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From:   Muchun Song <songmuchun@bytedance.com>
Date:   Sat, 17 Apr 2021 10:55:05 +0800
Message-ID: <CAMZfGtXpQ1WPRHX27tq++3AqCwUGNna7zvVh2toNDTfHkceY9Q@mail.gmail.com>
Subject: Re: [External] Re: [PATCH v20 4/9] mm: hugetlb: free the vmemmap
 pages associated with each HugeTLB page
To:     Mike Kravetz <mike.kravetz@oracle.com>
Cc:     Jonathan Corbet <corbet@lwn.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, bp@alien8.de,
        X86 ML <x86@kernel.org>, hpa@zytor.com,
        dave.hansen@linux.intel.com, luto@kernel.org,
        Peter Zijlstra <peterz@infradead.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>, paulmck@kernel.org,
        pawan.kumar.gupta@linux.intel.com,
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        anshuman.khandual@arm.com, jroedel@suse.de,
        Mina Almasry <almasrymina@google.com>,
        David Rientjes <rientjes@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        Oscar Salvador <osalvador@suse.de>,
        Michal Hocko <mhocko@suse.com>,
        "Song Bao Hua (Barry Song)" <song.bao.hua@hisilicon.com>,
        David Hildenbrand <david@redhat.com>,
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        Chen Huang <chenhuang5@huawei.com>,
        Bodeddula Balasubramaniam <bodeddub@amazon.com>
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On Sat, Apr 17, 2021 at 5:10 AM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 4/15/21 1:40 AM, Muchun Song wrote:
> > Every HugeTLB has more than one struct page structure. We __know__ that
> > we only use the first 4 (__NR_USED_SUBPAGE) struct page structures
> > to store metadata associated with each HugeTLB.
> >
> > There are a lot of struct page structures associated with each HugeTLB
> > page. For tail pages, the value of compound_head is the same. So we can
> > reuse first page of tail page structures. We map the virtual addresses
> > of the remaining pages of tail page structures to the first tail page
> > struct, and then free these page frames. Therefore, we need to reserve
> > two pages as vmemmap areas.
> >
> > When we allocate a HugeTLB page from the buddy, we can free some vmemmap
> > pages associated with each HugeTLB page. It is more appropriate to do it
> > in the prep_new_huge_page().
> >
> > The free_vmemmap_pages_per_hpage(), which indicates how many vmemmap
> > pages associated with a HugeTLB page can be freed, returns zero for
> > now, which means the feature is disabled. We will enable it once all
> > the infrastructure is there.
> >
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > Reviewed-by: Oscar Salvador <osalvador@suse.de>
> > Tested-by: Chen Huang <chenhuang5@huawei.com>
> > Tested-by: Bodeddula Balasubramaniam <bodeddub@amazon.com>
> > Acked-by: Michal Hocko <mhocko@suse.com>
>
> There may need to be some trivial rebasing due to Oscar's changes
> when they go in.

Yeah, thanks for your reminder.

>
> Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
> --
> Mike Kravetz