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 D957DFF8873 for ; Tue, 28 Apr 2026 20:06:01 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 067DF6B0005; Tue, 28 Apr 2026 16:06:01 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id F34BC6B008A; Tue, 28 Apr 2026 16:06:00 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id DFAFA6B008C; Tue, 28 Apr 2026 16:06:00 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0010.hostedemail.com [216.40.44.10]) by kanga.kvack.org (Postfix) with ESMTP id C838C6B0005 for ; Tue, 28 Apr 2026 16:06:00 -0400 (EDT) Received: from smtpin26.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay08.hostedemail.com (Postfix) with ESMTP id 4C679140397 for ; Tue, 28 Apr 2026 20:06:00 +0000 (UTC) X-FDA: 84709045680.26.D506275 Received: from tor.source.kernel.org (tor.source.kernel.org [172.105.4.254]) by imf16.hostedemail.com (Postfix) with ESMTP id 7382E18000A for ; Tue, 28 Apr 2026 20:05:58 +0000 (UTC) Authentication-Results: imf16.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=ZTIpM6yO; dmarc=pass (policy=quarantine) header.from=kernel.org; spf=pass (imf16.hostedemail.com: domain of david@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=david@kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1777406758; 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=dsMGeykKWZhC9ONB+MyQNfMWXIcvfeAFiKgekfHXIRo=; b=2lPpgkP4k9L2fha8ZKFoWbVQ/SRSZ3zasRc+u12Kbbo9ApQ+zvFc8aZyVf4j9qrRvb/Iq9 MgPVgq+sgFGWefd1ee9t/k6zT76p37BkNNqUDL121Vb64KIvAEY+tudfVdf2qW9FVl12la YFoeEFbc5dYuDK+/cn3AL/ElQeQCQSg= ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1777406758; a=rsa-sha256; cv=none; b=JEiamQrEvN5sDw4f6qyt9uuycb3ao6RlqgsGR9dt5Lq1XyqUEGsrmAoeKKJQNPfL6WwpVm UyWOCnZNjpF15pb8KJuVUC//RYblNKAn7PT8+LiSRnDKLY52VMlM+h54yMmtGTFy/FLkZ1 KjTFngZNRaASzck+M/sAkF7yMbi5la8= ARC-Authentication-Results: i=1; imf16.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=ZTIpM6yO; dmarc=pass (policy=quarantine) header.from=kernel.org; spf=pass (imf16.hostedemail.com: domain of david@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=david@kernel.org Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by tor.source.kernel.org (Postfix) with ESMTP id C604F61119; Tue, 28 Apr 2026 20:05:57 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4D5FEC2BCB3; Tue, 28 Apr 2026 20:05:52 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1777406757; bh=i6VL9rEPIzlOa7TBHegHrUm94Rx4pZDl6vJEPIECtVk=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=ZTIpM6yOYwOhkfetphTYo6eCJ+EpjJaVIwaao3v6nme2uV+NzAwO7SMUJozXHNL3l 1Vmp2aOdajQFrM/XQXdT/h7R4nh3B9DTfnKrpRnAJ3fhTxkuEzP/SNTDhkwfbdObpW KKxTRdFQpFSxYHOP7DlSddNjdK43eAoLoVjULXzvYzhSyLzARXH1zCLpTaWPwLU4BL h6VesQ0phLHTeXd4cLvoNd4OFJiCCCSi4YUxFIaW9FEX+12MFD+DffbUmWm9HgWhIK LbD00pGmgbj4TyOk5gHMq5MLwM9N6InVOKOgQaJyaMYm97FPvJmKv0bg/m3LQa3FJR wJ+RC1HIT/RDQ== Message-ID: <8b29da5b-e260-4b77-a640-8abb447291d1@kernel.org> Date: Tue, 28 Apr 2026 22:05:49 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 1/2] mm/process_vm_access: pidfd and nowait support for process_vm_readv/writev To: Alban Crequy , Andrew Morton , Christian Brauner Cc: Lorenzo Stoakes , "Liam R . Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Alban Crequy , Peter Xu , Willy Tarreau , linux-kselftest@vger.kernel.org, shuah@kernel.org, Usama Arif , David Laight References: <20260428122826.339550-1-alban.crequy@gmail.com> <20260428122826.339550-2-alban.crequy@gmail.