proc: use down_read_killable mmap_sem for /proc/pid/clear_refs

Do not remain stuck forever if something goes wrong.  Using a killable
lock permits cleanup of stuck tasks and simplifies investigation.

Replace the only unkillable mmap_sem lock in clear_refs_write().

Link: http://lkml.kernel.org/r/156007493826.3335.5424884725467456239.stgit@buzz
Signed-off-by: Konstantin Khlebnikov <[email protected]>
Reviewed-by: Roman Gushchin <[email protected]>
Reviewed-by: Cyrill Gorcunov <[email protected]>
Reviewed-by: Kirill Tkhai <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Alexey Dobriyan <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Michal Koutný <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

fs/proc/task_mmu.c

@@ -1140,7 +1140,10 @@ static ssize_t clear_refs_write(struct file *file, const char __user *buf,
 			goto out_mm;
 		}
 
-		down_read(&mm->mmap_sem);
+		if (down_read_killable(&mm->mmap_sem)) {
+			count = -EINTR;
+			goto out_mm;
+		}
 		tlb_gather_mmu(&tlb, mm, 0, -1);
 		if (type == CLEAR_REFS_SOFT_DIRTY) {
 			for (vma = mm->mmap; vma; vma = vma->vm_next) {