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

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Message-ID: <20180830095747.491f7770@roar.ozlabs.ibm.com>
Date:   Thu, 30 Aug 2018 09:57:47 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
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 Wed, 29 Aug 2018 16:15:37 -0700
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Wed, Aug 29, 2018 at 4:12 PM Nicholas Piggin <npiggin@...il.com> wrote:
> >
> > Dirty micro fault seems to be the big one for my Skylake, takes 300
> > nanoseconds per access. Accessed takes about 100. (I think, have to
> > go over my benchmark a bit more carefully and re-test).  
> 
> Yeah, but they only happen for shared areas after fork, which sounds
> like it shouldn't be a big deal in most cases.

You might be right there.

> 
> And I'm not entirely objecting to your patch per se, I just would want
> to keep the accessed bit changes separate from the dirty bit ones.
> 
> *If* somebody has bisectable issues with it (performance or not), it
> will then be clearer what the exact issue is.

Yeah that makes a lot of sense. I'll do a bit more testing and send
Andrew a respin at least with those split (and a good comment for
the dirty bit vs unmap handling that you pointed out).

Thanks,
Nick