Date: Thu, 23 Apr 2009 15:44:09 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Mel Gorman <mel@csn.ul.ie>
Cc: dave@linux.vnet.ibm.com, linux-mm@kvack.org,
       kosaki.motohiro@jp.fujitsu.com, cl@linux-foundation.org,
       npiggin@suse.de, linux-kernel@vger.kernel.org, ming.m.lin@intel.com,
       yanmin_zhang@linux.intel.com, peterz@infradead.org,
       penberg@cs.helsinki.fi
Subject: Re: [PATCH 02/22] Do not sanity check order in the fast path
Message-Id: <20090423154409.92aaf809.akpm@linux-foundation.org>
In-Reply-To: <20090422171151.GF15367@csn.ul.ie>
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On Wed, 22 Apr 2009 18:11:51 +0100
Mel Gorman <mel@csn.ul.ie> wrote:

> > I depend on the allocator to tell me when I've fed it too high of an
> > order.  If we really need this, perhaps we should do an audit and then
> > add a WARN_ON() for a few releases to catch the stragglers.
> > 
> 
> I consider it buggy to ask for something so large that you always end up
> with the worst option - vmalloc().

Nevertheless, it's a pretty common pattern for initialisation code all
over the kernel to do

	while (allocate(huge_amount) == NULL)
		huge_amount /= 2;

and the proposed change will convert that from "works" to "either goes
BUG or mysteriously overindexes zone->free_area[] in
__rmqueue_smallest()".  The latter of which is really nasty.

> How about leaving it as a VM_BUG_ON
> to get as many reports as possible on who is depending on this odd
> behaviour?

That would be quite disruptive.  Even emitting a trace for each call
would be irritating.  How's about this:

--- a/mm/page_alloc.c~page-allocator-do-not-sanity-check-order-in-the-fast-path-fix
+++ a/mm/page_alloc.c
@@ -1405,7 +1405,8 @@ get_page_from_freelist(gfp_t gfp_mask, n
 
 	classzone_idx = zone_idx(preferred_zone);
 
-	VM_BUG_ON(order >= MAX_ORDER);
+	if (WARN_ON_ONCE(order >= MAX_ORDER))
+		return NULL;
 
 zonelist_scan:
 	/*
_


and then we revisit later?
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