linux-kernel - Re: [PATCH v2] mm: hold PTL from the first PTE while reclaiming a large folio

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Date: Tue, 26 Mar 2024 16:21:01 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Barry Song <21cnbao@...il.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org, chrisl@...nel.org,
	david@...hat.com, hanchuanhua@...o.com, hughd@...gle.com,
	linux-kernel@...r.kernel.org, mhocko@...e.com, ryan.roberts@....com,
	shy828301@...il.com, v-songbaohua@...o.com,
	wangkefeng.wang@...wei.com, xiang@...nel.org, ying.huang@...el.com,
	yuzhao@...gle.com
Subject: Re: [PATCH v2] mm: hold PTL from the first PTE while reclaiming a
 large folio

On Wed, Mar 06, 2024 at 10:52:19PM +1300, Barry Song wrote:
>  			if (folio_test_pmd_mappable(folio))
>  				flags |= TTU_SPLIT_HUGE_PMD;
> +			/*
> +			 * Without TTU_SYNC, try_to_unmap will only begin to hold PTL
> +			 * from the first present PTE within a large folio. Some initial
> +			 * PTEs might be skipped due to races with parallel PTE writes
> +			 * in which PTEs can be cleared temporarily before being written
> +			 * new present values. This will lead to a large folio is still
> +			 * mapped while some subpages have been partially unmapped after
> +			 * try_to_unmap; TTU_SYNC helps try_to_unmap acquire PTL from the
> +			 * first PTE, eliminating the influence of temporary PTE values.
> +			 */

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