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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=LyMjq1S2; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 4FA33606A326 for ; Sat, 7 Nov 2026 20:06:02 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=LyMjq1S2 Received: by smtp.kernel.org (Postfix) with ESMTPSA id 002ED1F0089F; Sat, 7 Nov 2026 20:05:56 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794081962; bh=z3oDyqgDBVuce1eKczb2I46fcE48/Nsk2k7Al2XC6OM=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=LyMjq1S2WAolYZd76EhbJB1FgqYDBs7DcZjYqeRiLVsq1170bDG7Y12zYDonsbPNT lg0M7URG6heOwJqV+kuEm9hYbeN2fBqeRkbCsLDZJb+9Wshgzn0MD9yugElny91E7X oniUOhMLu8+2yu/SGKlFq1RU5j2rwn+wrQSFKSk2Qqgm5Rul2aojLZkR0kPjBbu69b w4vuHY9dS2CsVAcqOpQSRfizM2bkHxp3rhKfZCEU1yb/CL1O3hFEMnwbldf+j6J2IJ bC4MhtCahSKTQugMI1U+suda+qb8+wMVnoYcMsBB8hSCjfIFajD72Drg9MwmxdrpWR sgwUb4M7M2R4A== Message-ID: <47fee2b0-091f-4c20-9496-eff844ab2f6f@kernel.org> Date: Sat, 7 Nov 2026 21:05:52 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: synchronize with PTE table reclaim for all folio sizes To: Baolin Wang , Lance Yang Cc: baohua@kernel.org, akpm@linux-foundation.org, ljs@kernel.org, riel@surriel.com, liam@infradead.org, vbabka@kernel.org, harry@kernel.org, jannh@google.com, qi.zheng@linux.dev, pfalcato@suse.de, ayushr@modal.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20261106023326.10838-1-lance.yang@linux.dev> <9916bea5-3b53-43a0-8f65-cc33995adb23@linux.alibaba.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/6/26 03:54, Baolin Wang wrote: > > > On 11/6/26 10:33 AM, Lance Yang wrote: >> >> On Fri, Nov 06, 2026 at 09:57:47AM +0800, Baolin Wang wrote: >>> >>> >>> >>> Thanks. >> >> Cheers! >> > > ....... > >>> >>> I'm afraid the memory barrier here is subtle. __folio_remove_rmap() is >>> complex, and in the CONFIG_NO_PAGE_MAPCOUNT branch, >>> folio_sub_return_large_mapcount() doesn't provide full ordering, so we >>> may still need smp_wmb(). >> >> Yeah, agreed for large folios. In the diff above, only order-0 folios >> use smp_rmb(). >> >> Note that order-0 folios take the fully ordered atomic_add_negative() >> branch and skip the CONFIG_NO_PAGE_MAPCOUNT branch entirely. > > Yes. > >> static __always_inline void __folio_remove_rmap(struct folio *folio, >> struct page *page, int nr_pages, struct vm_area_struct *vma, >> enum pgtable_level level) >> { >> ... >> switch (level) { >> case PGTABLE_LEVEL_PTE: >> if (!folio_test_large(folio)) { >> nr = atomic_add_negative(-1, &folio->_mapcount); >> break; >> } >> >> if (IS_ENABLED(CONFIG_NO_PAGE_MAPCOUNT)) { >> nr = folio_sub_return_large_mapcount(folio, nr_pages, vma); >> ... >> } >> ... >> } >> ... >> } >> >> And large folios still take the PMD lock :D > > IIUC, the memory barrier optimization targets all non-PMD-sized folios > (including large folios). That is, for PMD-sized large folios we still > use the original PMD lock, while for other non-PMD-sized folios we can > use memory barriers to sync. It would be nice to prototype a patch and document the required memory barriers and how it is expected to work. -- Cheers, David