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=1793613788; b=2hGKxPBuJhjjEUXTzvkQd9Xd6/nh3RrjXd/hcfik386Wvwbgzns/K0rZA9feVo8mB3R8 uBEEJBgKqdmoZIUc7mwUhy2HP96QAPENbh+ehkDQsJIoormdjpwvIIm/MMeizwA86f6Rs X2b/R8R+GadiBHwlPWIRhxmMAeOdILqAWOBc0qAT9Enuq9AKrHjmBac4AKbrlSATxdt23 reBVD6HvFXjmMAPMEiIzywdu3YYwsn/YiGKEHcvHPn9TbO8M5oXqTmyYas1/xVxLmvGK8 6H9bWdt5dxpzvlGesGcakc77RGUUbdp1cjR4wxAJ0H1DjWZekmH8+Dr4bJEXpS+nLww== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793613788; 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=XFsKGmo7DMNhEgVimoBDSNS68WkiA1IPOa/9oNM+BB4=; b=zakbLUp6XZCaeRFGy/jcFEODJsbEmG0iuHwMsmh9rll8wXHhsXbiNK2ADhL7ypdZuZZ9 4/RSZvC/ZVKEQ54aD2+Q2MZNJzzlaoHbIvX7AMa0VkdLxdHngcTILis6kY1rqD2uP4f2l if+lsQP8AkJvB3GfDSsU4TCqOg/niATD7GKweB/VZhdNoNsXOlbe7m7ZzRDH/2g+pr5pu 1a5kucSrAjLhlfNlKaWT6JCItU1GSByoVAz/E3de7vPdzi1zgWFCHqpoN6rwoKS84r82L WAoCYW1K4qIdl2RvQbhut6CaQHLydnzwzZv6y5IUOn/OeEBGxYmdZ+krqplvNe/1NGQ== 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="Dx+/iD7G"; 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 39FA2606A324 for ; Mon, 2 Nov 2026 10:03:08 +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=Dx+/iD7G Received: by smtp.kernel.org (Postfix) with ESMTPSA id 084631F00A16; Mon, 2 Nov 2026 10:03:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793613788; bh=XFsKGmo7DMNhEgVimoBDSNS68WkiA1IPOa/9oNM+BB4=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Dx+/iD7G5omWWNtLm9rXXBkqcFwFkpoV76w5JNFRfp9yaT0+bhh/RjhOIuG+7G4qN X2zVQRBD9lBhtZPVFF7juoB7NTlANCgV47sLwYMQge/HcVZqJ2CRX+cvKoubOocBgO lbE1CsF2C42viDYG5fOpNDKXC3qRw3dcYR4l3qyytWKTmTOGu1vicyG/Irc1yH0TUM dkXC0NIzX7c01rGN7UxzIoKewKteXkPkKNuqNEe+Vp8FCp6txT/L6jbX/NZqYzPTeZ 0LQp5enljFBJOzeYZNHSkO3LUZZ2dyChPkvodXurwerhhsXrTBCDJzPARuNRkxqBF6 i+6Enw55VMVlA== Message-ID: Date: Mon, 2 Nov 2026 11:03:03 +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 v2] mm/memory: use pmdp_get_lockless() for the pmd read in do_fault_around() To: Hui Zhu , Andrew Morton , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , linux-mm@kvack.org, linux-kernel@vger.kernel.org Cc: Hui Zhu References: <20261102093120.92654-1-hui.zhu@linux.dev> 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: <20261102093120.92654-1-hui.zhu@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/2/26 10:31, Hui Zhu wrote: > From: Hui Zhu > > KCSAN reports a data-race in do_read_fault / pmd_install; the same code > is in mainline: > > write (marked) to 0xffff8d52a75fb810 of 8 bytes by task 27487 on cpu 5: > native_set_pmd arch/x86/include/asm/pgtable_64.h:83 [inline] > pmd_populate arch/x86/include/asm/pgalloc.h:84 [inline] > pmd_install+0xe0/0x120 mm/memory.c:444 > finish_fault+0x5bc/0xb00 mm/memory.c:5204 > do_cow_fault mm/memory.c:5432 [inline] > do_fault+0x5d0/0x6e0 mm/memory.c:5526 > handle_mm_fault+0x211/0x4c0 mm/memory.c:6218 > __mm_populate+0x144/0x270 mm/gup.c:2207 > __do_sys_mlockall+0x2cd/0x380 mm/mlock.c:764 > > read to 0xffff8d52a75fb810 of 8 bytes by task 27527 on cpu 9: > do_fault_around mm/memory.c:5341 [inline] > do_read_fault+0x162/0x670 mm/memory.c:5381 > do_fault+0x31a/0x6e0 mm/memory.c:5524 > handle_mm_fault+0x211/0x4c0 mm/memory.c:6218 > faultin_page_range+0x227/0x530 mm/gup.c:2156 > __x64_sys_madvise+0x67/0x80 mm/madvise.c:1479 > > value changed: 0x0000000000000000 -> 0x000000001386c067 > > Both paths only hold mmap_lock for read, so do_fault_around() tests > vmf->pmd while pmd_install() populates it under the pmd lock. The race is > benign: a stale pmd_none() only wastes the prealloc_pte, which do_fault() > frees again, and a pmd seen as populated only makes ->map_pages() find no > PTE and do_read_fault() fall back to __do_fault(). > > The load still has to be lockless-safe. pmdp_get() is a plain READ_ONCE() > and can tear where pmd_t is wider than a word, as on i386-PAE where > pmd_install() publishes two words with a single cmpxchg8b. Use > pmdp_get_lockless(), the accessor for pmd reads without the pmd lock and > what __pte_offset_map() already uses on this very pmd a few instructions > later. It is pmdp_get() everywhere else, so the report is silenced the > same way. > > No functional change on architectures that can load a pmd in one go. > Do we have a Fixes: tag? I'll give this some review time on the list; once review is done I'll queue this for the next merge window. > Signed-off-by: Hui Zhu > --- > Changelog: > Use pmdp_get_lockless() instead of pmdp_get() (David Hildenbrand). > > mm/memory.c | 6 +++++- > 1 file changed, 5 insertions(+), 1 deletion(-) > > diff --git a/mm/memory.c b/mm/memory.c > index 0fb676dd2bd63..2c1e50b70faeb 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -6017,7 +6017,11 @@ static vm_fault_t do_fault_around(struct vm_fault *vmf) > to_pte = min3(from_pte + nr_pages, (pgoff_t)PTRS_PER_PTE, > pte_off + vma_pages(vmf->vma) - vma_off) - 1; > > - if (pmd_none(*vmf->pmd)) { > + /* > + * Lockless: pmd_install() rechecks under the pmd lock, so a stale > + * pmd_none() here only wastes the prealloc_pte allocated below. > + */ I assume this comment can be shortened. Can be changed when applying. -- Cheers, David