[PATCH][V6]make get_user_pages interruptible

[Ying Han <yinghan@google.com>]

changelog
[v6] replace the sigkill_pending() with fatal_signal_pending()
      add the check for cases current != tsk

From: Ying Han <yinghan@google.com>

make get_user_pages interruptible
The initial implementation of checking TIF_MEMDIE covers the cases of OOM
killing. If the process has been OOM killed, the TIF_MEMDIE is set and it
return immediately. This patch includes:

1. add the case that the SIGKILL is sent by user processes. The process can
try to get_user_pages() unlimited memory even if a user process has sent a
SIGKILL to it(maybe a monitor find the process exceed its memory limit and
try to kill it). In the old implementation, the SIGKILL won't be handled
until the get_user_pages() returns.

2. change the return value to be ERESTARTSYS. It makes no sense to return
ENOMEM if the get_user_pages returned by getting a SIGKILL signal.
Considering the general convention for a system call interrupted by a
signal is ERESTARTNOSYS, so the current return value is consistant to that.

Signed-off-by:	Paul Menage <menage@google.com>
Signed-off-by:	Ying Han <yinghan@google.com>

mm/memory.c                   |   13 ++-

diff --git a/mm/memory.c b/mm/memory.c
index 164951c..049a4f1 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1218,12 +1218,15 @@ int __get_user_pages(struct task_struct *tsk, struct m
 			struct page *page;

 			/*
-			 * If tsk is ooming, cut off its access to large memory
-			 * allocations. It has a pending SIGKILL, but it can't
-			 * be processed until returning to user space.
+			 * If we have a pending SIGKILL, don't keep
+			 * allocating memory. We check both current
+			 * and tsk to cover the cases where current
+			 * is allocating pages on behalf of tsk.
 			 */
-			if (unlikely(test_tsk_thread_flag(tsk, TIF_MEMDIE)))
-				return i ? i : -ENOMEM;
+			if (unlikely(fatal_signal_pending(current) ||
+				((current != tsk) &&
+				fatal_signal_pending(tsk))))
+				return i ? i : -ERESTARTSYS;

 			if (write)
 				foll_flags |= FOLL_WRITE;

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