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Date:   Tue, 13 Nov 2018 15:32:04 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Vlastimil Babka <vbabka@suse.cz>
Cc:     Michal Hocko <mhocko@kernel.org>, Kyungtae Kim <kt0755@gmail.com>,
        pavel.tatashin@microsoft.com, osalvador@suse.de,
        rppt@linux.vnet.ibm.com, aaron.lu@intel.com,
        iamjoonsoo.kim@lge.com, alexander.h.duyck@linux.intel.com,
        mgorman@techsingularity.net, lifeasageek@gmail.com,
        threeearcat@gmail.com, syzkaller@googlegroups.com,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Subject: Re: UBSAN: Undefined behaviour in mm/page_alloc.c
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On Wed, 14 Nov 2018 00:23:28 +0100 Vlastimil Babka <vbabka@suse.cz> wrote:

> On 11/14/18 12:15 AM, Andrew Morton wrote:
> > On Tue, 13 Nov 2018 10:43:05 +0100 Michal Hocko <mhocko@kernel.org> wrote:
> > 
> >> --- a/mm/page_alloc.c
> >> +++ b/mm/page_alloc.c
> >> @@ -4364,6 +4353,15 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order, int preferred_nid,
> >>  	gfp_t alloc_mask; /* The gfp_t that was actually used for allocation */
> >>  	struct alloc_context ac = { };
> >>  
> >> +	/*
> >> +	 * There are several places where we assume that the order value is sane
> >> +	 * so bail out early if the request is out of bound.
> >> +	 */
> >> +	if (unlikely(order >= MAX_ORDER)) {
> >> +		WARN_ON_ONCE(!(gfp_mask & __GFP_NOWARN));
> >> +		return NULL;
> >> +	}
> >> +
> > 
> > I know "everybody enables CONFIG_DEBUG_VM", but given this is fastpath,
> > we could help those who choose not to enable it by using
> > 
> > #ifdef CONFIG_DEBUG_VM
> > 	if (WARN_ON_ONCE(order >= MAX_ORDER && !(gfp_mask & __GFP_NOWARN)))
> > 		return NULL;
> > #endif
> 
> Hmm, but that would mean there's still potential undefined behavior for
> !CONFIG_DEBUG_VM, so I would prefer not to do it like that.
> 

What does "potential undefined behavior" mean here?