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

[Yosry Ahmed <yosryahmed@google.com>]

Somehow I was stupid enough to forget CC'ing Matthew :)

+Matthew Wilcox

On Mon, Mar 13, 2023 at 1:34 AM 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. This warning was added to catch fragile reads of
> a page memcg. Make page_cgroup_ino() get memcg from compound_head(page):
> that gives it the correct memcg for each subpage of a compound page,
> so is the right fix.
>
> I dithered between the right fix and the safer "fix": it's unlikely but
> conceivable that some userspace has learnt that /proc/kpagecgroup gives
> no memcg on tail pages, and compensates for that in some (racy) way: so
> continuing to give no memcg on tails, without warning, might be safer.
>
> But hwpoison_filter_task(), the only other user of page_cgroup_ino(),
> persuaded me.  It looks as if it currently leaves out tail pages of the
> selected memcg, by mistake: whereas hwpoison_inject() uses compound_head()
> and expects the tails to be included.  So hwpoison testing coverage has
> probably been restricted by the wrong output from page_cgroup_ino() (if
> that memcg filter is used at all): in the short term, it might be safer
> not to enable wider coverage there, but long term we would regret that.
>
> Signed-off-by: Hugh Dickins <hughd@google.com>
> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> ---
>
> (Yosry: Alternatively, we could modify page_memcg_check() to do
>  page_folio() like its sibling page_memcg(), as page_cgroup_ino() is the
>  only remaining caller other than print_page_owner_memcg(); and it already
>  excludes pages that have page->memcg_data = 0)
>
> ---
>  mm/memcontrol.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 5abffe6f8389..e3a55295725e 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -395,7 +395,7 @@ ino_t page_cgroup_ino(struct page *page)
>         unsigned long ino = 0;
>
>         rcu_read_lock();
> -       memcg = page_memcg_check(page);
> +       memcg = page_memcg_check(compound_head(page));
>
>         while (memcg && !(memcg->css.flags & CSS_ONLINE))
>                 memcg = parent_mem_cgroup(memcg);
> --
> 2.40.0.rc1.284.g88254d51c5-goog
>