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

[Johannes Weiner <hannes@cmpxchg.org>]

On Mon, Nov 02, 2020 at 02:49:27PM +0000, Matthew Wilcox wrote:
> 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@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?
> 
> 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.

Hm, no, the thread is a lengthy discussion about whether the store
could be split such that page->mapping is actually pointing to
something invalid (anon_vma without the PageAnon bit).

From his email:

        CPU 0                                                                           CPU 1

do_anonymous_page
  __page_set_anon_rmap
  /* out of order happened so SetPageLRU is done ahead */
  SetPageLRU(page)
  /* Compilr changed store operation like below */
  page->mapping = (struct address_space *) anon_vma;
  /* Big stall happens */
                                                                /* idletacking judged it as LRU page so pass the page
                                                                   in page_reference */
                                                                page_refernced
                                                                        page_rmapping return true because
                                                                        page->mapping has some vaule but not complete
                                                                        so it calls rmap_walk_file.
                                                                        it's okay to pass non-completed anon page in rmap_walk_file?