Date:	Mon,  9 Jul 2012 11:38:20 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Mel Gorman <mgorman@...e.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Minchan Kim <minchan@...nel.org>
Subject: [PATCH] mm: Warn about costly page allocation

Since lumpy reclaim was introduced at 2.6.23, it helped higher
order allocation.
Recently, we removed it at 3.4 and we didn't enable compaction
forcingly[1]. The reason makes sense that compaction.o + migration.o
isn't trivial for system doesn't use higher order allocation.
But the problem is that we have to enable compaction explicitly
while lumpy reclaim enabled unconditionally.

Normally, admin doesn't know his system have used higher order
allocation and even lumpy reclaim have helped it.
Admin in embdded system have a tendency to minimise code size so that
they can disable compaction. In this case, we can see page allocation
failure we can never see in the past. It's critical on embedded side
because...

Let's think this scenario.

There is QA team in embedded company and they have tested their product.
In test scenario, they can allocate 100 high order allocation.
(they don't matter how many high order allocations in kernel are needed
during test. their concern is just only working well or fail of their
middleware/application) High order allocation will be serviced well
by natural buddy allocation without lumpy's help. So they released
the product and sold out all over the world.
Unfortunately, in real practice, sometime, 105 high order allocation was
needed rarely and fortunately, lumpy reclaim could help it so the product
doesn't have a problem until now.

If they use latest kernel, they will see the new config CONFIG_COMPACTION
which is very poor documentation, and they can't know it's replacement of
lumpy reclaim(even, they don't know lumpy reclaim) so they simply disable
that option for size optimization. Of course, QA team still test it but they
can't find the problem if they don't do test stronger than old.
It ends up release the product and sold out all over the world, again.
But in this time, we don't have both lumpy and compaction so the problem
would happen in real practice. A poor enginner from Korea have to flight
to the USA for the fix a ton of products. Otherwise, should recall products
from all over the world. Maybe he can lose a job. :(

This patch adds warning for notice. If the system try to allocate
PAGE_ALLOC_COSTLY_ORDER above page and system enters reclaim path,
it emits the warning. At least, it gives a chance to look into their
system before the relase.

This patch avoids false positive by alloc_large_system_hash which
allocates with GFP_ATOMIC and a fallback mechanism so it can make
this warning useless.

[1] c53919ad(mm: vmscan: remove lumpy reclaim)

Signed-off-by: Minchan Kim <minchan@...nel.org>
---
 mm/page_alloc.c |   16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index a4d3a19..1155e00 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2276,6 +2276,20 @@ gfp_to_alloc_flags(gfp_t gfp_mask)
 	return alloc_flags;
 }
 
+#if defined(CONFIG_DEBUG_VM) && !defined(CONFIG_COMPACTION)
+static inline void check_page_alloc_costly_order(unsigned int order)
+{
+	if (unlikely(order > PAGE_ALLOC_COSTLY_ORDER)) {
+		printk_once("WARNING: You are tring to allocate %d-order page."
+		" You might need to turn on CONFIG_COMPACTION\n", order);
+	}
+}
+#else
+static inline void check_page_alloc_costly_order(unsigned int order)
+{
+}
+#endif
+
 static inline struct page *
 __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 	struct zonelist *zonelist, enum zone_type high_zoneidx,
@@ -2353,6 +2367,8 @@ rebalance:
 	if (!wait)
 		goto nopage;
 
+	check_page_alloc_costly_order(order);
+
 	/* Avoid recursion of direct reclaim */
 	if (current->flags & PF_MEMALLOC)
 		goto nopage;
-- 
1.7.9.5

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