Messages in this thread | | | From | David Hildenbrand <> | Subject | Re: [PATCH v2 2/3] mm: introduce process_mrelease system call | Date | Wed, 21 Jul 2021 10:02:19 +0200 |
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On 18.07.21 23:41, Suren Baghdasaryan wrote: > In modern systems it's not unusual to have a system component monitoring > memory conditions of the system and tasked with keeping system memory > pressure under control. One way to accomplish that is to kill > non-essential processes to free up memory for more important ones. > Examples of this are Facebook's OOM killer daemon called oomd and > Android's low memory killer daemon called lmkd. > For such system component it's important to be able to free memory > quickly and efficiently. Unfortunately the time process takes to free > up its memory after receiving a SIGKILL might vary based on the state > of the process (uninterruptible sleep), size and OPP level of the core > the process is running. A mechanism to free resources of the target > process in a more predictable way would improve system's ability to > control its memory pressure. > Introduce process_mrelease system call that releases memory of a dying > process from the context of the caller. This way the memory is freed in > a more controllable way with CPU affinity and priority of the caller. > The workload of freeing the memory will also be charged to the caller. > The operation is allowed only on a dying process. > > Previously I proposed a number of alternatives to accomplish this: > - https://lore.kernel.org/patchwork/patch/1060407 extending > pidfd_send_signal to allow memory reaping using oom_reaper thread; > - https://lore.kernel.org/patchwork/patch/1338196 extending > pidfd_send_signal to reap memory of the target process synchronously from > the context of the caller; > - https://lore.kernel.org/patchwork/patch/1344419/ to add MADV_DONTNEED > support for process_madvise implementing synchronous memory reaping.
To me, this looks a lot cleaner. Although I do wonder why we need two separate mechanisms to achieve the end goal
1. send sigkill 2. process_mrelease
As 2. doesn't make sense without 1. it somehow feels like it would be optimal to achieve both steps in a single syscall. But I remember there were discussions around that.
> > The end of the last discussion culminated with suggestion to introduce a > dedicated system call (https://lore.kernel.org/patchwork/patch/1344418/#1553875) > The reasoning was that the new variant of process_madvise > a) does not work on an address range > b) is destructive > c) doesn't share much code at all with the rest of process_madvise > From the userspace point of view it was awkward and inconvenient to provide > memory range for this operation that operates on the entire address space. > Using special flags or address values to specify the entire address space > was too hacky. > > The API is as follows, > > int process_mrelease(int pidfd, unsigned int flags); > > DESCRIPTION > The process_mrelease() system call is used to free the memory of > a process which was sent a SIGKILL signal. > > The pidfd selects the process referred to by the PID file > descriptor. > (See pidofd_open(2) for further information) > > The flags argument is reserved for future use; currently, this > argument must be specified as 0. > > RETURN VALUE > On success, process_mrelease() returns 0. On error, -1 is > returned and errno is set to indicate the error. > > ERRORS > EBADF pidfd is not a valid PID file descriptor. > > EAGAIN Failed to release part of the address space. > > EINVAL flags is not 0. > > EINVAL The task does not have a pending SIGKILL or its memory is > shared with another process with no pending SIGKILL. > > ENOSYS This system call is not supported by kernels built with no > MMU support (CONFIG_MMU=n). > > ESRCH The target process does not exist (i.e., it has terminated > and been waited on). > > Signed-off-by: Suren Baghdasaryan <surenb@google.com> > --- > mm/oom_kill.c | 55 +++++++++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 55 insertions(+) > > diff --git a/mm/oom_kill.c b/mm/oom_kill.c > index d04a13dc9fde..7fbfa70d4e97 100644 > --- a/mm/oom_kill.c > +++ b/mm/oom_kill.c > @@ -28,6 +28,7 @@ > #include <linux/sched/task.h> > #include <linux/sched/debug.h> > #include <linux/swap.h> > +#include <linux/syscalls.h> > #include <linux/timex.h> > #include <linux/jiffies.h> > #include <linux/cpuset.h> > @@ -755,10 +756,64 @@ static int __init oom_init(void) > return 0; > } > subsys_initcall(oom_init) > + > +SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags) > +{ > + struct pid *pid; > + struct task_struct *task; > + struct mm_struct *mm = NULL; > + unsigned int f_flags; > + long ret = 0;
Nit: reverse Christmas tree.
> + > + if (flags != 0) > + return -EINVAL; > + > + pid = pidfd_get_pid(pidfd, &f_flags); > + if (IS_ERR(pid)) > + return PTR_ERR(pid); > + > + task = get_pid_task(pid, PIDTYPE_PID); > + if (!task) { > + ret = -ESRCH; > + goto put_pid; > + } > + > + /* > + * If the task is dying and in the process of releasing its memory > + * then get its mm. > + */ > + task_lock(task); > + if (task_will_free_mem(task) && (task->flags & PF_KTHREAD) == 0) { > + mm = task->mm; > + mmget(mm); > + }
AFAIU, while holding the task_lock, task->mm won't change and we cannot see a concurrent exit_mm()->mmput(). So the mm structure and the VMAs won't go away while holding the task_lock(). I do wonder if we need the mmget() at all here.
Also, I wonder if it would be worth dropping the task_lock() while reaping - to unblock anybody else wanting to lock the task. Getting a hold of the mm and locking the mmap_lock would be sufficient I guess.
In general, looks quite good to me.
-- Thanks,
David / dhildenb
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