linux-kernel - Re: [PATCH][v2] PM / Hibernate: Feed the wathdog when creating snapshot

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Date:   Wed, 23 Aug 2017 09:14:52 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Chen Yu <yu.c.chen@...el.com>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Vlastimil Babka <vbabka@...e.cz>, Len Brown <lenb@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH][v2] PM / Hibernate: Feed the wathdog when creating
 snapshot

On Wed 23-08-17 11:44:39, Chen Yu wrote:
> On Tue, Aug 22, 2017 at 02:55:39PM +0200, Rafael J. Wysocki wrote:
> > On Tuesday, August 22, 2017 5:20:02 AM CEST Chen Yu wrote:
[...]
> > >  void mark_free_pages(struct zone *zone)
> > >  {
> > > -	unsigned long pfn, max_zone_pfn;
> > > +	unsigned long pfn, max_zone_pfn, page_num = 0;
> > 
> > +	unsigned long pfn, max_zone_pfn, page_count = WD_PAGE_COUNT;
> > 
> > >  	unsigned long flags;
> > >  	unsigned int order, t;
> > >  	struct page *page;
> > > @@ -2552,6 +2559,9 @@ void mark_free_pages(struct zone *zone)
> > >  		if (pfn_valid(pfn)) {
> > >  			page = pfn_to_page(pfn);
> > >  
> > > +			if (!((page_num++) % WD_INTERVAL_PAGE))
> > > +				touch_nmi_watchdog();
> > > +
> > 
> > ->
> > 
> > 	if (!--page_count) {
> > 		touch_nmi_watchdog();
> > 		page_count = WD_PAGE_COUNT;
> > 	}
> >
> I guess this is to avoid the possible overflow if the page number is too large?

Even if the page_count overflown it wouldn't be a problem because of the
unsigned arithmetic. To be honest I find yours modulo based approach
easier to follow. Maybe it even compiles to a better code but I haven't
checked. Anyway one way or the other both ways are reasonable so
whatever Rafael (as the maintainer) prefers.

-- 
Michal Hocko
SUSE Labs