Re: [PATCH 3/6] mm: free zapped tail pages when splitting isolated thp

David Hildenbrand
2024-07-30T15:14:38Z

On 30.07.24 14:46, Usama Arif wrote:
> From: Yu Zhao <yuzhao@google.com>
>
> 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>
> Signed-off-by: Usama Arif <usamaarif642@gmail.com>
> ---
> mm/huge_memory.c | 26 ++++++++++++++++++++++++++
> 1 file changed, 26 insertions(+)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 0167dc27e365..76a3b6a2b796 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2923,6 +2923,8 @@ static void __split_huge_page(struct page *page, struct list_head *list,
> unsigned int new_nr = 1 << new_order;
> int order = folio_order(folio);
> unsigned int nr = 1 << order;
> + LIST_HEAD(pages_to_free);
> + int nr_pages_to_free = 0;
>
> /* complete memcg works before add pages to LRU */
> split_page_memcg(head, order, new_order);
> @@ -3007,6 +3009,24 @@ static void __split_huge_page(struct page *page, struct list_head *list,
> if (subpage == page)
> continue;
> folio_unlock(new_folio);
> + /*
> + * 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);
> +

No VM_BUG_*, VM_WARN is good enough.

> + ClearPageActive(subpage);
> + ClearPageUnevictable(subpage);
> + list_move(&subpage->lru, &pages_to_free);

Most checks here should operate on new_folio instead of subpage.

--
Cheers,

David / dhildenb