linux-kernel - Re: [patch 2/2] mm: pgtable -- Require X86

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Message-ID: <20140506170331.GQ28248@moon>
Date:	Tue, 6 May 2014 21:03:31 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	torvalds@...ux-foundation.org, mgorman@...e.de, mingo@...nel.org,
	steven@...inklabs.net, riel@...hat.com, david.vrabel@...rix.com,
	akpm@...ux-foundation.org, peterz@...radead.org,
	xemul@...allels.com
Subject: Re: [patch 2/2] mm: pgtable -- Require X86_64 for soft-dirty tracker

On Tue, May 06, 2014 at 09:28:07AM -0700, H. Peter Anvin wrote:
> On 04/25/2014 01:10 AM, Cyrill Gorcunov wrote:
> > Tracking dirty status on 2 level pages requires very ugly macros
> > and taking into account how old the machines who can operate
> > without PAE mode only are, lets drop soft dirty tracker from
> > them for code simplicity (note I can't drop all the macros
> > from 2 level pages by now since _PAGE_BIT_PROTNONE and
> > _PAGE_BIT_FILE are still used even without tracker).
> > 
> > Linus proposed to completely rip off softdirty support on
> > x86-32 (even with PAE) and since for CRIU we're not planning
> > to support native x86-32 mode, lets do that.
> > 
> > (Softdirty tracker is relatively new feature which mostly used
> >  by CRIU so I don't expect if such API change would cause problems
> >  on userspace).
> 
> I have to wonder which one is more likely to actually matter on whatever
> legacy 32-bit are going to remain.  This pretty much comes down to what
> kind of advanced features are going to matter in deep embedded
> applications in the future: checkpoint/restart or NUMA.  My guess is
> that it is actually checkpoint/restart...
> 
> How much does it actually simplify to leave this feature in for PAE?  I
> could care less about non-PAE... NX has pretty much killed that off cold.

At the current state -- not much I would say. Initially the idea was to
drop x86-32 and use page-soft-dirty-bit (ie 11) inside swap entries dropping
off page-swap-soft-dirty bit completely, this would simplify all the things
but eventually I realized that if I do so the number of maximum swap entries
will get more shrinked which is inacceptable I think.

Thus, currently (ie even with this patches) we can work on x86-32+PAE but
desided to drop x86-32 completely to simplify things in future.

Peter, deep embedded applications I guess would be working on systems
with really small amount of memory installed in a system I suppose and
I doubt if they would need c/r?

If we deside to leave x86-32+PAE then don't apply this patch, I will
need to update it, (The first patch is safe to apply anyway).
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