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

[Matthew Wilcox]

On Mon, Nov 02, 2020 at 09:41:10AM -0500, Johannes Weiner wrote:
> On Thu, Oct 29, 2020 at 06:44:53PM +0800, Alex Shi wrote:
> > From: Hugh Dickins <hughd@xxxxxxxxxx>
> > 
> > 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?

I don't think it can point to an anon_vma without the ANON bit set.
Minchan's concern in that email was that it might still be NULL.