On Tue, Apr 28, 2026 at 10:05:49PM +0200, David Hildenbrand (Arm) wrote: > On 4/28/26 14:28, Alban Crequy wrote: > > From: Alban Crequy <[email protected]> > > 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 <[email protected]> > > --- > > 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 <linux/ptrace.h> > > #include <linux/slab.h> > > #include <linux/syscalls.h> > > +#include <linux/process_vm.h> > > + > > +#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)
No, please don't. Let's not start overwriting errnos that are actually useful information for userspace.
