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 1:03 AM Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
>
> 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.

Here is a discussion [1] from a few months ago.

[1] https://lore.kernel.org/all/CAMZfGtWfz8DcwKBLdf3j0x9Dt6ZvOd+MvjX6yXrAoKDeXxW95w@xxxxxxxxxxxxxx/