Re: [PATCH v7 1/2] mm: introduce process_mrelease system call

[David Hildenbrand <david@redhat.com>]

On 05.08.21 19:49, Suren Baghdasaryan wrote:
> On Thu, Aug 5, 2021 at 10:29 AM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 05.08.21 19:08, 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.
>>>
>>> After previous discussions [1, 2, 3] the decision was made [4] to introduce
>>> a dedicated system call to cover this use case.
>>>
>>> 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
>>>             an exiting process.
>>>
>>>             The pidfd selects the process referred to by the PID file
>>>             descriptor.
>>>             (See pidfd_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 memory of the task cannot be released because the
>>>                    process is not exiting, the address space is shared
>>>                    with another live process or there is a core dump in
>>>                    progress.
>>>
>>>             ENOSYS This system call is not supported, for example, without
>>>                    MMU support built into Linux.
>>>
>>>             ESRCH  The target process does not exist (i.e., it has terminated
>>>                    and been waited on).
>>>
>>> [1] https://lore.kernel.org/lkml/20190411014353.113252-3-surenb@google.com/
>>> [2] https://lore.kernel.org/linux-api/20201113173448.1863419-1-surenb@google.com/
>>> [3] https://lore.kernel.org/linux-api/20201124053943.1684874-3-surenb@google.com/
>>> [4] https://lore.kernel.org/linux-api/20201223075712.GA4719@lst.de/
>>>
>>> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
>>> ---
>>> changes in v7:
>>> - Fixed pidfd_open misspelling, per Andrew Morton
>>> - Fixed wrong task pinning after find_lock_task_mm() issue, per Michal Hocko
>>> - Moved MMF_OOM_SKIP check before task_will_free_mem(), per Michal Hocko
>>>
>>>    mm/oom_kill.c | 73 +++++++++++++++++++++++++++++++++++++++++++++++++++
>>>    1 file changed, 73 insertions(+)
>>>
>>> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
>>> index c729a4c4a1ac..a4d917b43c73 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 +1142,75 @@ 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;
>>> +     struct task_struct *p;
>>> +     unsigned int f_flags;
>>> +     struct pid *pid;
>>> +     long ret = 0;
>>> +
>>> +     if (flags)
>>> +             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.
>>> +      */
>>> +     p = find_lock_task_mm(task);
>>> +     if (!p) {
>>> +             ret = -ESRCH;
>>> +             goto put_pid;
>>> +     }
>>> +     if (task != p) {
>>> +             get_task_struct(p);
>>
>>
>> Wouldn't we want to obtain the mm from p ? I thought that was the whole
>> exercise of going via find_lock_task_mm().
> 
> Yes, that's what we do after checking task_will_free_mem().
> task_will_free_mem() requires us to hold task_lock and
> find_lock_task_mm() achieves that ensuring that mm is still valid, but
> it might return a task other than the original one. That's why we do
> this dance with pinning the new task and unpinning the original one.
> The same dance is performed in __oom_kill_process(). I was
> contemplating adding a parameter to find_lock_task_mm() to request
> this unpin/pin be done within that function but then decided to keep
> it simple for now.
> Did I address your question or did I misunderstand it?

Excuse my tired eyes, I missed the "task = p;"

Feel free to carry my ack along, even if there are minor changes.


-- 
Thanks,

David / dhildenb