Re: [PATCH v7 1/2] mm: introduce process_mrelease system call
From: David Hildenbrand
Date: Thu Aug 05 2021 - 13:55:35 EST
On 05.08.21 19:49, Suren Baghdasaryan wrote:
On Thu, Aug 5, 2021 at 10:29 AM David Hildenbrand <david@xxxxxxxxxx> 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@xxxxxxxxxx/
[2] https://lore.kernel.org/linux-api/20201113173448.1863419-1-surenb@xxxxxxxxxx/
[3] https://lore.kernel.org/linux-api/20201124053943.1684874-3-surenb@xxxxxxxxxx/
[4] https://lore.kernel.org/linux-api/20201223075712.GA4719@xxxxxx/
Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
---
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