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: <20260428122826.339550-2-alban.crequy@gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 7382E18000A X-Stat-Signature: gnkgd3rh7ss5bishgd9b3r7wps7y8pdg X-Rspam-User: X-HE-Tag: 1777406758-717174 X-HE-Meta: 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 QS31zlSn NhQQtUlYPQzWa9fSIU1BqE/P4pZ2FNp2g1s9fRumacwkDZq+8LJXOYkj+G/JWIwtIK7CYZksd7At0ht1I2c3eE7VJTdiWYu9Kxqk9hSra2Hg7Fkd16MYEW18o61NDk/c6T0x/6BrO3HChcfjtqGTNFoSn675tGTCqNa1vT+tF6jCDeNZIKDYufO6djYlxog8ytT0xYFVolGrAjp6F4779kWAkIxVu0BbYsQk10TDvy+JsapGfKm32FPG7ntF6r+UnEtSHfMy5CLXrDQzLHdptgHdH7aAoJeHddcHugXuN0snFeFOzBsuG8fpOX5/2HkD8wwkcnNkffc2UNKe1T3kQm2zfXkP9s/YXe8RLq9lJpQr8pZ/dsZc0UUVptzG1jf+HShNdNCBlENMhmsFuUHnvbhS84Q== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 4/28/26 14:28, Alban Crequy wrote: > From: Alban Crequy Hi, some more smaller comments. Overall, LGTM. > > There are two categories of users for process_vm_readv: > > 1. Debuggers like GDB or strace. > > When a debugger attempts to read the target memory and triggers a > page fault, the page fault needs to be resolved so that the debugger > can accurately interpret the memory. A debugger is typically attached > to a single process. > > 2. Profilers like OpenTelemetry eBPF Profiler. > > The profiler uses a perf event to get stack traces from all > processes at 20Hz (20 stack traces to resolve per second). For > interpreted languages (Ruby, Python, etc.), the profiler uses > process_vm_readv to get the correct symbols. In this case, > performance is the most important. It is fine if some stack traces > cannot be resolved as long as it is not statistically significant. > > The current behaviour of process_vm_readv is to resolve page faults in > the target VM. This is as desired for debuggers, but unwelcome for > profilers because the page fault resolution could take a lot of time > depending on the backing filesystem. Additionally, since profilers > monitor all processes, we don't want a slow page fault resolution for > one target process slowing down the monitoring for all other target > processes. > > This patch adds the flag PROCESS_VM_NOWAIT, so the caller can choose to > not block on IO if the memory access causes a page fault. What is the expected return value to user space if we run into this case? And in the same context: Will you send a man page update? :) > > Additionally, this patch adds the flag PROCESS_VM_PIDFD to refer to the > remote process via PID file descriptor instead of PID. Such a file > descriptor can be obtained with pidfd_open(2). This is useful to avoid > the pid number being reused. It is unlikely to happen for debuggers > because they can monitor the target process termination in other ways > (ptrace), but can be helpful in some profiling scenarios. > > If a given flag is unsupported, the syscall returns the error EINVAL > without checking the buffers. This gives a way to userspace to detect > whether the current kernel supports a specific flag: > > process_vm_readv(pid, NULL, 1, NULL, 1, PROCESS_VM_PIDFD) > -> EINVAL if the kernel does not support the flag PROCESS_VM_PIDFD > (before this patch) > -> EFAULT if the kernel supports the flag (after this patch) > > Signed-off-by: Alban Crequy > --- > v3: > - Fix ERR_PTR handling for pidfd_get_task(): use IS_ERR()/PTR_ERR() > for the pidfd path, matching process_madvise() (Usama Arif, Sashiko) > > v2: > - Expand commit message with use-case motivation (David Hildenbrand) > - Use unsigned long consistently for pvm_flags parameter (David Hildenbrand) > - Add PROCESS_VM_SUPPORTED_FLAGS kernel-internal define (David Hildenbrand) > - Keep (1UL << N) in UAPI header: BIT() is defined in vdso/bits.h > which is not exported to userspace, so UAPI headers using BIT() would > break when included from userspace programs (David Hildenbrand) > > MAINTAINERS | 1 + > include/uapi/linux/process_vm.h | 9 +++++++++ > mm/process_vm_access.c | 34 ++++++++++++++++++++++++--------- > 3 files changed, 35 insertions(+), 9 deletions(-) > create mode 100644 