linux-kernel - Re: [PATCH 02/22] Do not sanity check order in the fast path

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Date:	Thu, 23 Apr 2009 15:44:09 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Mel Gorman <mel@....ul.ie>
Cc:	dave@...ux.vnet.ibm.com, linux-mm@...ck.org,
	kosaki.motohiro@...fujitsu.com, cl@...ux-foundation.org,
	npiggin@...e.de, linux-kernel@...r.kernel.org,
	ming.m.lin@...el.com, yanmin_zhang@...ux.intel.com,
	peterz@...radead.org, penberg@...helsinki.fi
Subject: Re: [PATCH 02/22] Do not sanity check order in the fast path

On Wed, 22 Apr 2009 18:11:51 +0100
Mel Gorman <mel@....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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