Re: [PATCH 2/3] mm: free zapped tail pages when splitting isolated thp

[Yang Shi <shy828301@gmail.com>]

On Fri, Jul 30, 2021 at 11:39 PM Yu Zhao <yuzhao@google.com> wrote:
>
> If a tail page has only two references left, one inherited from the
> isolation of its head and the other from lru_add_page_tail() which we
> are about to drop, it means this tail page was concurrently zapped.
> Then we can safely free it and save page reclaim or migration the
> trouble of trying it.
>
> Signed-off-by: Yu Zhao <yuzhao@google.com>
> Tested-by: Shuang Zhai <zhais@google.com>
> ---
>  include/linux/vm_event_item.h |  1 +
>  mm/huge_memory.c              | 28 ++++++++++++++++++++++++++++
>  mm/vmstat.c                   |  1 +
>  3 files changed, 30 insertions(+)
>
> diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h
> index ae0dd1948c2b..829eeac84094 100644
> --- a/include/linux/vm_event_item.h
> +++ b/include/linux/vm_event_item.h
> @@ -99,6 +99,7 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
>  #ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
>                 THP_SPLIT_PUD,
>  #endif
> +               THP_SPLIT_FREE,
>                 THP_ZERO_PAGE_ALLOC,
>                 THP_ZERO_PAGE_ALLOC_FAILED,
>                 THP_SWPOUT,
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index d8b655856e79..5120478bca41 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2432,6 +2432,8 @@ static void __split_huge_page(struct page *page, struct list_head *list,
>         struct address_space *swap_cache = NULL;
>         unsigned long offset = 0;
>         unsigned int nr = thp_nr_pages(head);
> +       LIST_HEAD(pages_to_free);
> +       int nr_pages_to_free = 0;
>         int i;
>
>         VM_BUG_ON_PAGE(list && PageLRU(head), head);
> @@ -2506,6 +2508,25 @@ static void __split_huge_page(struct page *page, struct list_head *list,
>                         continue;
>                 unlock_page(subpage);
>
> +               /*
> +                * If a tail page has only two references left, one inherited
> +                * from the isolation of its head and the other from
> +                * lru_add_page_tail() which we are about to drop, it means this
> +                * tail page was concurrently zapped. Then we can safely free it
> +                * and save page reclaim or migration the trouble of trying it.
> +                */
> +               if (list && page_ref_freeze(subpage, 2)) {
> +                       VM_BUG_ON_PAGE(PageLRU(subpage), subpage);
> +                       VM_BUG_ON_PAGE(PageCompound(subpage), subpage);
> +                       VM_BUG_ON_PAGE(page_mapped(subpage), subpage);
> +
> +                       ClearPageActive(subpage);
> +                       ClearPageUnevictable(subpage);
> +                       list_move(&subpage->lru, &pages_to_free);
> +                       nr_pages_to_free++;
> +                       continue;
> +               }

Yes, such page could be freed instead of swapping out. But I'm
wondering if we could have some simpler implementation. Since such
pages will be re-added to page list, so we should be able to check
their refcount in shrink_page_list(). If the refcount is 1, the
refcount inc'ed by lru_add_page_tail() has been put by later
put_page(), we know it is freed under us since the only refcount comes
from isolation, we could just jump to "keep" (the label in
shrink_page_list()), then such page will be freed later by
shrink_inactive_list().

For MADV_PAGEOUT, I think we could add some logic to handle such page
after shrink_page_list(), just like what shrink_inactive_list() does.

Migration already handles refcount == 1 page, so should not need any change.

Is this idea feasible?

> +
>                 /*
>                  * Subpages may be freed if there wasn't any mapping
>                  * like if add_to_swap() is running on a lru page that
> @@ -2515,6 +2536,13 @@ static void __split_huge_page(struct page *page, struct list_head *list,
>                  */
>                 put_page(subpage);
>         }
> +
> +       if (!nr_pages_to_free)
> +               return;
> +
> +       mem_cgroup_uncharge_list(&pages_to_free);
> +       free_unref_page_list(&pages_to_free);
> +       count_vm_events(THP_SPLIT_FREE, nr_pages_to_free);
>  }
>
>  int total_mapcount(struct page *page)
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index b0534e068166..f486e5d98d96 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -1300,6 +1300,7 @@ const char * const vmstat_text[] = {
>  #ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
>         "thp_split_pud",
>  #endif
> +       "thp_split_free",
>         "thp_zero_page_alloc",
>         "thp_zero_page_alloc_failed",
>         "thp_swpout",
> --
> 2.32.0.554.ge1b32706d8-goog
>