On Fri, May 27, 2022 at 04:04:51PM +0800, Miaohe Lin wrote:
> If folios were freed from under us, there's no need to reclaim them. Skip
> these folios to save lots of cpu cycles and avoid possible unnecessary
> disk IO.
>
> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
> ---
> mm/vmscan.c | 8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index f7d9a683e3a7..646dd1efad32 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1556,12 +1556,18 @@ static unsigned int shrink_page_list(struct list_head *page_list,
> folio = lru_to_folio(page_list);
> list_del(&folio->lru);
>
> + nr_pages = folio_nr_pages(folio);
> + if (folio_ref_count(folio) == 1) {
> + /* folio was freed from under us. So we are done. */
> + WARN_ON(!folio_put_testzero(folio));
What? No. This can absolutely happen. We have a refcount on the folio,
which means that any other thread can temporarily raise the refcount,
so this WARN_ON can trigger. Also, we don't hold the folio locked,
or an extra reference, so nr_pages is unstable because it can be split.
> + goto free_it;
> + }
> +
> if (!folio_trylock(folio))
> goto keep;
>
> VM_BUG_ON_FOLIO(folio_test_active(folio), folio);
>
> - nr_pages = folio_nr_pages(folio);
>
> /* Account the number of base pages */
> sc->nr_scanned += nr_pages;
> --
> 2.23.0
>
>
