linux-kernel - Re: [PATCH RESEND v2 3/4] mm: sparsemem: use page table lock to protect kernel pmd operations

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Message-ID: <CAMZfGtW1_U2d=zQFCYBEDfKWB=EkoYTGgVcCHauX-iv4VD7U8Q@mail.gmail.com>
Date:   Sat, 18 Sep 2021 18:51:07 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Barry Song <21cnbao@...il.com>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...e.de>,
        Michal Hocko <mhocko@...e.com>,
        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>,
        Matthew Wilcox <willy@...radead.org>,
        Xiongchun duan <duanxiongchun@...edance.com>,
        fam.zheng@...edance.com, Muchun Song <smuchun@...il.com>,
        Qi Zheng <zhengqi.arch@...edance.com>,
        linux-doc@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH RESEND v2 3/4] mm: sparsemem: use page table lock to
 protect kernel pmd operations

On Sat, Sep 18, 2021 at 1:07 PM Barry Song <21cnbao@...il.com> wrote:
>
> On Sat, Sep 18, 2021 at 12:09 AM Muchun Song <songmuchun@...edance.com> wrote:
> >
> > The init_mm.page_table_lock is used to protect kernel page tables, we
> > can use it to serialize splitting vmemmap PMD mappings instead of mmap
> > write lock, which can increase the concurrency of vmemmap_remap_free().
> >
>
> Curious what is the actual benefit we get in user scenarios from this patch,
> 1. we set bootargs to reserve hugetlb statically
> 2. we "echo" some figures to sys or proc.
>
> In other words, Who is going to care about this concurrency?

Actually, It increase the concurrency between allocations of
HugeTLB pages. But it is not my first consideration. There are
a lot of users of mmap read lock of init_mm. The mmap write
lock is holding through vmemmap_remap_free(), I want to make
it does not affect other users of mmap read lock.

I suppose a lot of developers are trying to avoid using mmap write
lock. I am also one of them.

> Can we have some details on this to put in the commit log?

For sure. Those judgments above should be placed in the
commit log.

Thanks.