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

[Matthew Wilcox <willy@infradead.org>]

On Wed, Mar 15, 2023 at 05:25:49PM -0700, Yosry Ahmed wrote:
[snipped 80 lines.  please learn to trim]
> I think instead of explicitly checking page->memcg_data, we can check
> PageTail() and return explicitly for tail pages tails, check
> PageSlab() to print the message for slab pages, then get the page's
> memcg through folio_memcg_check(page_folio(page)).
> 
> Something like:
> 
> static inline int print_page_owner_memcg(char *kbuf, size_t count, int ret,
> struct page *page)
> {
>     ...
>     rcu_read_lock();
> 
>     /* Only head pages hold refs to a memcg */
>     if (PageTail(page))
>         goto out_unlock;
> 
>     if (PageSlab(page))
>         ret += scnprintf(kbuf + ret, count - ret, "Slab cache page\n");
> 
>     memcg = folio_memcg_check(page_folio(page));
>     if (!memcg)
>         goto out_unlock;
>     ...
> }
> 
> Matthew, What do you think?

Brrr, this is hard.  read_page_owner() holds no locks or references,
so pages can transform between being head/tail/order-0 while we're
running.

It _tries_ to skip over tail pages in the most inefficient way possible:

                if (!IS_ALIGNED(pfn, 1 << page_owner->order))
                        goto ext_put_continue;

But any attempt to use folio APIs is going to risk tripping the
assertions in the folio code that it's not a tail.  This requires
more thought.