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=1793784269; b=CJARFRz3FX3TT7oMO0Ylc5IvcM7BMSXmEdGdO9ja9QZq13SPwRS5BWSaw3MADDETPQCZ lGPa3ia/GR5wqk1rJX1s2lG6MLQiZO3wyneiFHRvFiGSHK27Nl3MdaMI6TAAz7jwJPAFx p8JF7yvFWC5EpgIR+sRPuTK00wI1nq90/mzx2jo+w/dJjPpphTvaDMA+CJdHemLhScN0B EPYipmNxmZcuGdcFO+ARa9Ed7qq2Qmwphe00bwXgI4JYvnnHafmLnx86amc8QprXZ3m9F Km31bTHVidRAqfGiF+iLrNvSn1fQh+f5tcjSholvGo/Ub7l0lmHUp9zd+3JhbOrYrQQ== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793784269; 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=W74eU1IM1bdOgq49kFD3NW38F3uMSzpXAuObYdVLCfc=; b=IFk1YgtNlZMmmyeyIUIJQgHbeM1gS1/6f1T14WQqTLdSzbwXC7V8/rF5ibrZijqCxrsH RAm5mw1MqflIocuIrCOvG88YEo9FG1eoVdm6br1MwK+fZ2mLfP1tisf4u2hQJL/Y2i15O RX8c6L4cIA/Ano4U4Hl6Zs4SnArhja2fuGd796e37Vq9RziywnNPpe+bVVHkeaJlgr7KT /3gdoaXVe8TPVS6BDCqI5GfSAwokt5Dc74sZn8sGLvsfvsY5nsmGGP0nnhM05loKysqWr RsycaP4Dj8WOaSQWtiER6YLBiMJiJuJzN6eHNfqg25FzWQJ2GuK9diffMc6eHl6higw== 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=eJr3DKqJ; 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 77A58606A32E for ; Wed, 4 Nov 2026 09:24:29 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=eJr3DKqJ Received: by smtp.kernel.org (Postfix) with ESMTPSA id 0DD8E1F000C1; Wed, 4 Nov 2026 09:24:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793784269; bh=W74eU1IM1bdOgq49kFD3NW38F3uMSzpXAuObYdVLCfc=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=eJr3DKqJrhbx0YqNDJ+clNMUgF9C7b7x0qUdDIjRgH304POz6YAL7D78pq8eqAWlo iZPki2kDG+5d4hzx5sZl1maQOlB0nxP0PkSmvdqzTqkmUeHc68nGe8292JfeCDVAJJ JMjTqwz7c9/Pf4/PlsoSdaMMHhUMlRpULrQl+JJSYmboYzksDb7Y8In67oNx0MA8Eb GCVlNuak48odSzQWQzwBhb9g+lBOYZBfY4ICDRNQOOJzdNRah3IOvFT9pTpMsJrzt1 /SxzYBjwILsoTLhs9c2Dipv4iM8prSpaovL4GzdWfSedhqK58PkocGbEEZ3qZkQ3+v TSNm1wZrl8OfA== Message-ID: <15b9dc96-d3eb-4ce7-8db0-589c2798ed41@kernel.org> Date: Wed, 4 Nov 2026 10:24:23 +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: [PATCH v3] mm/memory: use pmdp_get_lockless() for the pmd read in do_fault_around() To: "Lorenzo Stoakes (ARM)" Cc: Hui Zhu , Andrew Morton , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Hui Zhu References: <20261103014749.96764-1-hui.zhu@linux.dev> <61f8a583-5ea5-4fcc-bf30-ea9cafe94d65@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 11/3/26 21:27, Lorenzo Stoakes (ARM) wrote: > On Tue, Nov 03, 2026 at 08:51:38PM +0100, David Hildenbrand (Arm) wrote: >> On 11/3/26 19:24, Lorenzo Stoakes (ARM) wrote: >>> It turns out we're as wildly inconsistent on pmdp_get_lockless() as we are on >>> pmdp_get(), so TL;DR my main concern is moot so: >> >> Yeah, I know that we use it in some core places already properly, like in >> GUP-fast, __handle_mm_fault() or our complicated __pte_offset_map(). > > Yup, but also inconsistently, basically mostly interchangeably with > pmdp_get(). Rarely is this value actually used. > >>> * But it turns out it's all a giant mess (of course)... for >>> CONFIG_GUP_GET_PXX_LOW_HIGH arches you really actually need to also do: >>> >>> pmdp_get_lockless_start(); >>> pmd = pmdp_get_lockless(...); >>> pmdp_get_lockless_end(); >>> >>> And that's because, as Hugh points out in commit 146b42e07494 ("mm/pgtable: add >>> PAE safety to __pte_offset_map()")), you need IRQs disabled for these arches for >>> it to be safe. >> >> Yes, there is this pmdp_get_lockless_start() magic .... > > One of those things that, the more you look into it, the more stuff you find... > >> It's really a mess. I wonder if we can remove that somehow completely ... >> >> arch/mips/Kconfig: select GUP_GET_PXX_LOW_HIGH if CPU_MIPS32 && >> PHYS_ADDR_T_64BIT >> arch/sh/Kconfig: select GUP_GET_PXX_LOW_HIGH if X2TLB >> arch/x86/Kconfig: select GUP_GET_PXX_LOW_HIGH if X86_PAE > > Yeah that'd be ideal. > > But actually could we not just simplify and allow these arches to be a little > bit slower (who cares for museum pieces)? > > The only user of the start/end magic is __pte_offset_map(). And it does: > > irqflags = pmdp_get_lockless_start(); > pmdval = pmdp_get_lockless(pmd); > pmdp_get_lockless_end(irqflags); > > So that could just get folded into pmdp_get_lockless() for these arches. > > That would mean the places that don't require IRQs to be disabled (optimstic > checks) would now be needlessly enabling/disabling, but hey - use a system that > doesn't live in a museum :) It would be interesting to know the performance impact. IOW, whether this is just a slight slowdown or whether it makes these installations unusable. It looks like an interesting thing. > > That way we could safely convert every non-PMD lock holding caller to > pmdp_get_lockless() without needing another pmdp_get*() variant + fix confusion > about whether the start/end magic is required. > > And this would work even in places where the IRQs are already disabled as it's > fine with nesting. > > So you'd just update thusly: > > static inline pmd_t pmdp_get_lockless(pmd_t *pmdp) > { > pmd_t pmd; > + unsigned long flags; > > + local_irq_save(flags); > do { > pmd.pmd_low = pmdp->pmd_low; > smp_rmb(); > pmd.pmd_high = pmdp->pmd_high; > smp_rmb(); > } while (unlikely(pmd.pmd_low != pmdp->pmd_low)); > - > + local_irq_restore(flags); > return pmd; > } > > ... > > pte_t *__pte_offset_map(pmd_t *pmd, unsigned long addr, pmd_t *pmdvalp) > { > - unsigned long irqflags; > pmd_t pmdval; > > rcu_read_lock(); > - irqflags = pmdp_get_lockless_start(); > pmdval = pmdp_get_lockless(pmd); > - pmdp_get_lockless_end(irqflags); > > ... > } > > What do you think? Sounds reasonable. I guess we should be able to disable IRQs on all callsites. I am quite surprised that x86 shouldn't have an instruction to read/write PTEs atomically. I think if we require MMX we could just do atomic read+write, and let x86 implement pmdp_get_lockless() / pmdp_get() itself. But MMX seems to be a bit younger than PAE. Apparently there is a hacky way to read through CMPXCHG8B. Doesn't sound like a good use of our time ... but removing that handling entirely would be very nice. Simplifying is nice as well. -- Cheers, David