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 98C05E7DEEE for ; Mon, 2 Feb 2026 14:38:08 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id ACBAF6B00B2; Mon, 2 Feb 2026 09:38:07 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id A79246B00B5; Mon, 2 Feb 2026 09:38:07 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 985A36B00B6; Mon, 2 Feb 2026 09:38:07 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0016.hostedemail.com [216.40.44.16]) by kanga.kvack.org (Postfix) with ESMTP id 83E616B00B2 for ; Mon, 2 Feb 2026 09:38:07 -0500 (EST) Received: from smtpin17.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay08.hostedemail.com (Postfix) with ESMTP id 2219B1401BD for ; Mon, 2 Feb 2026 14:38:07 +0000 (UTC) X-FDA: 84399771414.17.B233077 Received: from out-180.mta1.migadu.com (out-180.mta1.migadu.com [95.215.58.180]) by imf06.hostedemail.com (Postfix) with ESMTP id 3A8DA18000F for ; Mon, 2 Feb 2026 14:38:05 +0000 (UTC) Authentication-Results: imf06.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=UgCRKXtO; spf=pass (imf06.hostedemail.com: domain of lance.yang@linux.dev designates 95.215.58.180 as permitted sender) smtp.mailfrom=lance.yang@linux.dev; dmarc=pass (policy=none) header.from=linux.dev ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1770043085; 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=+YuikHIpXYtVLQeTjpDEpyPPM+rwsAsIsRE/N+8+UvM=; b=A4FIPpHQ3IkGR/74UwrzEbcgpm7yYwpYCq4rw3AQmlUdU2e97k4rFlurabotBa1bkfCrgk Yq1KhxqQ2k0xjpEN2Uca/nKiOLBa8BPsLDZ31kd3qz844tvZDPPzS+jE4eOHLOgg82+9/g CpgcxX01tTQWCdsVb4UMzvItE8NwVjU= ARC-Authentication-Results: i=1; imf06.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=UgCRKXtO; spf=pass (imf06.hostedemail.com: domain of lance.yang@linux.dev designates 95.215.58.180 as permitted sender) smtp.mailfrom=lance.yang@linux.dev; dmarc=pass (policy=none) header.from=linux.dev ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1770043085; a=rsa-sha256; cv=none; b=LtxFVbtQNoXXeoYdfNzzOTFF9cgN0b3MiEywLr/RVo7eVLasU6m4n2k8mR/iBKYSUkx1wF ENNO0sTUIrK/gj9tojgswl4/baIWlohRqhjflsUzqxI0DYd9XomPAlCfPEOsjAWSSZAiAy wT19sdSj6n4036cNLu9uwFSnZXy/BMU= Message-ID: <540adec9-c483-460a-a682-f2076cf015c2@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1770043082; h=from:from: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; bh=+YuikHIpXYtVLQeTjpDEpyPPM+rwsAsIsRE/N+8+UvM=; b=UgCRKXtOZlfN+/7cORPTlSVzR8UJHh7JqxgK0CfCBKekoSTHFgghh9HracXNSrSC3awjkU H+AQz66kUh63gKnKZVfMkiJJ+X9mVe1TcLsZJJK3NjU7aPXYAA3puXscll30N4PFjBYfdj QKaUx6BxL4dSPBiJMateUtbngiHNtnw= Date: Mon, 2 Feb 2026 22:37:39 +0800 MIME-Version: 1.0 Subject: Re: [PATCH v4 0/3] targeted TLB sync IPIs for lockless page table Content-Language: en-US To: Peter Zijlstra Cc: Liam.Howlett@oracle.com, akpm@linux-foundation.org, aneesh.kumar@kernel.org, arnd@arndb.de, baohua@kernel.org, baolin.wang@linux.alibaba.com, boris.ostrovsky@oracle.com, bp@alien8.de, dave.hansen@intel.com, dave.hansen@linux.intel.com, david@kernel.org, dev.jain@arm.com, hpa@zytor.com, hughd@google.com, ioworker0@gmail.com, jannh@google.com, jgross@suse.com, kvm@vger.kernel.org, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, lorenzo.stoakes@oracle.com, mingo@redhat.com, npache@redhat.com, npiggin@gmail.com, pbonzini@redhat.com, riel@surriel.com, ryan.roberts@arm.com, seanjc@google.com, shy828301@gmail.com, tglx@linutronix.de, virtualization@lists.linux.dev, will@kernel.org, x86@kernel.org, ypodemsk@redhat.com, ziy@nvidia.com References: <20260202095414.GE2995752@noisy.programming.kicks-ass.net> <20260202110329.74397-1-lance.yang@linux.dev> <20260202125030.GB1395266@noisy.programming.kicks-ass.net> <4700e7ba-8456-4a93-9e28-7e5a3ca2a1be@linux.dev> <20260202133713.GF1395266@noisy.programming.kicks-ass.net> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Lance Yang In-Reply-To: <20260202133713.GF1395266@noisy.programming.kicks-ass.net> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 3A8DA18000F X-Stat-Signature: 174834r93r5if4io48mq1jpg6nkpzzhh X-Rspam-User: X-HE-Tag: 1770043085-850966 X-HE-Meta: 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 h8Luma9u G8/YaawoweLw09AqqWsFgjwlrW0xhmp0JjbJqEPeO6K4AoW86BEDshAp8IRbw5jrszG95d6wKs21TsFoS6QU5qp3+gBMQUp7pyTf2s7G29MtGJigx8YoYxaXgQISwT2KxRlxeAylWY7vqRsMr4MMjSwtsMkgR7lEl281TbKa2gqoR0PzFb7CdmA7E/3LCr4QkVFB/ 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 2026/2/2 21:37, Peter Zijlstra wrote: > On Mon, Feb 02, 2026 at 09:07:10PM +0800, Lance Yang wrote: > >>>> Right, but if we can use full RCU for PT_RECLAIM, why can't we do so >>>> unconditionally and not add overhead? >>> >>> The sync (IPI) is mainly needed for unshare (e.g. hugetlb) and collapse >>> (khugepaged) paths, regardless of whether table free uses RCU, IIUC. >> >> In addition: We need the sync when we modify page tables (e.g. unshare, >> collapse), not only when we free them. RCU can defer freeing but does >> not prevent lockless walkers from seeing concurrent in-place >> modifications, so we need the IPI to synchronize with those walkers >> first. > > Currently PT_RECLAIM=y has no IPI; are you saying that is broken? If > not, then why do we need this at all? PT_RECLAIM=y does have IPI for unshare/collapse — those paths call tlb_flush_unshared_tables() (for hugetlb unshare) and collapse_huge_page() (in khugepaged collapse), which already send IPIs today (broadcast to all CPUs via tlb_remove_table_sync_one()). What PT_RECLAIM=y doesn't need IPI for is table freeing ( __tlb_remove_table_one() uses call_rcu() instead). But table modification (unshare, collapse) still needs IPI to synchronize with lockless walkers, regardless of PT_RECLAIM. So PT_RECLAIM=y is not broken; it already has IPI where needed. This series just makes those IPIs targeted instead of broadcast. Does that clarify? Thanks, Lance