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[91.125.157.104]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-486eb32e6f1sm28315322f8f.6.2026.09.14.12.55.18 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 14 Sep 2026 12:55:19 -0700 (PDT) Date: Mon, 14 Sep 2026 12:55:08 -0700 (PDT) From: Hugh Dickins To: "Vlastimil Babka (SUSE)" cc: Hugh Dickins , Andrew Morton , Ackerley Tng , Alexander Viro , Alexandre Ghiti , Baolin Wang , Barry Song , Binbin Wu , Christian Brauner , Christoph Hellwig , Christoph Lameter , Claudio Imbrenda , David Hildenbrand , JP Kobryn , Jan Kara , Jens Axboe , Johannes Weiner , Kairui Song , Kiryl Shutsemau , Lance Yang , Leonardo Bras , Lorenzo Stoakes , Marcelo Tosatti , Matthew Wilcox , Mel Gorman , Miaohe Lin , Michal Hocko , Minchan Kim , Muchun Song , Oscar Salvador , Peter Zijlstra , Qi Zheng , Rik van Riel , Sebastian Andrzej Siewior , Shakeel Butt , Suren Baghdasaryan , Yang Shi , Yu Zhao , Zach O'Keefe , Zi Yan , linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH v2 07/26] mm/fbatch: LRU_NEXT_ACTIVATE bit to optimize folio_activate() In-Reply-To: Message-ID: <60bac57a-e486-c0af-1a8e-583fd6e939cc@google.com> References: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 8E904C0008 X-Stat-Signature: hkdxrrw99c5kro3pf1fkbmjtocx6ws4k X-HE-Tag: 1789415722-229165 X-HE-Meta: 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 ocdHDfo3 T6YBNcZMiWoNreJTMGkI4g/4CJ3qBa35jzXXXMQVWvmzNl+h+VV6yPvK8ovPjBf9EELiMw/HQzmdK4NCmizJaEZk3dwkISTJrHM4s9dDQy9iiwJyiJhlL5ez0Nil1aJOlcMJvX5ZQYah+ucvwgrNYi0/vsNfVv2Mw/Y0V5cUru67OaWSmHlZjLXUUZg1lt+7Ox8Or7UT+RhYxu2WJVxcZD307sE3BXjNbWs7/nxYdpjZa1MXvaX4bw4faxSgMA2Eb+8JB7LexgaNqx42L7m/Gta85oQ9jbwLoVyyjEBQdIphYk75F6iYE1gHfRMqYn8SKaaFZyxljyz3qiEofVOzJYaHNbO1HEGtNdQsb5n6dmH4OySceYHgB1nONaM095U/FMlhHwDntdaDvmJSyrfYdd5lvE/SQH3zNs/CgBCXLBa4Il3J3OoseGPme02wH+G8RaafvDcu/JnvAyU8guOwAN4G3ADmD+sm1OBOSlpCAvnAOnzQ= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Sat, 12 Sep 2026, Hugh Dickins wrote: ... > I have kept the address for debugging, and do put it to some use in > the paranoid vmstats patch 27/26. On 32-bit, with 4-byte pointers, > this LRU_NEXT_ACTIVATE bit does then leave one pointer covering two > adjacent entries, when it's deciding whether the folio points back > to this lru_add entry. Is it possible for two adjacent entries to > hold the same folio (and so perhaps miscount the stat)? Yes, it is > possible (if that folio is freed and reused and readded immediately); > but so unlikely that it's of no importance when gathering stats. It doesn't matter at all, it doesn't affect the choices made here, but I do want to correct my estimation of the likelihood of identical adjacent entries. Perhaps the most common occurrence would be, not when freed+reused+readded, but when isolate_migratepages_block() does that little lru_add_del_folio() + folio_add_lru() dance. It could be that the entry it "deletes" (by breaking the linkage from folio to fbatch) is the one most recently added to that fbatch, then folio_add_lru() will fill the next slot with that same folio. Hugh