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

[Muchun Song]

On Tue, Mar 8, 2022 at 12:35 AM Luis Chamberlain <mcgrof@xxxxxxxxxx> 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.