From: SeongJae Park <sjpark@xxxxxxxxx>
On Thu, 10 Jun 2021 04:43:18 +0100 Matthew Wilcox <willy@xxxxxxxxxxxxx> 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
