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Date:   Fri, 9 Nov 2018 11:22:35 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Vlastimil Babka <vbabka@suse.cz>
Cc:     Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
        Kyungtae Kim <kt0755@gmail.com>, akpm@linux-foundation.org,
        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 Fri 09-11-18 11:10:00, Vlastimil Babka wrote:
> On 11/9/18 10:56 AM, Michal Hocko wrote:
> > On Fri 09-11-18 18:41:53, Tetsuo Handa wrote:
> >> On 2018/11/09 17:43, Michal Hocko wrote:
> >>> @@ -4364,6 +4353,17 @@ __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 = { };
> >>>  
> >>> +	/*
> >>> +	 * In the slowpath, we sanity check order to avoid ever trying to
> >>
> >> Please keep the comment up to dated.
> > 
> > Does this following look better?
> > 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 9fc10a1029cf..bf9aecba4222 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -4354,10 +4354,8 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order, int preferred_nid,
> >  	struct alloc_context ac = { };
> >  
> >  	/*
> > -	 * In the slowpath, we sanity check order to avoid ever trying to
> > -	 * reclaim >= MAX_ORDER areas which will never succeed. Callers may
> > -	 * be using allocators in order of preference for an area that is
> > -	 * too large.
> > +	 * 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 (order >= MAX_ORDER) {
> >  		WARN_ON_ONCE(!(gfp_mask & __GFP_NOWARN));
> 
> Looks ok, but I'd add unlikely(), although it doesn't currently seem to
> make any difference.
> 
> You can add Acked-by: Vlastimil Babka <vbabka@suse.cz>

OK, I have added both. Thanks!

-- 
Michal Hocko
SUSE Labs