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 ec706da ("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 <[email protected]>
Reported-by: Moussa Ba <[email protected]>
Acked-by: Hugh Dickins <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Russell King <[email protected]>
Acked-by: Nicolas Pitre <[email protected]>
Cc: Matt Mackall <[email protected]>
Cc: <[email protected]>		[2.6.37+]
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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);