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Message-ID: <95eff329-a7b1-dc2d-026c-fd61e476c846@redhat.com>
Date: Tue, 3 Aug 2021 09:48:06 +0200
From: David Hildenbrand <david@...hat.com>
To: Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org
Cc: mhocko@...nel.org, mhocko@...e.com, rientjes@...gle.com,
willy@...radead.org, hannes@...xchg.org, guro@...com,
riel@...riel.com, minchan@...nel.org, christian@...uner.io,
hch@...radead.org, oleg@...hat.com, jannh@...gle.com,
shakeelb@...gle.com, luto@...nel.org, christian.brauner@...ntu.com,
fweimer@...hat.com, jengelh@...i.de, timmurray@...gle.com,
linux-api@...r.kernel.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v4 1/2] mm: introduce process_mrelease system call
[...]
> 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.
> 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/
"
>
> 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.
>
> 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."
>
> ESRCH The target process does not exist (i.e., it has terminated
> and been waited on).
>
> Signed-off-by: Suren Baghdasaryan <surenb@...gle.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)
> + 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@...hat.com>
--
Thanks,
David / dhildenb
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