since the vectors page is required for handling faults, clearing the YOUNG bit (and subsequently writing a faulting pte) means that we lose the vectors page *globally* and cannot fault it back in. This results in a system deadlock on the next exception. This patch avoids clearing the aforementioned bits for reserved pages, therefore leaving the vectors page intact on ARM. Since reserved pages are not candidates for swap, this change should not have any impact on the usefulness of clear_refs. Cc: David Rientjes Cc: Andrew Morton Cc: Nicolas Pitre Reported-by: Moussa Ba Signed-off-by: Will Deacon --- An aside: if you want to see this problem in action, just run: $ echo 1 > /proc/self/clear_refs on an ARM platform (as any user) and watch your system hang. I think this has been the case since 2.6.37, so I'll CC stable once people are happy with the fix. fs/proc/task_mmu.c | 3 +++ 1 files changed, 3 insertions(+), 0 deletions(-) diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c index e418c5a..7dcd2a2 100644 --- a/fs/proc/task_mmu.c +++ b/fs/proc/task_mmu.c @@ -518,6 +518,9 @@ static int clear_refs_pte_range(pmd_t *pmd, unsigned long addr, if (!page) continue; + if (PageReserved(page)) + continue; + /* Clear accessed and referenced bits. */ ptep_test_and_clear_young(vma, addr, pte); ClearPageReferenced(page); -- 1.7.4.1 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org