On Mon, Jul 25, 2011 at 10:38:41PM +0200, Johannes Weiner wrote:

quoted

With defrag mode enabled per default, huge page allocations pass
__GFP_WAIT and may drop compaction into sync-mode where they wait for
pages under writeback.

I observe applications hang for several minutes(!) when they fault in
huge pages and compaction starts to wait on in-"flight" USB stick IO.

This patch disables defrag mode for page fault allocations unless the
VMA is madvised explicitely.  Khugepaged will continue to allocate
with __GFP_WAIT per default, but stalls are not a problem of
application responsiveness there.

Seems drastic. You could just avoid sync migration for transparent
hugepage allocations with something like the patch below? There still
is a stall as some order-0 pages will be reclaimed before compaction
is tried again but it will nothing like a sync migration.

=== CUT HERE ===

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 1fac154..40f2a9b 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c

@@ -2174,7 +2174,14 @@ rebalance:
 					sync_migration);
 	if (page)
 		goto got_pg;
-	sync_migration = true;
+
+	/*
+	 * Do not use sync migration for transparent hugepage allocations as
+	 * it could stall writing back pages which is far worse than simply
+	 * failing to promote a page.
+	 */
+	if (!(gfp_mask & __GFP_NO_KSWAPD))
+		sync_migration = true;