linux-kernel - Re: [External] Re: [PATCH v9 02/11] mm/hugetlb: Introduce a new config HUGETLB_PAGE_FREE

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Message-ID: <CAMZfGtU9c3DmZzrGxh2oo-GMjamP5VFLerZf_FCCx7A8KO6NQA@mail.gmail.com>
Date:   Wed, 16 Dec 2020 11:52:50 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>, mingo@...hat.com,
        bp@...en8.de, x86@...nel.org, hpa@...or.com,
        dave.hansen@...ux.intel.com, luto@...nel.org,
        Peter Zijlstra <peterz@...radead.org>, viro@...iv.linux.org.uk,
        Andrew Morton <akpm@...ux-foundation.org>, paulmck@...nel.org,
        mchehab+huawei@...nel.org, pawan.kumar.gupta@...ux.intel.com,
        Randy Dunlap <rdunlap@...radead.org>, oneukum@...e.com,
        anshuman.khandual@....com, jroedel@...e.de,
        Mina Almasry <almasrymina@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        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>,
        Xiongchun duan <duanxiongchun@...edance.com>,
        linux-doc@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [External] Re: [PATCH v9 02/11] mm/hugetlb: Introduce a new
 config HUGETLB_PAGE_FREE_VMEMMAP

On Wed, Dec 16, 2020 at 11:45 AM Mike Kravetz <mike.kravetz@...cle.com> wrote:
>
> On 12/15/20 5:03 PM, Mike Kravetz wrote:
> > On 12/13/20 7:45 AM, Muchun Song wrote:
> >> diff --git a/fs/Kconfig b/fs/Kconfig
> >> index 976e8b9033c4..4c3a9c614983 100644
> >> --- a/fs/Kconfig
> >> +++ b/fs/Kconfig
> >> @@ -245,6 +245,21 @@ config HUGETLBFS
> >>  config HUGETLB_PAGE
> >>      def_bool HUGETLBFS
> >>
> >> +config HUGETLB_PAGE_FREE_VMEMMAP
> >> +    def_bool HUGETLB_PAGE
> >> +    depends on X86_64
> >> +    depends on SPARSEMEM_VMEMMAP
> >> +    depends on HAVE_BOOTMEM_INFO_NODE
> >> +    help
> >> +      When using HUGETLB_PAGE_FREE_VMEMMAP, the system can save up some
> >> +      memory from pre-allocated HugeTLB pages when they are not used.
> >> +      6 pages per HugeTLB page of the pmd level mapping and (PAGE_SIZE - 2)
> >> +      pages per HugeTLB page of the pud level mapping.
> >> +
> >> +      When the pages are going to be used or freed up, the vmemmap array
> >> +      representing that range needs to be remapped again and the pages
> >> +      we discarded earlier need to be rellocated again.
> >
> > I see the previous discussion with David about wording here.  How about
> > leaving the functionality description general, and provide a specific
> > example for x86_64?  As mentioned we can always update when new arch support
> > is added.  Suggested text?
> >
> >       The option HUGETLB_PAGE_FREE_VMEMMAP allows for the freeing of
> >       some vmemmap pages associated with pre-allocated HugeTLB pages.
> >       For example, on X86_64 6 vmemmap pages of size 4KB each can be
> >       saved for each 2MB HugeTLB page.  4094 vmemmap pages of size 4KB
> >       each can be saved for each 1GB HugeTLB page.
> >
> >       When a HugeTLB page is allocated or freed, the vmemmap array
> >       representing the range associated with the page will need to be
> >       remapped.  When a page is allocated, vmemmap pages are freed
> >       after remapping.  When a page is freed, previously discarded
> >       vmemmap pages must be allocated before before remapping.
>
> Sorry, I am slowly coming up to speed with discussions when I was away.
>
> It appears vmemmap is not being mapped with huge pages if the boot option
> hugetlb_free_vmemmap is on.   Is that correct?

Right.

>
> If that is correct, we should document the trade off of increased page
> table pages needed to map vmemmap vs the savings from freeing struct page
> pages.  If a user/sysadmin only uses a small number of hugetlb pages (as
> a percentage of system memory) they could end up using more memory with
> hugetlb_free_vmemmap on as opposed to off.  Perhaps, it should be part of
> the documentation for hugetlb_free_vmemmap?  If this is true, and people

Right, it is better to document it around hugetlb_free_vmemmap.
This should be a part of pathe #8. Thanks.


> think this should be documented, I can try to come up with something.
>
> --
> Mike Kravetz



-- 
Yours,
Muchun