Re: [PATCH v2 1/2] mm/memcg: rename mem_cgroup_split_huge_fixup to split_page_memcg

[Hugh Dickins]

On Thu, 11 Mar 2021, Singh, Balbir wrote:
> On 9/3/21 7:28 pm, Michal Hocko wrote:
> > On Tue 09-03-21 09:37:29, Balbir Singh wrote:
> >> On 4/3/21 6:40 pm, Zhou Guanghui wrote:
> > [...]
> >>> -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> >>>  /*
> >>> - * Because page_memcg(head) is not set on compound tails, set it now.
> >>> + * Because page_memcg(head) is not set on tails, set it now.
> >>>   */
> >>> -void mem_cgroup_split_huge_fixup(struct page *head)
> >>> +void split_page_memcg(struct page *head, unsigned int nr)
> >>>  {
> >>
> >> Do we need input validation on nr? Can nr be aribtrary or can we enforce
> >>
> >> VM_BUG_ON(!is_power_of_2(nr));
> > 
> > In practice this will be power of 2 but why should we bother to sanitze
> > that? 
> > 
> 
> Just when DEBUG_VM is enabled to ensure the contract is valid, given that
> nr is now variable, we could end up with subtle bugs unless we can audit
> all callers. Even the power of 2 check does not catch the fact that nr
> is indeed what we expect, but it still checks a large range of invalid
> inputs.

I think you imagine this is something it's not.

"all callers" are __split_huge_page() and split_page() (maybe Matthew
will have a third caller, maybe not).  It is not something drivers will
be calling directly themselves, and it won't ever get EXPORTed to them.

Hugh