On Wed, Dec 14, 2016 at 04:07:06PM +0100, Michal Hocko wrote:

From: Michal Hocko <mhocko@suse.com>

Tetsuo Handa has pointed out that 0a0337e0d1d1 ("mm, oom: rework oom
detection") has subtly changed semantic for costly high order requests
with __GFP_NOFAIL and withtout __GFP_REPEAT and those can fail right now.
My code inspection didn't reveal any such users in the tree but it is
true that this might lead to unexpected allocation failures and
subsequent OOPs.

__alloc_pages_slowpath wrt. GFP_NOFAIL is hard to follow currently.
There are few special cases but we are lacking a catch all place to be
sure we will not miss any case where the non failing allocation might
fail. This patch reorganizes the code a bit and puts all those special
cases under nopage label which is the generic go-to-fail path. Non
failing allocations are retried or those that cannot retry like
non-sleeping allocation go to the failure point directly. This should
make the code flow much easier to follow and make it less error prone
for future changes.

While we are there we have to move the stall check up to catch
potentially looping non-failing allocations.

Signed-off-by: Michal Hocko <mhocko@suse.com>
Acked-by: Vlastimil Babka <redacted>

It's not the nicest thing that we have to duplicate all the conditions
to warn on, but it's preferable over unreliable GFP_NOFAIL handling.
Consolidating the handling of this flag makes a lot of sense to me.

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

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