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

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Message-ID: <CA+55aFzG=B3t_YaoCY_H1jmEgs+cYd--ZHz7XhGeforMRvNfEQ@mail.gmail.com>
Date:	Tue, 1 Apr 2014 08:13:31 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Rik van Riel <riel@...hat.com>
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 Mon, Mar 31, 2014 at 8:34 AM, Rik van Riel <riel@...hat.com> wrote:
>
> However, clearing the accessed bit does not lead to any
> consistency issues, there is no reason to flush the TLB
> immediately. The TLB flush can be deferred until some
> later point in time.

Ugh. I absolutely detest this patch.

If we're going to leave the TLB dirty, then dammit, leave it dirty.
Don't play some half-way games.

Here's the patch you should just try:

 int ptep_clear_flush_young(struct vm_area_struct *vma,
        unsigned long address, pte_t *ptep)
 {
     return ptep_test_and_clear_young(vma, address, ptep);
 }

instead of complicating things.

Rationale: if the working set is so big that we start paging things
out, we sure as hell don't need to worry about TLB flushing. It will
flush itself.

And conversely - if it doesn't flush itself, and something stays
marked as "accessed" in the TLB for a long time even though we've
cleared it in the page tables, we don't care, because clearly there
isn't enough memory pressure for the accessed bit to matter.

                  Linus
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