linux-kernel - Re: [External] Re: [PATCH v5 00/21] Free some vmemmap pages of hugetlb page

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Message-ID: <20201123124513.GI4327@casper.infradead.org>
Date:   Mon, 23 Nov 2020 12:45:13 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Muchun Song <songmuchun@...edance.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 00/21] Free some vmemmap pages of
 hugetlb page

On Mon, Nov 23, 2020 at 11:42:58AM +0100, Michal Hocko wrote:
> On Mon 23-11-20 18:36:33, Muchun Song wrote:
> > > No I really mean that pfn_to_page will give you a struct page pointer
> > > from pages which you release from the vmemmap page tables. Those pages
> > > might get reused as soon sa they are freed to the page allocator.
> > 
> > We will remap vmemmap pages 2-7 (virtual addresses) to page
> > frame 1. And then we free page frame 2-7 to the buddy allocator.
> 
> And this doesn't really happen in an atomic fashion from the pfn walker
> POV, right? So it is very well possible that 
> 
> struct page *page = pfn_to_page();
> // remapping happens here
> // page content is no longer valid because its backing memory can be
> // reused for whatever purpose.

pfn_to_page() returns you a virtual address.  That virtual address
remains a valid pointer to exactly the same contents, it's just that
the page tables change to point to a different struct page which has
the same compound_head().