[patch] mm, page_alloc: allow __GFP_NOFAIL to allocate below watermarks after reclaim

[David Rientjes <rientjes@google.com>]

If direct reclaim has failed to free memory, __GFP_NOFAIL allocations
can potentially loop forever in the page allocator.  In this case, it's
better to give them the ability to access below watermarks so that they
may allocate similar to the same privilege given to GFP_ATOMIC
allocations.

We're careful to ensure this is only done after direct reclaim has had
the chance to free memory, however.

Signed-off-by: David Rientjes <rientjes@google.com>
---
 mm/page_alloc.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2629,6 +2629,11 @@ rebalance:
 						pages_reclaimed)) {
 		/* Wait for some write requests to complete then retry */
 		wait_iff_congested(preferred_zone, BLK_RW_ASYNC, HZ/50);
+
+		/* Allocations that cannot fail must allocate from somewhere */
+		if (gfp_mask & __GFP_NOFAIL)
+			alloc_flags |= ALLOC_HARDER;
+
 		goto rebalance;
 	} else {
 		/*

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