Re: [PATCH v2 1/4] mm: free zapped tail pages when splitting isolated thp

[David Hildenbrand <david@redhat.com>]

On 07.08.24 15:46, 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 | 27 +++++++++++++++++++++++++++
>   1 file changed, 27 insertions(+)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 0167dc27e365..35c1089d8d61 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2923,7 +2923,9 @@ 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;
> +	struct folio_batch free_folios;
>   
> +	folio_batch_init(&free_folios);
>   	/* complete memcg works before add pages to LRU */
>   	split_page_memcg(head, order, new_order);
>   
> @@ -3007,6 +3009,26 @@ static void __split_huge_page(struct page *page, struct list_head *list,
>   		if (subpage == page)
>   			continue;
>   		folio_unlock(new_folio);
> +		/*
> +		 * If a folio 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
> +		 * folio 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_WARN_ON_ONCE_FOLIO(folio_test_lru(new_folio), new_folio);
> +			VM_WARN_ON_ONCE_FOLIO(folio_test_large(new_folio), new_folio);
> +			VM_WARN_ON_ONCE_FOLIO(folio_mapped(new_folio), new_folio);
> +
> +			folio_clear_active(new_folio);
> +			folio_clear_unevictable(new_folio);
> +			if (folio_batch_add(&free_folios, folio) == 0) {

nit:  "!folio_batch_add(&free_folios, folio)"

Nothing else jumped at me.

-- 
Cheers,

David / dhildenb