From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 1E90EEA4FB8 for ; Mon, 23 Feb 2026 13:02:59 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 5B5246B008C; Mon, 23 Feb 2026 08:02:59 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 55F156B0092; Mon, 23 Feb 2026 08:02:59 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 423BF6B0093; Mon, 23 Feb 2026 08:02:59 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0011.hostedemail.com [216.40.44.11]) by kanga.kvack.org (Postfix) with ESMTP id 31B6A6B008C for ; Mon, 23 Feb 2026 08:02:59 -0500 (EST) Received: from smtpin23.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay08.hostedemail.com (Postfix) with ESMTP id E8E27140143 for ; Mon, 23 Feb 2026 13:02:58 +0000 (UTC) X-FDA: 84475736436.23.41FBEE0 Received: from sea.source.kernel.org (sea.source.kernel.org [172.234.252.31]) by imf04.hostedemail.com (Postfix) with ESMTP id 077FC4000A for ; Mon, 23 Feb 2026 13:02:56 +0000 (UTC) Authentication-Results: imf04.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b="jkG6/V+6"; spf=pass (imf04.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1771851777; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=a53a4jiZX+4siH/+N7ov7PbqbJocA0mCy0U0f6hjhgY=; b=QY+6uMDfU6OcyFWW2WHEDL/xKszu4eaXraQBU5AWMbpD7Dkdfo98FzRdkrG4dedTQCJJtp akVpGMcwnLqDqvd/8jZcKq7QDoRKfRxikvQkobi6ZStGSay8TQtYYFKZepKlm4zbCwiib6 xfL+t8U2b65qfJBhUMKKlYxk4au4LAw= ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1771851777; a=rsa-sha256; cv=none; b=2cZsAXErYTRS6g8OqqKQTGmPDAeWoPFxrceLhurLJ48EcyYZejdzb9c41ralkz8BCQBC4m 5KE5UvsxP1RhHjb1fhse6jEpyDQlYnhujHxJAzfwxH3rHUv0XZXDSzP4dZmcBGHdgVJ2Fq V8SNoa7NLqg08yFhbtmkeshexJqvqgI= ARC-Authentication-Results: i=1; imf04.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b="jkG6/V+6"; spf=pass (imf04.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by sea.source.kernel.org (Postfix) with ESMTP id 14D5544454; Mon, 23 Feb 2026 13:02:56 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 2FE70C116C6; Mon, 23 Feb 2026 13:02:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1771851775; bh=ptRUSre2ZAhwCRyTmqafxxaRaPN+lKYGNkEB7a3hP9U=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=jkG6/V+6CCoPx7QwGGY9LjtWnT3Cx0CYRQlMmmzR0dlwY85ydB5NBfBqJVlsQ8Jvo XnmHrTgR0m8vGgr7+JNzCaSi7tYRFoi2oP8HfTjyNa3gBkJawjebb3bfRm0zwDUaYq xUGP4O3sa5d5ql/2minx/yt1S8tJRHns70MApU30iiXLsCH6Rt+b1hgtq3AfvYdqmM 139yVFJe9vUKEZZDk0wYn9uf4oc1GUhfL5cMWtOI0bz/qu5KGof0Ftw8khjLBb5TJz T33weFbOYqqoVo0JDNfsSX6DzyOPSuqG/eY48ouFCRQgSM/94O5XwTwCKK68kDPNb9 snkjHk91pLUaA== Message-ID: Date: Mon, 23 Feb 2026 14:02:49 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/1] mm/mmu_gather: replace IPI with synchronize_rcu() when batch allocation fails To: Lance Yang Cc: akpm@linux-foundation.org, aneesh.kumar@kernel.org, dave.hansen@intel.com, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, npiggin@gmail.com, peterz@infradead.org, will@kernel.org References: <2a6c4e62-1663-4a98-9adc-406a6a1ebfd3@kernel.org> <20260223125826.28207-1-lance.yang@linux.dev> 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: <20260223125826.28207-1-lance.yang@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Stat-Signature: sbw1njftibmjm6xdmtwto16e6f777c34 X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 077FC4000A X-HE-Tag: 1771851776-116651 X-HE-Meta: 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 uezprIxr 6CWPz5yEd8FcGr41MaZUkdDpFLCuSkEWuayY7WsLZl5rlriEF8SdcaLYVU4onUH5mAz4b4qLrQnWLCkvpvcsolqxTHjHSXW8LtEaFGDgHHFuvn/gv44zSj+CxdaqD1o/7mvkOi0LiK+yqvEp7tLYN2n8uoksh9VqUbdbRzgAzA/5dPaYl39IF2Bp+xEgwuiXkhM94C6J2C0/1fj5CffxDXM2/357h0jHtJsKYX05L5/9yfaE7vb39RMjIMkwCqdIFoniA5d1ebFOHSqzQCOdcZ+MOvv5zSNviKGhCCT0rBNcfiVftfBUHhcG3IB5V6nM45YfSs6nMolYvY1sCaIR1964j7XvPBoOj6fz83BOy8zXiT9JsbMnYychfFwbDNvzVzOONOVPtxThpCZKHmfUK+F+Y+lyMfinv6VNhBBoolPwEhxk= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2/23/26 13:58, Lance Yang wrote: > > On Mon, Feb 23, 2026 at 10:29:56AM +0100, David Hildenbrand (Arm) wrote: >> On 2/23/26 04:36, Lance Yang wrote: >>> From: Lance Yang >>> >>> When freeing page tables, we try to batch them. If batch allocation fails >>> (GFP_NOWAIT), __tlb_remove_table_one() immediately frees the one without >>> batching. >>> >>> On !CONFIG_PT_RECLAIM, the fallback sends an IPI to all CPUs via >>> tlb_remove_table_sync_one(). It disrupts all CPUs even when only a single >>> process is unmapping memory. IPI broadcast was reported to hurt RT >>> workloads[1]. >>> >>> tlb_remove_table_sync_one() synchronizes with lockless page-table walkers >>> (e.g. GUP-fast) that rely on IRQ disabling. These walkers use >>> local_irq_disable(), which is also an RCU read-side critical section. >>> synchronize_rcu() waits for all such sections to complete, providing the >>> same guarantee as IPI but without disrupting all CPUs. >>> >>> Since batch allocation already failed, we are in a way slow path, so >>> replacing the IPI with synchronize_rcu() is fine. >>> >>> We are in process context (unmap_region, exit_mmap) with only mmap_lock >>> held, a sleeping lock. synchronize_rcu() will catch any invalid context >>> via might_sleep(). >>> >>> [1] https://lore.kernel.org/linux-mm/1b27a3fa-359a-43d0-bdeb-c31341749367@kernel.org/ >>> >>> Link: https://lore.kernel.org/linux-mm/20260202150957.GD1282955@noisy.programming.kicks-ass.net/ >>> Link: https://lore.kernel.org/linux-mm/dfdfeac9-5cd5-46fc-a5c1-9ccf9bd3502a@intel.com/ >>> Link: https://lore.kernel.org/linux-mm/bc489455-bb18-44dc-8518-ae75abda6bec@kernel.org/ >>> Suggested-by: Peter Zijlstra >>> Suggested-by: Dave Hansen >>> Suggested-by: David Hildenbrand (Arm) >> >> I think it was primarily Peter and Dave suggesting that :) > > :) > >>> Signed-off-by: Lance Yang >>> --- >>> mm/mmu_gather.c | 3 ++- >>> 1 file changed, 2 insertions(+), 1 deletion(-) >>> >>> diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c >>> index fe5b6a031717..df670c219260 100644 >>> --- a/mm/mmu_gather.c >>> +++ b/mm/mmu_gather.c >>> @@ -339,7 +339,8 @@ static inline void __tlb_remove_table_one(void *table) >>> #else >>> static inline void __tlb_remove_table_one(void *table) >>> { >>> - tlb_remove_table_sync_one(); >>> + if (IS_ENABLED(CONFIG_MMU_GATHER_RCU_TABLE_FREE)) >>> + synchronize_rcu(); >> >> That should work. >> >> Reading all the comments for tlb_remove_table_smp_sync(), I wonder >> whether we should wrap that in a tlb_remove_table_sync_rcu() function, >> with a proper kerneldoc for the CONFIG_MMU_GATHER_RCU_TABLE_FREE variant >> where we discuss how this relates to tlb_remove_table_sync_one (and >> tlb_remove_table_smp_sync() . > > Good point! That would be cleaner and better ;) > > How about the following: > > ---8<--- > diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c > index fe5b6a031717..ea5503d3e650 100644 > --- a/mm/mmu_gather.c > +++ b/mm/mmu_gather.c > @@ -296,6 +296,24 @@ static void tlb_remove_table_free(struct mmu_table_batch *batch) > call_rcu(&batch->rcu, tlb_remove_table_rcu); > } > > +/** > + * tlb_remove_table_sync_rcu() - synchronize with software page-table walkers > + * > + * Like tlb_remove_table_sync_one() but uses RCU grace period instead of IPI > + * broadcast. Should be used in slow paths where sleeping is acceptable. > + * > + * Software/Lockless page-table walkers use local_irq_disable(), which is also > + * an RCU read-side critical section. synchronize_rcu() waits for all such > + * sections, providing the same guarantee as tlb_remove_table_sync_one() but > + * without disrupting all CPUs with IPIs. > + * > + * Context: Can sleep/block. Cannot be called from any atomic context. > + */ > +static void tlb_remove_table_sync_rcu(void) > +{ > + synchronize_rcu(); > +} > + > #else /* !CONFIG_MMU_GATHER_RCU_TABLE_FREE */ > > static void tlb_remove_table_free(struct mmu_table_batch *batch) > @@ -303,6 +321,10 @@ static void tlb_remove_table_free(struct mmu_table_batch *batch) > __tlb_remove_table_free(batch); > } > > +static void tlb_remove_table_sync_rcu(void) > +{ > +} > + > #endif /* CONFIG_MMU_GATHER_RCU_TABLE_FREE */ > > /* > @@ -339,7 +361,7 @@ static inline void __tlb_remove_table_one(void *table) > #else > static inline void __tlb_remove_table_one(void *table) > { > - tlb_remove_table_sync_one(); > + tlb_remove_table_sync_rcu(); > __tlb_remove_table(table); > } > #endif /* CONFIG_PT_RECLAIM */ > --- > > Thanks for the suggestion! > Lance LGTM, but let's hear other options. -- Cheers, David