include/uapi/linux/process_vm.h > > diff --git a/MAINTAINERS b/MAINTAINERS > index 2fb1c75afd16..0f6ce21d6235 100644 > --- a/MAINTAINERS > +++ b/MAINTAINERS > @@ -16786,6 +16786,7 @@ F: include/linux/ptdump.h > F: include/linux/vmpressure.h > F: include/linux/vmstat.h > F: fs/proc/meminfo.c > +F: include/uapi/linux/process_vm.h We try to sort this alphabetically. Sometimes we failed. Likely this should just go to one more line up. > F: kernel/fork.c > F: mm/Kconfig > F: mm/debug.c > diff --git a/include/uapi/linux/process_vm.h b/include/uapi/linux/process_vm.h > new file mode 100644 > index 000000000000..4168e09f3f4e > --- /dev/null > +++ b/include/uapi/linux/process_vm.h Thinking out loud: the c file is called "process_vm_access.c", should we name the header like that as well? > @@ -0,0 +1,9 @@ > +/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */ > +#ifndef _UAPI_LINUX_PROCESS_VM_H > +#define _UAPI_LINUX_PROCESS_VM_H > + > +/* Flags for process_vm_readv/process_vm_writev */ > +#define PROCESS_VM_PIDFD (1UL << 0) > +#define PROCESS_VM_NOWAIT (1UL << 1) Should we use BIT here? I see some usage in other uapi headers (e.g., tcp.h) > + > +#endif /* _UAPI_LINUX_PROCESS_VM_H */ > diff --git a/mm/process_vm_access.c b/mm/process_vm_access.c > index 656d3e88755b..dacef50be0be 100644 > --- a/mm/process_vm_access.c > +++ b/mm/process_vm_access.c > @@ -14,6 +14,9 @@ > #include > #include > #include > +#include > + > +#define PROCESS_VM_SUPPORTED_FLAGS (PROCESS_VM_PIDFD | PROCESS_VM_NOWAIT) > > /** > * process_vm_rw_pages - read/write pages from task specified > @@ -68,6 +71,7 @@ static int process_vm_rw_pages(struct page **pages, > * @mm: mm for task > * @task: task to read/write from > * @vm_write: 0 means copy from, 1 means copy to > + * @pvm_flags: PROCESS_VM_* flags > * Returns 0 on success or on failure error code > */ > static int process_vm_rw_single_vec(unsigned long addr, > @@ -76,7 +80,8 @@ static int process_vm_rw_single_vec(unsigned long addr, > struct page **process_pages, > struct mm_struct *mm, > struct task_struct *task, > - int vm_write) > + int vm_write, > + unsigned long pvm_flags) > { > unsigned long pa = addr & PAGE_MASK; > unsigned long start_offset = addr - pa; > @@ -91,6 +96,8 @@ static int process_vm_rw_single_vec(unsigned long addr, > > if (vm_write) > flags |= FOLL_WRITE; > + if (pvm_flags & PROCESS_VM_NOWAIT) > + flags |= FOLL_NOWAIT; > > while (!rc && nr_pages && iov_iter_count(iter)) { > int pinned_pages = min_t(unsigned long, nr_pages, PVM_MAX_USER_PAGES); > @@ -141,7 +148,7 @@ static int process_vm_rw_single_vec(unsigned long addr, > * @iter: where to copy to/from locally > * @rvec: iovec array specifying where to copy to/from in the other process > * @riovcnt: size of rvec array > - * @flags: currently unused > + * @flags: process_vm_readv/writev flags > * @vm_write: 0 if reading from other process, 1 if writing to other process > * > * Returns the number of bytes read/written or error code. May > @@ -163,6 +170,7 @@ static ssize_t process_vm_rw_core(pid_t pid, struct iov_iter *iter, > unsigned long nr_pages_iov; > ssize_t iov_len; > size_t total_len = iov_iter_count(iter); > + unsigned int f_flags; > > /* > * Work out how many pages of struct pages we're going to need > @@ -194,10 +202,18 @@ static ssize_t process_vm_rw_core(pid_t pid, struct iov_iter *iter, > } > > /* Get process information */ > - task = find_get_task_by_vpid(pid); > - if (!task) { > - rc = -ESRCH; > - goto free_proc_pages; > + if (flags & PROCESS_VM_PIDFD) { > + task = pidfd_get_task(pid, &f_flags); > + if (IS_ERR(task)) { > + rc = PTR_ERR(task); This could return -EBADF or -ESRCH. We should document both in the man page. (or decide to always return -ESRCH, dunno) -- Cheers, David