linux-kernel - Re: [External] Re: [PATCH v2 0/3] Split huge PMD mapping of vmemmap pages

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Date:   Tue, 15 Jun 2021 13:37:15 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Oscar Salvador <osalvador@...e.de>, Michal Hocko <mhocko@...e.com>,
        "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>,
        David Hildenbrand <david@...hat.com>,
        Chen Huang <chenhuang5@...wei.com>,
        "Bodeddula, Balasubramaniam" <bodeddub@...zon.com>,
        Jonathan Corbet <corbet@....net>,
        Xiongchun duan <duanxiongchun@...edance.com>,
        fam.zheng@...edance.com, linux-doc@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: [External] Re: [PATCH v2 0/3] Split huge PMD mapping of vmemmap pages

On Tue, Jun 15, 2021 at 11:52 AM Mike Kravetz <mike.kravetz@...cle.com> wrote:
>
> On 6/14/21 6:12 PM, Andrew Morton wrote:
> > On Sat, 12 Jun 2021 17:45:52 +0800 Muchun Song <songmuchun@...edance.com> wrote:
> >
> >> In order to reduce the difficulty of code review in series[1]. We disable
> >> huge PMD mapping of vmemmap pages when that feature is enabled. In this
> >> series, we do not disable huge PMD mapping of vmemmap pages anymore. We
> >> will split huge PMD mapping when needed. When HugeTLB pages are freed from
> >> the pool we do not attempt coalasce and move back to a PMD mapping because
> >> it is much more complex.
> >>
> >> [1] https://lore.kernel.org/linux-doc/20210510030027.56044-1-songmuchun@bytedance.com/
> >
> > [1] had a nice [0/n] description but the v2 series lost that.  I could
> > copy/paste the v1 changelogging but I am unsure that it has been
> > maintained appropriately for the v2 series.
> >
> > I think I'll pass on this v2 pending additional review input.  Please reinstate
> > the [0/n] overview if/when resending?
>
> There may be some confusion.
>
> This series is a follow on optimization for the functionality provided by
> [1].  Early in the development of [1], it was decided to drop some code
> for ease of review.  Specifically, splitting vmemmap PMD mappings to PTE
> mappings as required when hugetlb pages were allocated.  The
> 'simplification' in [1] is that if the feature is enabled then vmemmap
> will only be mapped with PTEs.
>
> This series provides the ability to split PMD mappings 'on demand' as
> hugetlb pages are allocated.  As mentioned, it really is a follow on and
> optimization to functionality provided in [1].  As such, I am not sure
> that repeating the [0/n] description from 1 is necessary here.
>
> In any case, this should be clearly stated in the [0/n] description of
> this series.

Thanks for the clarification for me. I totally agree with you.

>
> BTW- I did get through the series today, and did not discover any
> issues.  However, I want to sleep on it before signing off.
> --
> Mike Kravetz