linux-kernel - Re: [LKP] efad4e475c [ 40.308255] Oops: 0000 [#1] PREEMPT SMP PTI

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Date:   Mon, 18 Feb 2019 19:11:55 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Mike Rapoport <rppt@...ux.ibm.com>,
        Rong Chen <rong.a.chen@...el.com>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        linux-kernel@...r.kernel.org,
        Linux Memory Management List <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>, LKP <lkp@...org>,
        Oscar Salvador <osalvador@...e.de>
Subject: Re: [LKP] efad4e475c [ 40.308255] Oops: 0000 [#1] PREEMPT SMP PTI

On Mon 18-02-19 09:57:26, Matthew Wilcox wrote:
> On Mon, Feb 18, 2019 at 06:05:58PM +0100, Michal Hocko wrote:
> > +	end_pfn = min(start_pfn + nr_pages,
> > +			zone_end_pfn(page_zone(pfn_to_page(start_pfn))));
> >  
> >  	/* Check the starting page of each pageblock within the range */
> > -	for (; page < end_page; page = next_active_pageblock(page)) {
> > -		if (!is_pageblock_removable_nolock(page))
> > +	for (; start_pfn < end_pfn; start_pfn = next_active_pageblock(start_pfn)) {
> > +		if (!is_pageblock_removable_nolock(start_pfn))
> 
> If you have a zone which contains pfns that run from ULONG_MAX-n to ULONG_MAX,
> end_pfn is going to wrap around to 0 and this loop won't execute.

Is this a realistic situation to bother?

> I think
> you should use:
> 
> 	max_pfn = min(start_pfn + nr_pages,
> 			zone_end_pfn(page_zone(pfn_to_page(start_pfn)))) - 1;
> 
> 	for (; start_pfn <= max_pfn; ...)

I do not really care strongly, but we have more places were we do
start_pfn + nr_pages and then use it as pfn < end_pfn construct. I
suspect we would need to make a larger audit and make the code
consistent so unless there are major concerns I would stick with what
I have for now and leave the rest for the cleanup. Does that sound
reasonable?

-- 
Michal Hocko
SUSE Labs