From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 B86B33AC0F3 for ; Wed, 3 Jun 2026 11:35:25 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780486526; cv=none; b=OaSZGPIbGt0aAT+Qi9IzNfGjy9wVANTmPLDDufg7IS8e7k7IEwuUzxT3j9NHWkuKOdTTXL/HqCUF9rqGHvuFwwsVSt3NlYsVgRhvnrzAVb5a9sNpo0ZG4oZGOwr4emAI5wXVSAs1ZJbZSGTQqsE8X/BDDZ9SmbMhDIG0aCOCbuI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780486526; c=relaxed/simple; bh=3488XlKW8FbN0vygCxPmDfImdXuvOFScgw+3WPJzRVY=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=upXV4B0AhEmQ7i/YEbpd7yCqoHF+6E4MeEmONpLPPxrrIPt6aSi0jSTMHAibwGJAqOaKQyYhRS86RJLIBlv9lHX4o8aa1FGU2jgZx3ZlczK3kAiqtblGxhlkceG39Xv6gY+STJtDg3IxOXjo5ZhSfHsN4QDza+5GTEEtKN+duaU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=AOsCxC5N; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="AOsCxC5N" Received: by smtp.kernel.org (Postfix) with ESMTPSA id E9FC51F00893; Wed, 3 Jun 2026 11:35:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780486525; bh=TThOnuJSX9KK+vQX9xQjNnIrM9wL88++yVRScE8ravo=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=AOsCxC5Nj4eKY5rrTt8ykBjjVHP4EjQOuRlgU5qmQzYo1YB4+C7EEc8AaEbhjxhhy nEiE+j4wrwQNpSaW2xj6+OPs6ZAy+18GHkt7Y1G0Z2IW1rgdoDEV/r3eWhxac1K7TA Bh847xSZpq2c3knIERsiuIWDrE1cra2FMwJHSj3DUBaIgaDA+CAvniDmMCHNYZ48sA cQAd6eL7ENlUe5S/9fIQG6IRMhxCp/F5c/bVWUdXvtO0ZMP3jVCuU4RgBTN67luAql 7O4eHyO6Xopz1U5hdpoIDcluFaHHb4CR7YLQ+C+hmBGjAqjXzeJHbh711sWDtEKj0Y WmwJu6nMX+Xbg== Message-ID: <10274d6e-0af2-4f9c-9788-2896065ec16a@kernel.org> Date: Wed, 3 Jun 2026 13:35:19 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 0/6] vdso/datastore: Allow prefaulting by mlockall() To: =?UTF-8?Q?Thomas_Wei=C3=9Fschuh?= Cc: Anna-Maria Behnsen , Frederic Weisbecker , Andy Lutomirski , Vincenzo Frascino , Thomas Gleixner , Nam Cao , Sebastian Andrzej Siewior , linux-kernel@vger.kernel.org, Andrew Morton , Jason Gunthorpe , John Hubbard , Peter Xu , linux-mm@kvack.org, Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko References: <20260518-vdso-mlockall-v3-0-bf4f2af2b41f@linutronix.de> <20260603114217-9eaaa386-a766-4336-8930-250c2ff91d19@linutronix.de> 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: <20260603114217-9eaaa386-a766-4336-8930-250c2ff91d19@linutronix.de> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 6/3/26 12:10, Thomas Weißschuh wrote: > On Tue, Jun 02, 2026 at 08:41:31PM +0200, David Hildenbrand (Arm) wrote: >> On 5/18/26 11:49, Thomas Weißschuh wrote: >>> While mlockall() is meant to lock page *memory*, effectively it will >>> also create and lock the corresponding page table entries. >>> Latency-sensitive applications expect not to experience any pagefaults >>> after calling mlockall(). However mlockall() ignores VM_IO mappings, >>> which is used by the generic vDSO datastore. >>> While the fault handler itself is very fast, going through the full >>> pagefault exception handling is much slower, on the order of 20us in a >>> test machine. >>> >>> Since the memory behind the datastore mappings is always present and >>> accessible it is not necessary to use VM_IO for them. >> >> GUP also refuses VM_IO, so this change unlocks GUP'ing these pages and using >> them for DIRECT_IO etc. >> >> Well, I assume that GUP-fast (no VMA available) would be able to GUP them >> already. But mechanisms that don't use GUP-fast would get reliably blocked for now. > > I am not sure about all of the side-effects of GUP. > Userspace is allowed to read this data in any way it wants. Ok, good! > It is *not* allowed > to interpret it however, which is obviously not enforcable anyways. > *All* writes from userspace need to be rejected. But those are forbidden via > VMA flags which as far as I can see are respected by GUP. Yes, even for FOLL_FORCE (see below). > >> I've been wondering for a while, whether the VM_IO for GUP is still required. > > If that restriction goes away, mlockall() would work automatically for the vDSO > data pages, no? Yes. But I guess there is this "debugger should not read/write memory with side-effects thing. Again, the GUP-fast interaction is a bit confusing, because that should still succeed. But debuggers don't use that. So the VM_IO protects that for now. > >> IIRC, we want to disallow read/write access that could have side effects through >> ptrace, including process_vm_*(). For memory-mapped I/O that might certainly the >> case. > > There are no side-effects, this is just global shared memory. > (But as mentioned before, writes can not be allowed) > Accesses from other tasks need to be prevented, which is what > patch 4 tries to do based on FAULT_FLAG_REMOTE. > > The only data in these pages which is not globally shared and accessible > are the time namespace offsets. But /proc/$PID/timens_offsets is > world-readable anyways. > >> With your change, you'd also unlock get_dump_page() on vdso pages. > > We use VM_DONTDUMP on the VMA, shouldn't that prevent dumping? > Dumping these pages is not a (security) issue, just pointless. Ah, right. So we still have VM_READ | VM_MAYREAD | VM_DONTDUMP | VM_MIXEDMAP ... So even FOLL_FORCE cannot write to it. It might be good to document the change (pages can not be GUP'ed, including being read by ptrace etc). But I agree that it should likely be fine. -- Cheers, David