linux-kernel - using highmem for atomic copy of lowmem was Re: [PATCHv2 2/5] vmscan: Kill hibernation specific reclaim logic and unify it

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Message-ID: <20091112123339.GA1546@ucw.cz>
Date:	Thu, 12 Nov 2009 13:33:39 +0100
From:	Pavel Machek <pavel@....cz>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Nigel Cunningham <ncunningham@...a.org.au>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Rik van Riel <riel@...hat.com>,
	linux-mm <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: using highmem for atomic copy of lowmem was Re: [PATCHv2 2/5]
	vmscan: Kill hibernation specific reclaim logic and unify it

Hi!

> > >> (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.

Yes, it does, but I wonder if it can ever work...?

copy_data_page() takes great care not to modify any memory -- like
using handmade loop instead of memcpy() -- yet it uses kmap_atomic()
and friends.

If kmap_atomic()+kunmap_atomic() pair is guaranteed not to change any
memory, thats probably safe, but...

(Actually, plain memcpy() should be usable for copying highmem pages;
those should not contain task_structs or anything else touched by
memcpy()).
								Pavel 

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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