Date: Thu, 12 Nov 2009 13:33:39 +0100
From: Pavel Machek <pavel@ucw.cz>
To: "Rafael J. Wysocki" <rjw@sisk.pl>
Cc: Nigel Cunningham <ncunningham@crca.org.au>,
       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>
Subject: using highmem for atomic copy of lowmem was Re: [PATCHv2 2/5]
	vmscan: Kill hibernation specific reclaim logic and unify it
Message-ID: <20091112123339.GA1546@ucw.cz>
References: <20091102000855.F404.A69D9226@jp.fujitsu.com> <200911031230.20344.rjw@sisk.pl> <4AF09CB2.9030500@crca.org.au> <200911032300.14790.rjw@sisk.pl>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <200911032300.14790.rjw@sisk.pl>
User-Agent: Mutt/1.5.18 (2008-05-17)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1985
Lines: 48

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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/