[patch v2 4/4] mm, thp: do not perform sync compaction on pagefault

[David Rientjes]

Synchronous memory compaction can be very expensive: it can iterate an enormous 
amount of memory without aborting, constantly rescheduling, waiting on page
locks and lru_lock, etc, if a pageblock cannot be defragmented.

Unfortunately, it's too expensive for pagefault for transparent hugepages and 
it's much better to simply fallback to pages.  On 128GB machines, we find that 
synchronous memory compaction can take O(seconds) for a single thp fault.

Now that async compaction remembers where it left off without strictly relying
on sync compaction, this makes thp allocations best-effort without causing
egregious latency during pagefault.

Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
---
 mm/page_alloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2656,7 +2656,7 @@ rebalance:
 		/* Wait for some write requests to complete then retry */
 		wait_iff_congested(preferred_zone, BLK_RW_ASYNC, HZ/50);
 		goto rebalance;
-	} else {
+	} else if (!(gfp_mask & __GFP_NO_KSWAPD)) {
 		/*
 		 * High-order allocations do not necessarily loop after
 		 * direct reclaim and reclaim/compaction depends on compaction
--
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