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

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Message-ID: <20210804142257.tsgzpfc3zq7um7iu@box.shutemov.name>
Date:   Wed, 4 Aug 2021 17:22:57 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Yu Zhao <yuzhao@...gle.com>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Yang Shi <shy828301@...il.com>, Zi Yan <ziy@...dia.com>,
        linux-kernel@...r.kernel.org, Shuang Zhai <zhais@...gle.com>
Subject: Re: [PATCH 2/3] mm: free zapped tail pages when splitting isolated
 thp

On Sat, Jul 31, 2021 at 12:39:37AM -0600, Yu Zhao wrote:
> @@ -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);

Why touch PG_Active/PG_Unevictable?

> +			list_move(&subpage->lru, &pages_to_free);
> +			nr_pages_to_free++;
> +			continue;
> +		}
> +

-- 
 Kirill A. Shutemov