From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 437462EAB64; Thu, 5 Feb 2026 16:46:29 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770309989; cv=none; b=B0CLYUj1ZmCOYui7xt9dNWjJ0ifOOFrbhtkNoOvBmEbzq6RQiqegMruZaGNmxoQOOhexjxuky955JT93Wu+ILPdTtL8K2rkCJU0Ndz6Hism8R0tNtgzrycykgj6WQmMWZBvrfJdR6/TZaHaH5qJd9vnHPr1eJY3mZnVWuAxxFHI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770309989; c=relaxed/simple; bh=xZ40A2XkLWxNuM0wtPdSv5FXxZ2RAwf3Y0brB27dkCk=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=NBQ0lbGApBo5Oayej1E8RKNHPysL2jHbzcy1TTt77ggK9Xbn42/Swf/2v69x+SkRQBHuTz6d+6p+o9O1J9lKmtNTmd1rgHRUp1sCds7i0LkkoNqhB2VkJ4ZZz6Wh09JTjQ5dBVF8Vr1vOv8tHoB+wC+1XoNh7Ta3T1lol8lnNNo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=YJCyzbg2; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="YJCyzbg2" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 36EEDC4CEF7; Thu, 5 Feb 2026 16:46:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1770309989; bh=xZ40A2XkLWxNuM0wtPdSv5FXxZ2RAwf3Y0brB27dkCk=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=YJCyzbg2VBKutZnbfY5RGMyskD50lSS2TpqXqwLIjEQMZfJ1jHKWwl3AaxAi20GLL VsGf/m0Hd/URQaPfrKzsixUikbp0H2YSTNHc21IbAiVac7NXyTxzw6EzMAIj4PRCC1 IndCBmMBh777Pe6GqDfPwzb/JEaGHX4ZnkELgAgNN8DyDWkaVFMsLJZWHO2m6tNS+2 kTKs4EMqZxe9hCk3CmYekOfAHv1ElOEFy6WWzpQOgbFY+fkp4al9lyra46gHKRJY6h up9qpJfI5r2ydgFJqHfE3VchHNqJ7eQFa7A7bcQQumBV7jsWqu38n/x4dSFyI+6Su0 Az2gcwSK0iYSw== Message-ID: <8fcfe565-4371-47ee-a0de-a9d3e1efa0a1@kernel.org> Date: Thu, 5 Feb 2026 17:46:18 +0100 Precedence: bulk X-Mailing-List: virtualization@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 0/3] targeted TLB sync IPIs for lockless page table To: Lance Yang , Dave Hansen , 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@linux.intel.com, 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> <540adec9-c483-460a-a682-f2076cf015c2@linux.dev> <20260202150957.GD1282955@noisy.programming.kicks-ass.net> <06d48a52-e4ec-47cd-b3fb-0fccd4dc49f4@kernel.org> <3026ad8d-92ad-4683-8c3e-733d4070d033@linux.dev> <64f3a75a-30ff-4bee-833c-be5dba05f72b@intel.com> <647cbe2e-a034-4a75-9492-21ea1708eccc@intel.com> <99237729-f2b0-4a7a-8213-65a2f1c57744@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 2/5/26 17:30, Lance Yang wrote: > > > On 2026/2/5 23:41, Dave Hansen wrote: >> On 2/5/26 07:31, Lance Yang wrote: >>> >>> Without IPIs or synchronize_rcu(), IIUC, we have no way to know if there >>> are ongoing concurrent lockless page-table walks — the walkers just >>> disable >>> IRQs and walk. >> >> Yeah, but one aim of RCU is ensuring that readers see valid data but not >> necessarily the most up to date data. >> >> Are there cases where ongoing concurrent lockless page-table walks need >> to see the writes and they can't tolerate seeing valid but slightly >> stale data? > > The issue is we're about to free the page table (e.g. > pmdp_collapse_flush()). > > We have to ensure no walker is still doing a lockless page-table walk > when the page directories are freed, otherwise we get use-after-free. Right, and walking a page table that is suddenly no longer a page table is the real fun :) ... or trying to lookup the page of something that is not even a page. > >> Don't forget that we also have pesky concurrent lockless page-table >> walkers called CPUs. They're extra pesky in that they don't even stop >> for IPIs. ;) > > I assume those walkers that don't disable IRQs only read the PMD and > don't walk into the table; otherwise the current sync wouldn't work > for them. CPU page table walkers are much easier to control in that regard :) -- Cheers, David