Re: [PATCH v4 1/2] mm: introduce process_mrelease system call

[Suren Baghdasaryan]

On Tue, Aug 3, 2021 at 12:48 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> [...]
>
> > Previously I proposed a number of alternatives to accomplish this:
> > - https://lore.kernel.org/patchwork/patch/1060407 extending
>
> I have no idea how stable these links are. Referencing via message id is
> the common practice. For this link, we'd use
>
> https://lkml.kernel.org/r/20190411014353.113252-3-surenb@xxxxxxxxxx/
>
> instead.

Ack.

>
> > 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.
> >
> > 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.
>
> I'd condense this description and only reference previous discussions to
> put a main focus on what this patch actually does. Like
>
> "
> After previous discussions [1, 2, 3] the decision was made to introduce
> a dedicated system call to cover this use case.
>
> ...
>
> [1] https://lkml.kernel.org/r/20190411014353.113252-3-surenb@xxxxxxxxxx/
> "
>

Ack.

> >
> > 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.
> >
> >            EINTR  The call was interrupted by a signal; see signal(7).
> >
> >            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.
>
> Hm, I do wonder if it would make sense to have a mode (e.g., via a flag)
> to reap all but shared memory from a dying process. Future work.

Agree. Let's keep it simple for now and will expand when the need arises.

>
> >
> >            ENOSYS This system call is not supported by kernels built with no
> >                   MMU support (CONFIG_MMU=n).
>
> Maybe "This system call is not supported, for example, without MMU
> support built into Linux."

Ack.

>
> >
> >            ESRCH  The target process does not exist (i.e., it has terminated
> >                   and been waited on).
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> > ---
> > changes in v4:
> > - Replaced mmap_read_lock() with mmap_read_lock_killable(), per Michal Hocko
> > - Added EINTR error in the manual pages documentation
> >
> >   mm/oom_kill.c | 58 +++++++++++++++++++++++++++++++++++++++++++++++++++
> >   1 file changed, 58 insertions(+)
> >
> > diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> > index c729a4c4a1ac..86727794b0a8 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>
> > @@ -1141,3 +1142,60 @@ void pagefault_out_of_memory(void)
> >       out_of_memory(&oc);
> >       mutex_unlock(&oom_lock);
> >   }
> > +
> > +SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags)
> > +{
> > +#ifdef CONFIG_MMU
> > +     struct mm_struct *mm = NULL;
> > +     struct task_struct *task;
> > +     unsigned int f_flags;
> > +     struct pid *pid;
> > +     long ret = 0;
> > +
> > +     if (flags != 0)
>
> if (flags)

Ack.

>
> > +             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);
> > +     }
> > +     task_unlock(task);
> > +     if (!mm) {
> > +             ret = -EINVAL;
> > +             goto put_task;
> > +     }
> > +
> > +     if (mmap_read_lock_killable(mm)) {
> > +             ret = -EINTR;
> > +             goto put_mm;
> > +     }
> > +     if (!__oom_reap_task_mm(mm))
> > +             ret = -EAGAIN;
>
> I'm not an expert on __oom_reap_task_mm(), but the whole approach makes
> sense to. So feel free to add my
>
> Acked-by: David Hildenbrand <david@xxxxxxxxxx>

Thanks! I see Michal also asked for some documentation changes and a
simple code change, so I won't roll your Acked-by automatically into
the next version but would appreciate it on the final version :)
Will post the next rev later today or tomorrow morning.
Thanks for the review!

>
> --
> Thanks,
>
> David / dhildenb
>