linux-kernel - Re: [PATCH 2/3] mm/cow: optimise pte dirty/accessed bits handling in fork

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Date:   Wed, 29 Aug 2018 08:42:09 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Nick Piggin <npiggin@...il.com>
Cc:     linux-mm <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        ppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 2/3] mm/cow: optimise pte dirty/accessed bits handling in fork

On Tue, Aug 28, 2018 at 4:20 AM Nicholas Piggin <npiggin@...il.com> wrote:
>
> fork clears dirty/accessed bits from new ptes in the child. This logic
> has existed since mapped page reclaim was done by scanning ptes when
> it may have been quite important. Today with physical based pte
> scanning, there is less reason to clear these bits.

Can you humor me, and make the dirty/accessed bit patches separate?

There is actually a difference wrt the dirty bit: if we unmap an area
with dirty pages, we have to do the special synchronous flush.

So a clean page in the virtual mapping is _literally_ cheaper to have.

> This eliminates a major source of faults powerpc/radix requires to set
> dirty/accessed bits in ptes, speeding up a fork/exit microbenchmark by
> about 5% on POWER9 (16600 -> 17500 fork/execs per second).

I don't think the dirty bit matters.

The accessed bit I think may be worth keeping, so by all means remove the mkold.

                  Linus