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From:   Muchun Song <songmuchun@bytedance.com>
Date:   Fri, 11 Mar 2022 15:22:29 +0800
Message-ID: <CAMZfGtUybuDSnwQKos7A9zB=WQc_avrFK0eZo3bgeD7tnWzf3Q@mail.gmail.com>
Subject: Re: [PATCH v3 1/4] mm: hugetlb: disable freeing vmemmap pages when
 struct page crosses page boundaries
To:     Luis Chamberlain <mcgrof@kernel.org>
Cc:     Jonathan Corbet <corbet@lwn.net>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>,
        Iurii Zaikin <yzaikin@google.com>,
        Oscar Salvador <osalvador@suse.de>,
        David Hildenbrand <david@redhat.com>,
        Linux Doc Mailing List <linux-doc@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        Xiongchun duan <duanxiongchun@bytedance.com>,
        Muchun Song <smuchun@gmail.com>
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On Fri, Mar 11, 2022 at 5:31 AM Luis Chamberlain <mcgrof@kernel.org> wrote:
>
> On Tue, Mar 08, 2022 at 01:03:08AM +0800, Muchun Song wrote:
> > On Tue, Mar 8, 2022 at 12:35 AM Luis Chamberlain <mcgrof@kernel.org> wrote:
> > >
> > > On Mon, Mar 07, 2022 at 09:07:05PM +0800, Muchun Song wrote:
> > > > If the size of "struct page" is not the power of two and this
> > > > feature is enabled, then the vmemmap pages of HugeTLB will be
> > > > corrupted after remapping (panic is about to happen in theory).
> > >
> > > Huh what? If a panic is possible best we prevent this in kconfig
> > > all together. I'd instead just put some work into this instead of
> > > adding all this run time hacks.
> >
> > If the size of `struct page` is not power of 2, then those lines added
> > by this patch will be optimized away by the compiler, therefore there
> > is going to be no extra overhead to detect this.
> >
> > >
> > > Can you try to add kconfig magic to detect if a PAGE_SIZE is PO2?
> > >
> >
> > I agree with you that it is better if we can move this check
> > into Kconfig. I tried this a few months ago. It is not easy to
> > do this. How to check if a `struct page size` is PO2 in
> > Kconfig? If you have any thoughts please let me know.
>
> Can you query this with a script?
>
> config HAS_PAGE_SIZE_PO2
>         bool
>         default $(shell, scripts/check_po2_page_size.sh arguments_are_allowed)
>

Excellent. I'll try this approach.

Thanks very much.