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

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Message-ID: <20180911121128.ikwptix6e4slvpt2@suse.de>
Date:   Tue, 11 Sep 2018 14:12:22 +0200
From:   Joerg Roedel <jroedel@...e.de>
To:     Meelis Roos <mroos@...ux.ee>
Cc:     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, 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? It
shouldn't be too big because the top-level of the page-table only has 4
entries and is completly cached in the CPU. This makes %cr3 switches
slower, but the page-walk itself still only needs 2 memory accesses.

The page-table entries are also 8 bytes instead of 4 bytes, so that
there is less locality in page-walks and probably a higher cache-miss
rate.

Regards,

	Joerg