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Date: Tue, 26 Mar 2024 16:21:01 +0000
From: Matthew Wilcox <willy@infradead.org>
To: Barry Song <21cnbao@gmail.com>
Cc: akpm@linux-foundation.org, linux-mm@kvack.org, chrisl@kernel.org,
	david@redhat.com, hanchuanhua@oppo.com, hughd@google.com,
	linux-kernel@vger.kernel.org, mhocko@suse.com, ryan.roberts@arm.com,
	shy828301@gmail.com, v-songbaohua@oppo.com,
	wangkefeng.wang@huawei.com, xiang@kernel.org, ying.huang@intel.com,
	yuzhao@google.com
Subject: Re: [PATCH v2] mm: hold PTL from the first PTE while reclaiming a
 large folio
Message-ID: <ZgL17TSjiUtrmzk8@casper.infradead.org>
References: <20240306095219.71086-1-21cnbao@gmail.com>
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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.
> +			 */

Please wrap comments at <80 columns.  The easiest way to do this is just
to pipe the block of text through "fmt -p \*" (need to quote the * to
prevent it from being a glob/regex)