Re: [PATCH v20 08/20] mm: page_idle_get_page() does not need lru_lock

Johannes Weiner
2020-11-02T14:42:59Z

On Thu, Oct 29, 2020 at 06:44:53PM +0800, Alex Shi wrote:
> From: Hugh Dickins <hughd@google.com>
>
> It is necessary for page_idle_get_page() to recheck PageLRU() after
> get_page_unless_zero(), but holding lru_lock around that serves no
> useful purpose, and adds to lru_lock contention: delete it.
>
> See https://lore.kernel.org/lkml/20150504031722.GA2768@blaptop for the
> discussion that led to lru_lock there; but __page_set_anon_rmap() now
> uses WRITE_ONCE(),

That doesn't seem to be the case in Linus's or Andrew's tree. Am I
missing a dependent patch series?

> and I see no other risk in page_idle_clear_pte_refs() using
> rmap_walk() (beyond the risk of racing PageAnon->PageKsm, mostly but
> not entirely prevented by page_count() check in ksm.c's
> write_protect_page(): that risk being shared with page_referenced()
> and not helped by lru_lock).

Isn't it possible, as per Minchan's description, for page->mapping to
point to a struct anon_vma without PAGE_MAPPING_ANON set, and rmap
thinking it's looking at a struct address_space?