Re: [PATCH] memcg: page_cgroup_ino() get memcg from compound_head(page)

[Michal Hocko <mhocko@suse.com>]

On Mon 13-03-23 14:08:53, Yosry Ahmed wrote:
> On Mon, Mar 13, 2023 at 12:44 PM Andrew Morton
> <akpm@linux-foundation.org> wrote:
> >
> > On Mon, 13 Mar 2023 08:34:52 +0000 Yosry Ahmed <yosryahmed@google.com> wrote:
> >
> > > From: Hugh Dickins <hughd@google.com>
> > >
> > > In a kernel with added WARN_ON_ONCE(PageTail) in page_memcg_check(), we
> > > observed a warning from page_cgroup_ino() when reading
> > > /proc/kpagecgroup.
> >
> > If this is the only known situation in which page_memcg_check() is
> > passed a tail page, why does page_memcg_check() have
> >
> >         if (PageTail(page))
> >                 return NULL;
> >
> > ?  Can we remove this to simplify, streamline and clarify?
> 
> I guess it's a safety check so that we don't end up trying to cast a
> tail page to a folio. My opinion is to go one step further and change
> page_memcg_check() to do return the memcg of the head page, i.e:
> 
> static inline struct mem_cgroup *page_memcg_check(struct page *page)
> {
>     return folio_memcg_check(page_folio(page));
> }

I would just stick with the existing code and put a comment that this
function shouldn't be used in any new code and the folio counterpart
should be used instead.

-- 
Michal Hocko
SUSE Labs