[RFC 1/8] mm, oom: Give __GFP_NOFAIL allocations access to memory reserves

[mhocko]

From: Michal Hocko <mhocko@xxxxxxxx>

__GFP_NOFAIL is a big hammer used to ensure that the allocation
request can never fail. This is a strong requirement and as such
it also deserves a special treatment when the system is OOM. The
primary problem here is that the allocation request might have
come with some locks held and the oom victim might be blocked
on the same locks. This is basically an OOM deadlock situation.

This patch tries to reduce the risk of such a deadlocks by giving
__GFP_NOFAIL allocations a special treatment and let them dive into
memory reserves after oom killer invocation. This should help them
to make a progress and release resources they are holding. The OOM
victim should compensate for the reserves consumption.

Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
---
 mm/page_alloc.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 1f9ffbb087cb..ee69c338ca2a 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2732,8 +2732,16 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order,
 	}
 	/* Exhausted what can be done so it's blamo time */
 	if (out_of_memory(ac->zonelist, gfp_mask, order, ac->nodemask, false)
-			|| WARN_ON_ONCE(gfp_mask & __GFP_NOFAIL))
+			|| WARN_ON_ONCE(gfp_mask & __GFP_NOFAIL)) {
 		*did_some_progress = 1;
+
+		if (gfp_mask & __GFP_NOFAIL) {
+			page = get_page_from_freelist(gfp_mask, order,
+					ALLOC_NO_WATERMARKS|ALLOC_CPUSET, ac);
+			WARN_ONCE(!page, "Unable to fullfil gfp_nofail allocation."
+				    " Consider increasing min_free_kbytes.\n");
+		}
+	}
 out:
 	mutex_unlock(&oom_lock);
 	return page;
-- 
2.5.0

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