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From:   Joel Fernandes <joel@joelfernandes.org>
Date:   Thu, 16 Jul 2020 14:47:35 -0400
Message-ID: <CAEXW_YRWDwUVrvedLtoMssFcrwAnWHAQ0CWNbR4UF=GS18B3wA@mail.gmail.com>
Subject: Re: [PATCHv2] mm: Fix warning in move_normal_pmd()
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     "Kirill A. Shutemov" <kirill@shutemov.name>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux-MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Naresh Kamboju <naresh.kamboju@linaro.org>,
        William Kucharski <william.kucharski@oracle.com>
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On Thu, Jul 16, 2020 at 1:55 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Thu, Jul 16, 2020 at 6:16 AM Kirill A. Shutemov <kirill@shutemov.name> wrote:
> >
> > It can also lead to performance regression: for small mremap() if only one
> > side of the range got aligned and there's no PMD_SIZE range to move,
> > kernel will still iterate over PTEs, but it would need to handle more
> > pte_none()s than without the patch.
>
> Ack, I've dropped the patch from my queue of experiments, because it
> doesn't work for the case I wanted to do, and the other cases could
> regress, as you say.
>
> Plus considering how many problems that patch had, I decided it wasn't
> as simple as I initially thought it would be anyway ;)
>
> Joel - while it's gone from my mind, if you're still interested in
> this, maybe you can do something _similar_ that patch, except perhaps
> also start out checking that the initial size is large enough for this
> to make sense even when one of the sides doesn't align, for example.
>
> (It might be as simple as checking that the initial 'len' is at least
> PMD_SIZE - then you're guaranteed that whichever side gets aligned,
> it's not doing extra work because the other side didn't).

Hi Linus,
Yes I'm quite interested in doing a similar patch and also adding some
test cases to make sure it always works properly. Probably I can add
some kselftest cases doing mremap on various ranges.

I think it will be a great optimization to trigger the PMD move more
often. I'll work on it and get back hopefully soon on this, Thanks!

 - Joel