From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1794235391; b=ynKmqU/03gZRJYWJQVFasa/Gvc6uXRTVOLNmi+eX4Ouy3fHeVRgw6wYo0psP3TegyoRE TdBP1tayy+HuF+FaRAdRdfqBY0d7yiHBtgiIxCyz+k46CCqgn1X/4rUuu5/ml58HmbLMA UlpcHolh5zW8VK2p4lQQpRhXXfjxlhB/s2OS4yDy36xg841uyRTZtjTbxZYFmfSbccQqO LK0VmbThf3pxjMWpCIKJX+ih9OXQrvPDVpnUnnYwEhxqXOYGOhY5Zl1GMGXM9qfKixSOx MI4t3n/W9yutIvIeT53TrNRAAbABHIJ5XvgHRjTES38B90qTWCBvtQaFxXwNyZM1gOA== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794235391; h=Received:DKIM-Signature:Message-ID:Date:MIME-Version:User-Agent: Subject:To:Cc:References:From:Content-Language:Autocrypt:In-Reply-To: Content-Type:Content-Transfer-Encoding; bh=8xp9qI2Ok6sylEAN+gwDtNeJcQqJMD2By0aR3Vam4ic=; b=EYonIdh7RsTSdNj0sqZkjhb4/V74Njz3IBFqauGwEvuLYOH4vw8vEidccY8r1NR9C0ag zeJOF15n4ovNxu56DEFdaV9a99nmWnr+3WK/RjnxcugMdfnMkxlImRMnf2ZgOfzkwOIah iKF2evnZhZMQDNi3E21Px6uvKjuq5oG7SCUVydfoD7NcLhSwLGl99S/d7R6ZF+55S8JFm QSGVOj+56oOFVy/NQm3P2OKGikK+8R9J7CNcXfN+wo2yAi1GVDCt95oXn3yvfY2JBYVcx z3fq9OHLItISZl+npxXzCbdnwhDu5SOK/HCIF1c0lGVDBsevFFjOeswI8xNq0fXLq4g== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=fiRcdJU3; 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 5C3FA606A331; Mon, 9 Nov 2026 14:43:11 +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=fiRcdJU3 Received: by smtp.kernel.org (Postfix) with ESMTPSA id B71801F0089F; Mon, 9 Nov 2026 14:43:00 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794235390; bh=8xp9qI2Ok6sylEAN+gwDtNeJcQqJMD2By0aR3Vam4ic=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=fiRcdJU3ll1oX0GL0kFkxzsBFsMHgUWajfOKORb5n5LGR0Ijd/a/ZTZV+msRt+7AE izmHSZ0aw7NmYNXW2GLtEsOVZAVfpyTgNpWMg0tnkNJIiyc5/KCWAsOooJ2agALkvo xMx1r2vihgzi5AJitWcAdZVUonHMXvaKgl9UVq27SVx9gYsvdThm6KBH+43D8zNpIr FMQ7SsNDb6DxIz8AlwOC0mr0jutSYBOHsbEGL+VL9IRKLrnSaOy3QQQC3fefk2V1Bc NeugOO2gVdweOhasQ4WB+Fp70fov7x2MciIpscO2WdjUB0pXFmsgdSIiWUkB+DBpqF g6u1UllN9MC0w== Message-ID: Date: Mon, 9 Nov 2026 15:42:58 +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: [RFC PATCH v2 01/18] mm: free PG_rcu_free folios only after an RCU grace period To: Kiryl Shutsemau Cc: Andrew Morton , Linus Torvalds , Matthew Wilcox , Vlastimil Babka , Lorenzo Stoakes , Jan Kara , Hugh Dickins , Christian Brauner , Alexander Viro , Harry Yoo , Miklos Szeredi , Miaohe Lin , Naoya Horiguchi , Oscar Salvador , "Liam R. Howlett" , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Johannes Weiner , Zi Yan , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kairui Song , Qi Zheng , Shakeel Butt , Axel Rasmussen , Yuanchu Xie , Wei Xu , Baoquan He , "Paul E. McKenney" , Pedro Falcato , Luis Chamberlain , Ilya Gladyshev , Jaegeuk Kim , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, fuse-devel@lists.linux.dev, kernel-team@meta.com References: <20261104130118.2507513-1-kas@kernel.org> <20261104130118.2507513-2-kas@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/6/26 11:23, Kiryl Shutsemau wrote: > On Thu, Nov 05, 2026 at 09:37:31PM +0100, David Hildenbrand (Arm) wrote: >>> diff --git a/mm/Kconfig b/mm/Kconfig >>> index 604c58199acb..6e5341b0bf64 100644 >>> --- a/mm/Kconfig >>> +++ b/mm/Kconfig >>> @@ -718,6 +718,21 @@ config PCP_BATCH_SCALE_MAX >>> latency. This option sets the upper limit of scale factor to limit >>> the maximum latency. >>> >>> +config PAGE_RCU_FREE >> >> I think we really want to focus this only on folios (like the current code does). >> >> FOLIO_RCU_FREE ? > > Sure. > >>> + def_bool y >>> + depends on 64BIT >>> + depends on !PREEMPT_RT >>> + help >>> + Reads from the page cache can copy from folios that are read >>> + again and again without taking a reference on them, so small >>> + reads of one file from many CPUs do not contend on the folio's >>> + reference count. The cost is that memory freed from such >>> + folios becomes reusable only after an RCU grace period. >> >> While that description is helpful, that's not really what this patch, or rather >> the basic infrastructure in page_alloc, does. >> >> I'd suggest to have a dedicated FOLIO_RCU_FREE config and maybe another one >> (that selects/depends on) FOLIO_RCU_FREE that actually enables the pagecache use >> and documents the