[PATCH 2/2] mm: do not loop over ALLOC_NO_WATERMARKS without triggering reclaim

[mhocko]

From: Michal Hocko <mhocko@xxxxxxxx>

__alloc_pages_slowpath is looping over ALLOC_NO_WATERMARKS requests if
__GFP_NOFAIL is requested. This is fragile because we are basically
relying on somebody else to make the reclaim (be it the direct reclaim
or OOM killer) for us. The caller might be holding resources (e.g.
locks) which block other other reclaimers from making any progress for
example. Remove the retry loop and rely on __alloc_pages_slowpath to
invoke all allowed reclaim steps and retry logic.

We have to be careful about __GFP_NOFAIL allocations from the
PF_MEMALLOC context even though this is a very bad idea to begin with
because no progress can be gurateed at all.  We shouldn't break the
__GFP_NOFAIL semantic here though. It could be argued that this is
essentially GFP_NOWAIT context which we do not support but PF_MEMALLOC
is much harder to check for existing users because they might happen
deep down the code path performed much later after setting the flag
so we cannot really rule out there is no kernel path triggering this
combination.

Acked-by: Mel Gorman <mgorman@xxxxxxx>
Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
---
 mm/page_alloc.c | 32 ++++++++++++++++++--------------
 1 file changed, 18 insertions(+), 14 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index b153fa3d0b9b..df7746280427 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3046,32 +3046,36 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 		 * allocations are system rather than user orientated
 		 */
 		ac->zonelist = node_zonelist(numa_node_id(), gfp_mask);
-		do {
-			page = get_page_from_freelist(gfp_mask, order,
-							ALLOC_NO_WATERMARKS, ac);
-			if (page)
-				goto got_pg;
-
-			if (gfp_mask & __GFP_NOFAIL)
-				wait_iff_congested(ac->preferred_zone,
-						   BLK_RW_ASYNC, HZ/50);
-		} while (gfp_mask & __GFP_NOFAIL);
+		page = get_page_from_freelist(gfp_mask, order,
+						ALLOC_NO_WATERMARKS, ac);
+		if (page)
+			goto got_pg;
 	}
 
 	/* Caller is not willing to reclaim, we can't balance anything */
 	if (!can_direct_reclaim) {
 		/*
-		 * All existing users of the deprecated __GFP_NOFAIL are
-		 * blockable, so warn of any new users that actually allow this
-		 * type of allocation to fail.
+		 * All existing users of the __GFP_NOFAIL are blockable, so warn
+		 * of any new users that actually allow this type of allocation
+		 * to fail.
 		 */
 		WARN_ON_ONCE(gfp_mask & __GFP_NOFAIL);
 		goto nopage;
 	}
 
 	/* Avoid recursion of direct reclaim */
-	if (current->flags & PF_MEMALLOC)
+	if (current->flags & PF_MEMALLOC) {
+		/*
+		 * __GFP_NOFAIL request from this context is rather bizarre
+		 * because we cannot reclaim anything and only can loop waiting
+		 * for somebody to do a work for us.
+		 */
+		if (WARN_ON_ONCE(gfp_mask & __GFP_NOFAIL)) {
+			cond_resched();
+			goto retry;
+		}
 		goto nopage;
+	}
 
 	/* Avoid allocations with no watermarks from looping endlessly */
 	if (test_thread_flag(TIF_MEMDIE) && !(gfp_mask & __GFP_NOFAIL))
-- 
2.6.2

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