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

[huang ying <huang.ying.caritas@gmail.com>]

On Thu, Nov 5, 2020 at 4:56 PM Alex Shi <alex.shi@linux.alibaba.com> 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(), 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@google.com>
> Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Alex Shi <alex.shi@linux.alibaba.com>
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
> ---
>  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,

Acked-by: "Huang, Ying" <ying.huang@intel.com>

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
>
>