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authorWill Deacon <will.deacon@arm.com>2012-01-20 14:34:09 -0800
committerGreg Kroah-Hartman <gregkh@suse.de>2012-01-25 17:25:05 -0800
commitc2c9f543718e15227a4aa0135e793480b94c4d97 (patch)
tree54abbd14151f4fd7900237125f92f21571a4a4c5 /fs/proc
parentb8f256956a063ebc60a674ec5795c7ca5aaa79c6 (diff)
downloadlwn-c2c9f543718e15227a4aa0135e793480b94c4d97.tar.gz
lwn-c2c9f543718e15227a4aa0135e793480b94c4d97.zip
proc: clear_refs: do not clear reserved pages
commit 85e72aa5384b1a614563ad63257ded0e91d1a620 upstream. /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. 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 <will.deacon@arm.com> Reported-by: Moussa Ba <moussaba@micron.com> Acked-by: Hugh Dickins <hughd@google.com> Cc: David Rientjes <rientjes@google.com> Cc: Russell King <rmk@arm.linux.org.uk> Acked-by: Nicolas Pitre <nico@linaro.org> Cc: Matt Mackall <mpm@selenic.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Diffstat (limited to 'fs/proc')
-rw-r--r--fs/proc/task_mmu.c3
1 files changed, 3 insertions, 0 deletions
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index c7d4ee663f14..3487b066500b 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -516,6 +516,9 @@ 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);