details. > > I will make the help describe what this patch does. The page cache is > the only user, and both symbols would be def_bool without a prompt, so > I would rather add the second one with a second user. > > Does that work for you? > >>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >>> index 1e5d68acf62a..25082949a4ca 100644 >>> --- a/mm/huge_memory.c >>> +++ b/mm/huge_memory.c >>> @@ -3743,6 +3743,7 @@ static void __split_folio_to_order(struct folio *folio, int old_order, >>> #endif >>> (1L << PG_dirty) | >>> (1L << PG_dropbehind) | >>> + __PG_RCU_FREE | >> >> That looks a bit odd at first, as it's very different to the other once. But no >> easy way around assuming that the flag is not always around. >> >> Maybe we should just have this be a const mask constructed somewhere using BIT() >> and __PG_RCU_FREE etc, instead of having this growing list in this user here. > > Makes sense. Something like the diff below, on top of this patch. > Introducing split_copy_flags would be a preparatory patch, and this one > would only add __PG_RCU_FREE to it. > >>> @@ -1327,6 +1417,19 @@ static __always_inline bool __free_pages_prepare(struct page *page, >>> >>> VM_BUG_ON_PAGE(PageTail(page), page); >>> >>> + /* >>> + * Not for a hwpoisoned order-0 page: it never reaches the allocator, >>> + * and page_handle_poison() takes a reference right after this free. >>> + */ >>> + if (unlikely(folio_test_rcu_free(folio)) && >>> + !(PageHWPoison(page) && !order)) { >>> + VM_WARN_ON_ONCE(fpi_flags & FPI_NOLOCK); >>> + __folio_clear_rcu_free(folio); >>> + count_vm_events(PGFREE_RCU, 1 << order); >>> + rcu_free_defer(folio); >>> + return false; >>> + } >> I'm wondering whether we want to intercept on the folio freeing path instead? >> That would be __folio_put(), before we go into free_frozen_pages() used by other >> machinery that does not apply (slab etc). > > Not all page cache folios are freed through __folio_put(). Reclaim > frees them with free_unref_folios() directly (shrink_folio_list(), > move_folios_to_lru()), and so do folios_put_refs() and the LRU batches. > __free_pages_prepare() is the one place they all pass. Can we just rework that code first to have some clear folio freeing path, IOW doing this in a folio-specific helper? folios_put_refs() is just a specialized and batched variant of the __folio_put() freeing path in the end. So we can hook into these two easily. For free_unref_folios() would need another hook or a cleaner rework. > > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h > index a1e52a1f8a52..6d4e534c9ccc 100644 > --- a/include/linux/page-flags.h > +++ b/include/linux/page-flags.h > @@ -1185,6 +1185,18 @@ static __always_inline void __ClearPageAnonExclusive(struct page *page) > #define __PG_MLOCKED 0 > #endif > > +#ifdef CONFIG_ARCH_USES_PG_ARCH_2 > +#define __PG_ARCH_2 BIT(PG_arch_2) > +#else > +#define __PG_ARCH_2 0 > +#endif > + > +#ifdef CONFIG_ARCH_USES_PG_ARCH_3 > +#define __PG_ARCH_3 BIT(PG_arch_3) > +#else > +#define __PG_ARCH_3 0 > +#endif > + > /* > * Flags checked when a page is freed. Pages being freed should not have > * these flags set. If they are, there is a problem. > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index 25082949a4ca..44a1aaa3f834 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -3689,6 +3689,23 @@ static void __split_folio_to_order(struct folio *folio, int old_order, > const bool handle_hwpoison = folio_test_has_hwpoisoned(folio) && new_order; > long new_nr_pages = 1 << new_order; > long nr_pages = 1 << old_order; > + const unsigned long split_copy_flags = > + BIT(PG_referenced) | > + BIT(PG_swapbacked) | > + BIT(PG_swapcache) | > + __PG_MLOCKED | > + BIT(PG_uptodate) | > + BIT(PG_active) | > + BIT(PG_workingset) | > + BIT(PG_locked) | > + BIT(PG_unevictable) | > + __PG_ARCH_2 | > + __PG_ARCH_3 | > + BIT(PG_dirty) | > + BIT(PG_dropbehind) | > + __PG_RCU_FREE | > + LRU_GEN_MASK | > + LRU_REFS_MASK; I was rather thinking of a #define FOLIO_SPLIT_FLAGS BIT(PG_referenced) | BIT(PG_swapbacked) | \ ... To make this function actually shorter :) (ofc, you could also have a static const unsigned long, if that's what we want) -- Cheers, David