linux-kernel - Re: [PATCH] x86,mm: delay TLB flush after clearing accessed bit

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Message-ID: <20140402074651.GA22772@gmail.com>
Date:	Wed, 2 Apr 2014 09:46:51 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Shaohua Li <shli@...nel.org>
Cc:	Rik van Riel <riel@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>, Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH] x86,mm: delay TLB flush after clearing accessed bit


* Shaohua Li <shli@...nel.org> wrote:

> On Tue, Apr 01, 2014 at 02:31:31PM -0400, Rik van Riel wrote:
> > On 04/01/2014 12:21 PM, Linus Torvalds wrote:
> > > On Tue, Apr 1, 2014 at 9:11 AM, Rik van Riel <riel@...hat.com> wrote:
> > >>
> > >> Memory pressure is not necessarily caused by the same process
> > >> whose accessed bit we just cleared. Memory pressure may not
> > >> even be caused by any process's virtual memory at all, but it
> > >> could be caused by the page cache.
> > > 
> > > If we have that much memory pressure on the page cache without having
> > > any memory pressure on the actual VM space, then the swap-out activity
> > > will never be an issue anyway.
> > > 
> > > IOW, I think all these scenarios are made-up. I'd much rather go for
> > > simpler implementation, and make things more complex only in the
> > > presence of numbers. Of which we have none.
> > 
> > We've been bitten by the lack of a properly tracked accessed
> > bit before, but admittedly that was with the KVM code and EPT.
> > 
> > I'll add my Acked-by: to Shaohua's original patch then, and
> > will keep my eyes open for any problems that may or may not
> > materialize...
> > 
> > Shaohua?
> 
> I'd agree to choose the simple implementation at current stage and check if
> there are problems really.
> 
> Andrew,
> can you please pick up my orginal patch "x86: clearing access bit don't
> flush tlb" (with Rik's Ack)? Or I can resend it if you preferred.

Please resend it so I can pick it up for this cycle, that approach 
obviously looks good.

Thanks,

	Ingo
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