process_mrelease: backport 5.15 syscall to 4.19

patches/process_mrelease/0001-process-mrelease-syscall.patch

@@ -0,0 +1,126 @@
+From 884a7e5964e06ed93c7771c0d7cf19c09a8946f1 Mon Sep 17 00:00:00 2001
+From: Pulsar Bot <pulsar@localhost>
+Date: Thu, 13 Aug 2026 11:00:00 +0000
+Subject: [PATCH] oom: backport process_mrelease syscall (5.15)
+
+Backport process_mrelease() from Linux 5.15 (syscall nr 448), adapted to
+Pulsar's 4.19 CAF base.
+
+The base already has pidfd_get_pid(), find_lock_task_mm(), task_will_free_mem()
+and __oom_reap_task_mm(), so the backport reuses those existing primitives.
+The mmap_read_lock_killable() API from newer kernels is expressed as the
+4.19 equivalent down_read_killable(&mm->mmap_sem).
+
+Also keep the MMF_OOM_SKIP recheck under mmap_sem, matching the upstream fix
+for process_mrelease racing with exit_mmap().
+
+Signed-off-by: Pulsar Bot <pulsar@localhost>
+---
+ arch/arm64/include/asm/unistd32.h |  2 ++
+ include/uapi/asm-generic/unistd.h |  2 ++
+ mm/oom_kill.c                     | 48 +++++++++++++++++++++++++++++++++++++++
+ 3 files changed, 52 insertions(+)
+
+diff --git a/arch/arm64/include/asm/unistd32.h b/arch/arm64/include/asm/unistd32.h
+--- a/arch/arm64/include/asm/unistd32.h
++++ b/arch/arm64/include/asm/unistd32.h
+@@ -837,6 +837,8 @@ __SYSCALL(__NR_pidfd_open, sys_pidfd_open)
+ __SYSCALL(__NR_process_madvise, sys_process_madvise)
+ #define __NR_epoll_pwait2 441
+ __SYSCALL(__NR_epoll_pwait2, compat_sys_epoll_pwait2)
++#define __NR_process_mrelease 448
++__SYSCALL(__NR_process_mrelease, sys_process_mrelease)
+ #define __NR_futex_waitv 449
+ __SYSCALL(__NR_futex_waitv, sys_futex_waitv)
+
+ /*
+  * Please add new compat syscalls above this comment and update
+
+diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
+--- a/include/uapi/asm-generic/unistd.h
++++ b/include/uapi/asm-generic/unistd.h
+@@ -754,6 +754,8 @@ __SYSCALL(__NR_process_madvise, sys_process_madvise)
+ __SC_COMP(__NR_epoll_pwait2, sys_epoll_pwait2, compat_sys_epoll_pwait2)
++#define __NR_process_mrelease 448
++__SYSCALL(__NR_process_mrelease, sys_process_mrelease)
+ #define __NR_futex_waitv 449
+ __SYSCALL(__NR_futex_waitv, sys_futex_waitv)
+
+ #undef __NR_syscalls
+ #define __NR_syscalls 450
+
+diff --git a/mm/oom_kill.c b/mm/oom_kill.c
+--- a/mm/oom_kill.c
++++ b/mm/oom_kill.c
+@@ -829,6 +829,54 @@ static bool task_will_free_mem(struct task_struct *task)
+
+ 	return ret;
+ }
++
++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;
++	bool reap = false;
++	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_TGID);
++	if (!task) {
++		ret = -ESRCH;
++		goto put_pid;
++	}
++
++	/* Make sure to choose a thread which still has an mm during group exit. */
++	p = find_lock_task_mm(task);
++	if (!p) {
++		ret = -ESRCH;
++		goto put_task;
++	}
++
++	mm = p->mm;
++	mmgrab(mm);
++
++	if (task_will_free_mem(p))
++		reap = true;
++	else if (!test_bit(MMF_OOM_SKIP, &mm->flags))
++		ret = -EINVAL;
++	task_unlock(p);
++
++	if (!reap)
++		goto drop_mm;
++
++	if (down_read_killable(&mm->mmap_sem)) {
++		ret = -EINTR;
++		goto drop_mm;
++	}
++
++	/* Serialize with exit_mmap() before walking mm->mmap. */
++	if (!test_bit(MMF_OOM_SKIP, &mm->flags) && !__oom_reap_task_mm(mm))
++		ret = -EAGAIN;
++	up_read(&mm->mmap_sem);
++
++drop_mm:
++	mmdrop(mm);
++put_task:
++	put_task_struct(task);
++put_pid:
++	put_pid(pid);
++	return ret;
++#else
++	return -ENOSYS;
++#endif /* CONFIG_MMU */
++}
+
+ static void __oom_kill_process(struct task_struct *victim)
+ {