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 E86FC75801 for ; Tue, 21 Jul 2026 18:12:42 +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=1784657564; cv=none; b=EjlB0cbHfHSXZ2nNc9Z7KCXoOuEBbTGHiljCCxgZwjoLdQasiGG0Dzntxbp0bo4pU6/8VSKRBwWBMiBZ+Zzpqfbk5VjrlCDHsca6uTjW3kIm/70kSSAAN79WLjdBsHQt5lfOJCW6c3cko1A/PcmkRSjKVLVLX3TvtU8YPQ75Tnc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784657564; c=relaxed/simple; bh=s072KJF256/hJhGDa4ROo5FBSGZUSC7bWlmeUi3HbQU=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=clG+Y73kSTQHe718UEQO53qdvvXRtdaj/9+Ey+YZrtn7jfVYoBb1vIDftlsBdDGvrm2cfw71/dLwlF+dkgOxFqcl75cWDr7z/4R/E1ucnnV/RpBvsiaw7EuvaPCCiyjQjz3M2mIYxBazwIp4h8/UnFnZQn29Ak1kEICn2fSbA/s= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=bvdgVv6s; 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="bvdgVv6s" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 25D111F000E9; Tue, 21 Jul 2026 18:12:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1784657562; bh=Kl0Lh6FjGEgG1H/3xD/a8p3nj1WAQN+duTO4F2foLOk=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=bvdgVv6sYnZOASG/ksH9sQhf/lJzzXdiBaiQw4qKoPbUwo8rmsFbdnFL7KJkOSzzo qkbWcYA+5g0tRRv8TBOdqhhU6lEtcvLeI0Uy8tgPF09Ws0ZSujd0n55HbnxKo0oC9u K5/0bXQjlcKvvBX2ny4JRMxCxlvrem0vBvhoKO8nOe3ve7vBr35CFb7J1GwzlVfePA z4Dj6v723Rdu0FdpkO32dxJn1+tiZN2cb6M/ynUx9RoiajTtjMavSg377RTAp6f5IF MS/I28YPA3bXOXRy9RdKD0fWo60E1iTo+iX7FaIwm0pMIXggQJ22eo19LY4DD8gdZs jOnKGEQ9jcW/w== Message-ID: Date: Tue, 21 Jul 2026 20:12:38 +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] mm: access remote process memory under the per-VMA lock To: Rik van Riel , linux-kernel@vger.kernel.org, Andrew Morton Cc: kernel-team@meta.com, Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , linux-mm@kvack.org References: <20260717170036.743149-1-riel@surriel.com> 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: <20260717170036.743149-1-riel@surriel.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 7/17/26 19:00, Rik van Riel wrote: > __access_remote_vm() holds mmap_read_lock() for the whole transfer. On > large machines with large multi-threaded applications, the mmap_lock > is often contended due to mixed accesses from readers and writers, like > mmap and munmap. When a lock holder gets stuck, system monitoring software > can get stuck behind that, resulting in a failure to log that the system > is in trouble. > > Take the per-VMA lock in __access_remote_vm() when the access falls > entirely within a single VMA. Fall back to the mmap lock when the access > crosses a VMA boundary, or when the page cannot be reached under the > per-VMA lock: a dropped fault, a userfaultfd VMA, a hard error, or memory > with no struct page that has to go through vma->vm_ops->access(). > > The bulk of the work is a new gup helper. __access_remote_vm() needs a > single page from a VMA it has already looked up and locked, faulting it in > when necessary, under either lock. Yes, much better. > > get_user_pages_remote() does not fit: it hard codes the mmap lock and > re-derives the VMA. get_user_page_vma() walks the page tables, faults a > missing page in, and returns it with a reference and the caller's lock > still held. Agreed, and that can be tackled later. > > The per-VMA path also closes a pre-existing gap. A COWed page in a > VM_IO/VM_PFNMAP VMA has a struct page, but the old code routed it to > ->access(), where generic_access_phys() ioremaps the PFN and ioremap of RAM > is rejected, so the read came up short. Yep. The GUP code rejected VM_IO/VM_PFNMAP for, though, because it did not properly handle vm_normal_page_pmd() etc so far. > > get_user_page_vma() now returns that page normally. Raw PFNs with no struct > page still reach ->access() under the mmap lock, as before. > > The series is arranged as: It will take me a bit to dig into the details; I'll be out the remainder of the week. But the general direction sounds good. -- Cheers, David