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=1793735504; b=AHdHDMzfJzErocNbjj4SmtL5yvx5lCKEfWZxlqfKn6kieke9D++hMEiqxZ9M3WgnbuM0 fdKmLDDYVgG0qJxBWByaPQWjyZ+ZcglzbLWC2QWNf/TyQ1UEbZnJwjGJMkJrM4drqlGnF rSZQJUN/xHg+Ksq+NsSXKnGRpqFje1WI0cSFFLctZgh5lNyoEa3U1obVBrnGZLVb3erL1 M/x4xWWkPq0KwnFJAgxKccrHV41ki+zmCJ5l9SbZLRt/zqMZfnFK0TpRSS2LoM5Ix/9aV 4lxFx6UfVStMrtQj3rl9dkYC+JyZ6fkTyGdacbbriPjbl5ezGf0BVharW+NShcdBOjw== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793735504; 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=n/PskOK3Y7Z0ycyR+laJWkqhqrbsZR+1MjVv+aAgnRs=; b=t7R4oXeV+9zNaP1eHqyLgrxsaBB0Y/cO43m2FU7IJzEDDJ5PwdudKulQUL6p+1QOwq/m jewz9saEU+N0eIQZgR7Cgan/3hQ0thpOcvaq+0xmgbMWgkua+Cbxu1BL8GLzjjGDuWAtB i1RJbMMdMTvlB2saJAkqTLmmK4ksbCkZtuGwjeig3qFpNLcGwrRhfxdpsDCTX6BFHF1gd oSg1ehNpYk7nllM/93tn9B558gxD22TP72Hn6s6Z75m0ebmazRvDPs1ti4d6pYdjcOi3+ HX+/Rr5QJxUvfJYPUPeFRaNs62T8SUYqg9B0ZmQCKOX7bfRZQqtj0cKtuZ8hPZh6nBA== 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=HAWhIOYQ; 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 F2095606A324 for ; Tue, 3 Nov 2026 19:51:43 +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=HAWhIOYQ Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4E1E01F000C1; Tue, 3 Nov 2026 19:51:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793735503; bh=n/PskOK3Y7Z0ycyR+laJWkqhqrbsZR+1MjVv+aAgnRs=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=HAWhIOYQspeIoWsCYj7aDYzrcrBssnG2n9TS3TvyWlXUOi4S+2f9m05ihQemddbiL 1vq+7Hg72EJhLloWSbVxrFpT0RBsfXIfkCEjxx5+ljJXJqTRA9GNoU3B91yaiQdsBa G6EkK9NuLDXflo7vW+fBc7g6VJHl2k9B3mFxyeb41YfTsKg7aRGDrBO4vwM0NzKcWy z05vVBw7S9JpXhWI/XRfbo4Iom2ajS0QHljgLqypcKyfMaOV+QHe3VdMnKs3n+m26F FmE1j+4qkcxhjYNZC1RusBTY+VG+vSNJPlig/lcVLdn7ge7xq89lYG9J0ayRZIXVel jKdHizrhlEODg== Message-ID: Date: Tue, 3 Nov 2026 20:51:38 +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 19:24, Lorenzo Stoakes (ARM) wrote: > On Tue, Nov 03, 2026 at 05:15:27PM +0100, David Hildenbrand (Arm) wrote: >> >>> >>> This pattern is definitely bad I think, we have it in many other places so >>> should probably convert those to pmdp_get() at least? >> >> Usama is working on this as part of separating HW and SW PMDs (he only sent out >> the PTE part so far). >> >>> >>> >>> It definitely seems right to use at least pmdp_get() here, but I wonder >>> about the need for pmdp_get_lockless() here. >> >> Whenever we're not holding the page table lock we should be using >> pmdp_get_lockless(). >> >> Even if they are semantically equivalent for most cases, it serves as a good >> reminder that there are no guarantees. >> >> And that you have to look again under PT lock. >> >>> >>> pmd_none() only checks the lower word, so a torn 64-bit word on 32-bit >>> (i.e. the PAE style cases we are worried about) makes no difference: >>> >>> From x86 pmd_none(): >>> >>> /* Only check low word on 32-bit platforms, since it might be >>> out of sync with upper half. */ >>> >>> And anyway I guess there is no valid PMD in such systems that would have >>> the lower word cleared and upper word not cleared. >>> >> >> I don't understand the discussion really. >> >> No PT lock -> use the lockless variant. >> >> Until we can remove the lockless variant. >> >> Are we consistent with that? Likely not. >> >> But that's all independent of this patch here. > > It turns out all of this is a complicated mess, I think I've nerd-sniped myself > into sending a series at some point about this... > > 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(). > > Acked-by: Lorenzo Stoakes (ARM) > > But for the non-TL;DR... (optional reading, will probably send something): > > * Agreed that the semantics matter - and the 'use pXXp_get_lockless() if no > locks, otherwise pXXp_get()' is preferable. > Ack. > * 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 .... > > The explanation is quite good: > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=146b42e07494e45f7c7bcf2cbf7afd1424afd78e > > And then of course there's also pmdp_get_lockless_sync() used by THP collapse > which is some kind of 'true' lockless usage that needs to synchronise with TLB > invalidation against GUP-fast and ugh. > > * So I think what matters is whether you actually NEED the value un-torn or > whether it's an optimistic check that you can live with (rechecked under lock > as needed). > > * So I wonder if we shouldn't just have e.g. pmdp_get_lockless_optimistic() or > something like that which is just a pmdp_get() in all cases with correct kdoc > description etc. > > * Then we can convert ALL lockless users to using a lockless form without having > to do the IRQ stuff. Including a ton of the existing lockless users. > > * I am not sure why I did this in bullets. :) 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 -- Cheers, David