mm, thp: avoid excessive compaction latency during fault

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 transparent hugepage page faults 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 fault.  We still need to retry async
compaction after reclaim, but this won't stall for seconds.
Signed-off-by: David Rientjes <rientjes@google.com>
Acked-by: Mel Gorman <mgorman@suse.de>
Cc: Greg Thelen <gthelen@google.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -2575,7 +2575,14 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 					&did_some_progress);
 	if (page)
 		goto got_pg;
-	migration_mode = MIGRATE_SYNC_LIGHT;
+
+	/*
+	 * It can become very expensive to allocate transparent hugepages at
+	 * fault, so use asynchronous memory compaction for THP unless it is
+	 * khugepaged trying to collapse.
+	 */
+	if (!(gfp_mask & __GFP_NO_KSWAPD) || (current->flags & PF_KTHREAD))
+		migration_mode = MIGRATE_SYNC_LIGHT;
 
 	/*
 	 * If compaction is deferred for high-order allocations, it is because