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References: <20220307130708.58771-1-songmuchun@bytedance.com>
 <20220307130708.58771-2-songmuchun@bytedance.com> <YiY0QkE+efo1hyda@bombadil.infradead.org>
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From:   Muchun Song <songmuchun@bytedance.com>
Date:   Tue, 8 Mar 2022 01:03:08 +0800
Message-ID: <CAMZfGtWHZgvFSE0P1LNprcjt0YAvC5XFAz=aUZw9vJFPGzUTbw@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 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.

Thanks.