From 94d77b732e45c92b53ce8ea525c2b479bdc76f0b Mon Sep 17 00:00:00 2001 From: Will Deacon Date: Fri, 20 Jan 2012 14:34:09 -0800 Subject: [PATCH] proc: clear_refs: do not clear reserved pages /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 45aa21fe ("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. 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 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. Signed-off-by: Will Deacon Reported-by: Moussa Ba Acked-by: Hugh Dickins Cc: David Rientjes Cc: Russell King Acked-by: Nicolas Pitre Cc: Matt Mackall Cc: [2.6.37+] Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- fs/proc/task_mmu.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c index e418c5abdb0ef..7dcd2a250495d 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); -- 2.39.5