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

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Date:	Tue, 01 Apr 2014 14:31:31 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>, shli@...nel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Hugh Dickins <hughd@...gle.com>, Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH] x86,mm: delay TLB flush after clearing accessed bit

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?

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