Re: Idle THPs

[SeongJae Park <sj38.park@gmail.com>]

From: SeongJae Park <sjpark@amazon.de>

On Thu, 10 Jun 2021 04:43:18 +0100 Matthew Wilcox <willy@infradead.org> wrote:

> As part of the folio work, I'm looking at PageIdle and PageYoung and
> they're defined to operate on PF_ANY.  So, for example, in
> pagecache_get_page(), we will call clear_page_idle() on the head page
> (actually, I changed this in a8cf7f272b5a -- before, it would call
> clear_page_idle() on the tail page).
> 
> However, we never actually call set_page_idle() on tail pages.  This is
> because we only call it here:
> 
>                         page = page_idle_get_page(pfn);
>                         if (page) {
>                                 page_idle_clear_pte_refs(page);
>                                 set_page_idle(page);
>                                 put_page(page);
>                         }
> 
> where page_idle_get_page() does:
> 
>         struct page *page = pfn_to_online_page(pfn);
> 
>         if (!page || !PageLRU(page) ||
>             !get_page_unless_zero(page))
>                 return NULL;
> 
> get_page_unless_zero() will always fail for tail pages (as it uses
> page_ref_add_unless(), which does not redirect to the head page's
> refcount).  So all tail pages read back as !idle in
> page_idle_bitmap_read().  Is this intended?  Should they rather
> mirror the state of their head page?

I think this is an intended behavior, as the document[1] says as below:

    For huge pages the idle flag is set only on the head page, so one has to
    read /proc/kpageflags in order to correctly count idle huge pages.

[1] https://www.kernel.org/doc/html/latest/admin-guide/mm/idle_page_tracking.html

Thanks,
SeongJae Park