From: "Rafael J. Wysocki" <rjw@sisk.pl>
To: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Subject: Re: [linux-pm] [RFC][PATCH] PM: Force GFP_NOIO during suspend/resume (was: Re: Memory allocations in .suspend became very unreliable)
Date: Mon, 18 Jan 2010 22:06:36 +0100
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Cc: Minchan Kim <minchan.kim@gmail.com>,
       Maxim Levitsky <maximlevitsky@gmail.com>,
       "linux-mm" <linux-mm@kvack.org>, linux-pm@lists.linux-foundation.org,
       LKML <linux-kernel@vger.kernel.org>,
       Andrew Morton <akpm@linux-foundation.org>
References: <1263745267.2162.42.camel@barrios-desktop> <201001180125.59413.rjw@sisk.pl> <20100118111703.AE36.A69D9226@jp.fujitsu.com>
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On Monday 18 January 2010, KOSAKI Motohiro wrote:
> > Index: linux-2.6/mm/page_alloc.c
> > ===================================================================
> > --- linux-2.6.orig/mm/page_alloc.c
> > +++ linux-2.6/mm/page_alloc.c
> > @@ -1963,10 +1963,13 @@ __alloc_pages_nodemask(gfp_t gfp_mask, u
> >  	page = get_page_from_freelist(gfp_mask|__GFP_HARDWALL, nodemask, order,
> >  			zonelist, high_zoneidx, ALLOC_WMARK_LOW|ALLOC_CPUSET,
> >  			preferred_zone, migratetype);
> > -	if (unlikely(!page))
> > +	if (unlikely(!page)) {
> > +		mm_lock_suspend(gfp_mask);
> >  		page = __alloc_pages_slowpath(gfp_mask, order,
> >  				zonelist, high_zoneidx, nodemask,
> >  				preferred_zone, migratetype);
> > +		mm_unlock_suspend(gfp_mask);
> > +	}
> >  
> >  	trace_mm_page_alloc(page, order, gfp_mask, migratetype);
> >  	return page;
> 
> I think we don't need read side lock at all. generally, no lock might makes race.
> But in this case, changing gfp_allowed_mask and nvidia suspend method should be
> serialized higher level. Why the above two code need to run concurrently?

The changing of gfp_allowed_mask is serialized with the suspend of devices,
so there's no concurrency here.

I was concerned about another problem, though, which is what happens if the
suspend process runs in parallel with a memory allocation that started earlier
and happens to do some I/O.  I that case the suspend process doesn't know
about the I/O done by the mm subsystem and may disturb it in principle.

That said, perhaps that should be a concern for the block devices subsystem to
prevent such situations from happening.

So, perhaps I'll remove the reader-side lock altogether and go back to
something like the first version of the patch.

Rafael
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