Re: [PATCH v2 1/3] mm: hugetlb: disable freeing vmemmap pages when struct page crosses page boundaries

[Muchun Song <songmuchun@bytedance.com>]

On Thu, Mar 3, 2022 at 5:21 AM Luis Chamberlain <mcgrof@kernel.org> wrote:
>
> On Wed, Mar 02, 2022 at 04:37:56PM +0800, Muchun Song wrote:
> > If CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON is enabled and the size
> > of "struct page" is not power of two, we cannot optimize vmemmap pages
> > of HugeTLB pages. We should disable this feature in this case.
>
> The commit log does not describe what happens if this is left enabled in
> that case? Is this a fix? Why would it be a fix? Was something failing?
> How did you spot this issue? What are the consequences of not applying
> this patch?
>

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).  But this only
exists when !CONFIG_MEMCG && CONFIG_SLAB on x86_64.
However, it is not a conventional configuration nowadays.  So it is
not a real word issue, just the result of a code review.  But we cannot
prevent someone from configuring that combined configure.  OK,
this information should go to the commit log. Will update it.

Thanks.