linux-kernel - Re: [External] Re: [PATCH v5 13/21] mm/hugetlb: Use PG

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Message-ID: <20201123074804.GC27488@dhcp22.suse.cz>
Date:   Mon, 23 Nov 2020 08:48:04 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     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>,
        Matthew Wilcox <willy@...radead.org>,
        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 13/21] mm/hugetlb: Use PG_slab to
 indicate split pmd

On Fri 20-11-20 17:30:27, Muchun Song wrote:
> On Fri, Nov 20, 2020 at 4:16 PM Michal Hocko <mhocko@...e.com> wrote:
> >
> > On Fri 20-11-20 14:43:17, Muchun Song wrote:
> > > When we allocate hugetlb page from buddy, we may need split huge pmd
> > > to pte. When we free the hugetlb page, we can merge pte to pmd. So
> > > we need to distinguish whether the previous pmd has been split. The
> > > page table is not allocated from slab. So we can reuse the PG_slab
> > > to indicate that the pmd has been split.
> >
> > PageSlab is used outside of the slab allocator proper and that code
> > might get confused by this AFAICS.
> 
> I got your concerns. Maybe we can use PG_private instead of the
> PG_slab.

Reusing a page flag arbitrarily is not that easy. Hugetlb pages have a
lot of spare room in struct page so I would rather use something else.
-- 
Michal Hocko
SUSE Labs