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

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Message-ID: <20240807194512.GA1828817@cmpxchg.org>
Date: Wed, 7 Aug 2024 15:45:12 -0400
From: Johannes Weiner <hannes@...xchg.org>
To: Usama Arif <usamaarif642@...il.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org, riel@...riel.com,
	shakeel.butt@...ux.dev, roman.gushchin@...ux.dev, yuzhao@...gle.com,
	david@...hat.com, baohua@...nel.org, ryan.roberts@....com,
	rppt@...nel.org, willy@...radead.org, cerasuolodomenico@...il.com,
	corbet@....net, linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org, kernel-team@...a.com,
	Shuang Zhai <zhais@...gle.com>
Subject: Re: [PATCH v2 1/4] mm: free zapped tail pages when splitting
 isolated thp

On Wed, Aug 07, 2024 at 02:46:46PM +0100, Usama Arif wrote:
> From: Yu Zhao <yuzhao@...gle.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@...gle.com>
> Tested-by: Shuang Zhai <zhais@...gle.com>
> Signed-off-by: Usama Arif <usamaarif642@...il.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)) {

folio_ref_freeze(new_folio, 2)?

Otherwise looks good to me

Acked-by: Johannes Weiner <hannes@...xchg.org>