Re: [PATCH v21 07/19] mm: page_idle_get_page() does not need lru_lock

[Vlastimil Babka]

On 11/11/20 9:17 AM, huang ying wrote:
> On Thu, Nov 5, 2020 at 4:56 PM Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx> 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(), 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).
> >
> > Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> > Signed-off-by: Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx>
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx>
> > Cc: Vlastimil Babka <vbabka@xxxxxxx>
> > Cc: Minchan Kim <minchan@xxxxxxxxxx>
> > Cc: Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx>
> > Cc: linux-mm@xxxxxxxxx
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > ---
> >  mm/page_idle.c | 4 ----
> >  1 file changed, 4 deletions(-)
> >
> > diff --git a/mm/page_idle.c b/mm/page_idle.c
> > index 057c61df12db..64e5344a992c 100644
> > --- a/mm/page_idle.c
> > +++ b/mm/page_idle.c
> > @@ -32,19 +32,15 @@
> >  static struct page *page_idle_get_page(unsigned long pfn)
> >  {
> >         struct page *page = pfn_to_online_page(pfn);
> > -       pg_data_t *pgdat;
> >
> >         if (!page || !PageLRU(page) ||
> >             !get_page_unless_zero(page))
> >                 return NULL;
> >
> > -       pgdat = page_pgdat(page);
> > -       spin_lock_irq(&pgdat->lru_lock);
>
> get_page_unless_zero() is a full memory barrier.  But do we need a
> compiler barrier here to prevent the compiler to cache PageLRU()
> results here?  Otherwise looks OK to me,

I think the compiler barrier is also implied by the full memory barrier and 
prevents the caching.

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

> Acked-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
>
> Best Regards,
> Huang, Ying
>
> >         if (unlikely(!PageLRU(page))) {
> >                 put_page(page);
> >                 page = NULL;
> >         }
> > -       spin_unlock_irq(&pgdat->lru_lock);
> >         return page;
> >  }
> >
> > --
> > 1.8.3.1