linux-kernel - Re: [PATCH v3 1/4] mm: hugetlb: disable freeing vmemmap pages when struct page crosses page boundaries

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Message-ID: <YipuORuIdxq89Lqg@bombadil.infradead.org>
Date:   Thu, 10 Mar 2022 13:31:37 -0800
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     Jonathan Corbet <corbet@....net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
        Oscar Salvador <osalvador@...e.de>,
        David Hildenbrand <david@...hat.com>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Xiongchun duan <duanxiongchun@...edance.com>,
        Muchun Song <smuchun@...il.com>
Subject: Re: [PATCH v3 1/4] mm: hugetlb: disable freeing vmemmap pages when
 struct page crosses page boundaries

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@...nel.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)

  Luis