From: "Rafael J. Wysocki" <rjw@sisk.pl>
To: Nigel Cunningham <ncunningham@crca.org.au>
Subject: Re: [PATCHv2 2/5] vmscan: Kill hibernation specific reclaim logic and unify it
Date: Tue, 3 Nov 2009 23:00:14 +0100
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Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
       LKML <linux-kernel@vger.kernel.org>, Rik van Riel <riel@redhat.com>,
       "linux-mm" <linux-mm@kvack.org>,
       Andrew Morton <akpm@linux-foundation.org>
References: <20091102000855.F404.A69D9226@jp.fujitsu.com> <200911031230.20344.rjw@sisk.pl> <4AF09CB2.9030500@crca.org.au>
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On Tuesday 03 November 2009, Nigel Cunningham wrote:
> Hi Rafael.
> 
> Rafael J. Wysocki wrote:
> > On Monday 02 November 2009, Nigel Cunningham wrote:
> >> Hi.
> > 
> > Hi,
> > 
> >> KOSAKI Motohiro wrote:
> >>>> I haven't given much thought to numa awareness in hibernate code, but I
> >>>> can say that the shrink_all_memory interface is woefully inadequate as
> >>>> far as zone awareness goes. Since lowmem needs to be atomically restored
> >>>> before we can restore highmem, we really need to be able to ask for a
> >>>> particular number of pages of a particular zone type to be freed.
> >>> Honestly, I am not suspend/hibernation expert. Can I ask why caller need to know
> >>> per-zone number of freed pages information? if hibernation don't need highmem.
> >>> following incremental patch prevent highmem reclaim perfectly. Is it enough?
> >> (Disclaimer: I don't think about highmem a lot any more, and might have
> >> forgotten some of the details, or swsusp's algorithms might have
> >> changed. Rafael might need to correct some of this...)
> >>
> >> Imagine that you have a system with 1000 pages of lowmem and 5000 pages
> >> of highmem. Of these, 950 lowmem pages are in use and 500 highmem pages
> >> are in use.
> >>
> >> In order to to be able to save an image, we need to be able to do an
> >> atomic copy of those lowmem pages.
> >>
> >> You might think that we could just copy everything into the spare
> >> highmem pages, but we can't because mapping and unmapping the highmem
> >> pages as we copy the data will leave us with an inconsistent copy.
> > 
> > This isn't the case any more for the mainline hibernate code.  We use highmem
> > for storing image data as well as lowmem.
> 
> Highmem for storing copies of lowmem pages?

It is possible in theory, but I don't think it happens in practice given the
way in which the memory is freed.  Still copy_data_page() takes this
possibility into account.

Thanks,
Rafael
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