linux-kernel - Re: [External] Re: [PATCH v5 21/21] mm/hugetlb: Disable freeing vmemmap if struct page size is not power of two

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Message-ID: <CAMZfGtW9drQ7OBhf0wMG4joVz=5UAN1d8P=GQxs2M4MjKoBwxw@mail.gmail.com>
Date:   Mon, 23 Nov 2020 11:14:59 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Michal Hocko <mhocko@...e.com>, Jonathan Corbet <corbet@....net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        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>,
        Oscar Salvador <osalvador@...e.de>,
        "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.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 v5 21/21] mm/hugetlb: Disable freeing
 vmemmap if struct page size is not power of two

On Mon, Nov 23, 2020 at 3:00 AM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Fri, Nov 20, 2020 at 09:25:52AM +0100, Michal Hocko wrote:
> > On Fri 20-11-20 14:43:25, Muchun Song wrote:
> > > We only can free the unused vmemmap to the buddy system when the
> > > size of struct page is a power of two.
> >
> > Can we actually have !power_of_2 struct pages?
>
> Yes.  On x86-64, if you don't enable MEMCG, it's 56 bytes.  On SPARC64,
> if you do enable MEMCG, it's 72 bytes.  On 32-bit systems, it's
> anything from 32-44 bytes, depending on MEMCG, WANT_PAGE_VIRTUAL and
> LAST_CPUPID_NOT_IN_PAGE_FLAGS.
>

On x86-64, even if you do not enable MEMCG, it's also 64 bytes. Because
CONFIG_HAVE_ALIGNED_STRUCT_PAGE is defined if we use SLUB.



-- 
Yours,
Muchun