linux-kernel - Re: 32-bit PTI with THP = userspace corruption

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Message-ID: <20181021123745.GA26042@amd>
Date:   Sun, 21 Oct 2018 14:37:45 +0200
From:   Pavel Machek <pavel@....cz>
To:     Alan Cox <gnomes@...rguk.ukuu.org.uk>
Cc:     Joerg Roedel <jroedel@...e.de>, Meelis Roos <mroos@...ux.ee>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux Kernel list <linux-kernel@...r.kernel.org>,
        linux-mm@...ck.org, Andrea Arcangeli <aarcange@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: 32-bit PTI with THP = userspace corruption

On Tue 2018-09-18 14:00:30, Alan Cox wrote:
> On Tue, 11 Sep 2018 14:12:22 +0200
> Joerg Roedel <jroedel@...e.de> wrote:
> 
> > On Tue, Sep 11, 2018 at 02:58:10PM +0300, Meelis Roos wrote:
> > > The machines where I have PAE off are the ones that have less memory. 
> > > PAE is off just for performance reasons, not lack of PAE. PAE should be 
> > > present on all of my affected machines anyway and current distributions 
> > > seem to mostly assume 686 and PAE anyway for 32-bit systems.  
> > 
> > Right, most distributions don't even provide a non-PAE kernel for their
> > users anymore.
> > 
> > How big is the performance impact of using PAE over legacy paging?
> 
> On what system. In the days of the original 36bit PAE Xeons it was around
> 10% when we measured it at Red Hat, but that was long ago and as you go
> newer it really ought to be vanishingly small.
> 
> There are pretty much no machines that don't support PAE and are still
> even vaguely able to boot a modern Linux kernel. The oddity is the
> Pentium-M but most distros shipped a hack to use PAE on the Pentium M
> anyway as it seems to work fine.

I do have some AMD Geode here, in form of subnotebook. Definitely
newer then Pentium Ms, but no PAE...

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

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