linux-kernel - Re: [PATCH 0/2] change_protection(): Count the number of pages affected

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Message-ID: <CA+55aFz_JnoR73O46YWhZn2A4t_CSUkGzMMprCUpvR79TVMCEQ@mail.gmail.com>
Date:	Wed, 14 Nov 2012 10:01:08 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>, Paul Turner <pjt@...gle.com>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Christoph Lameter <cl@...ux.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH 0/2] change_protection(): Count the number of pages affected

On Wed, Nov 14, 2012 at 12:50 AM, Ingo Molnar <mingo@...nel.org> wrote:
> What do you guys think about this mprotect() optimization?

Hmm..

If this is mainly about just avoiding the TLB flushing, I do wonder if
it might not be more interesting to try to be much more aggressive.

As noted elsewhere, we should just notice when vm_page_prot doesn't
change at all - even if 'flags' change, it is possible that the actual
low-level page protection bits do not (due to the X=R issue).

But even *more* aggressively, how about looking at

 - not flushing the TLB at all if the bits become  more permissive
(taking the TLB micro-fault and letting the CPU just update it on its
own)

 - even *more* aggressive: if the bits become strictly more
restrictive, how about not flushing the TLB at all, *and* not even
changing the page tables, and just teaching the page fault code to do
it lazily at fault time?

Now, the "change protections lazily" might actually be a huge
performance problem with the page fault overhead dwarfing any TLB
flush costs, but we don't really know, do we? It might be worth trying
out.

               Linus
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