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

On Tue, Aug 3, 2021 at 12:48 AM David Hildenbrand [off-list ref] wrote:

[...]

quoted

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@google.com/

instead.

Ack.

quoted

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@google.com/
"

Ack.

quoted

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.

quoted

           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.

quoted

           ESRCH  The target process does not exist (i.e., it has terminated
                  and been waited on).

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
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.

quoted

+             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 <redacted>

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