From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 115E2C4452B for ; Tue, 21 Jul 2026 18:13:03 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 17C3B6B007B; Tue, 21 Jul 2026 14:12:47 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 153446B008C; Tue, 21 Jul 2026 14:12:47 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 042C66B0095; Tue, 21 Jul 2026 14:12:47 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0017.hostedemail.com [216.40.44.17]) by kanga.kvack.org (Postfix) with ESMTP id 9E9416B007B for ; Tue, 21 Jul 2026 14:12:46 -0400 (EDT) Received: from smtpin04.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay01.hostedemail.com (Postfix) with ESMTP id 8E9751C0142 for ; Tue, 21 Jul 2026 18:12:45 +0000 (UTC) X-FDA: 85013579490.04.147A814 Received: from sea.source.kernel.org (sea.source.kernel.org [172.234.252.31]) by imf05.hostedemail.com (Postfix) with ESMTP id B65EA100006 for ; Tue, 21 Jul 2026 18:12:43 +0000 (UTC) Authentication-Results: imf05.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=bvdgVv6s; spf=pass (imf05.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1784657563; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=Kl0Lh6FjGEgG1H/3xD/a8p3nj1WAQN+duTO4F2foLOk=; b=Io5NNG45JdISYFqcXwv+p26xrhDWzo44XUoS2PmJ1A5T+XrI6ftRyHUpSHbPtLxuh93rzn LE/f2XH/XegTg416zBSLHaZPiJknUqN6nxyVhYWdPMVFY7NPs2oGfPfnnb1NUKQFWpSdZk M/enHimpOz5B4L8Egh5pcIn7IScGMiQ= ARC-Authentication-Results: i=1; imf05.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=bvdgVv6s; spf=pass (imf05.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1784657563; b=ge1YTKTCsSL9LVQQU2CZh+xGX60XBMw6Z4qQhn2P9Rk15DWzcWqngu8PxRHO0GrJUbr4kn jBXljuXVfB5/zQr282BFwRFI+lfnBmb2OrONEiWDrOXqB3SrJpXm0nkowGzH0Rtt53Mq1+ uRagOJQF7FzWAO7Ilx54UlhwWPwkS0w= Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id D976241395; Tue, 21 Jul 2026 18:12:42 +0000 (UTC) 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 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 X-Rspam-User: X-Rspamd-Queue-Id: B65EA100006 X-Rspamd-Server: rspam01 X-Stat-Signature: 4p8iufhc37nuk679asq1siipzrwn571x X-HE-Tag: 1784657563-979241 X-HE-Meta: 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 hcizJP6k YGIah4C/XdCUveHCdPEAzVgyRlF2lsSJ4nb6weBU5ioQz3TwkPnG8lu43w71dsGT5ybiTtUj4VrywRmBxb/XDK8qcbmJfL6jHK9pRg6NOmw5Y+fN+ODJh2Oq2L01NlktZoXR8t4XagVqif/ftfF+eM4CTtCXuznSG7K29rgJcY0KIbNAEnhq8xqa8Afc1WmUHF9WcF5JWCkHHApMJFqK8Iarrqd5fbpvJKzLwZDpYn2qtW6iW6mKyF0yR1cClDRn1TH6m Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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