mm: vmscan: check for fatal signals iff the process was throttled

Commit 5515061d

 ("mm: throttle direct reclaimers if PF_MEMALLOC
reserves are low and swap is backed by network storage") introduced a
check for fatal signals after a process gets throttled for network
storage.  The intention was that if a process was throttled and got
killed that it should not trigger the OOM killer.  As pointed out by
Minchan Kim and David Rientjes, this check is in the wrong place and too
broad.  If a system is in am OOM situation and a process is exiting, it
can loop in __alloc_pages_slowpath() and calling direct reclaim in a
loop.  As the fatal signal is pending it returns 1 as if it is making
forward progress and can effectively deadlock.

This patch moves the fatal_signal_pending() check after throttling to
throttle_direct_reclaim() where it belongs.  If the process is killed
while throttled, it will return immediately without direct reclaim
except now it will have TIF_MEMDIE set and will use the PFMEMALLOC
reserves.

Minchan pointed out that it may be better to direct reclaim before
returning to avoid using the reserves because there may be pages that
can easily reclaim that would avoid using the reserves.  However, we do
no such targetted reclaim and there is no guarantee that suitable pages
are available.  As it is expected that this throttling happens when
swap-over-NFS is used there is a possibility that the process will
instead swap which may allocate network buffers from the PFMEMALLOC
reserves.  Hence, in the swap-over-nfs case where a process can be
throtted and be killed it can use the reserves to exit or it can
potentially use reserves to swap a few pages and then exit.  This patch
takes the option of using the reserves if necessary to allow the process
exit quickly.

If this patch passes review it should be considered a -stable candidate
for 3.6.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Cc: David Rientjes <rientjes@google.com>
Cc: Luigi Semenzato <semenzato@google.com>
Cc: Dan Magenheimer <dan.magenheimer@oracle.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Sonny Rao <sonnyrao@google.com>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>