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 >
