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.
