[RFC 3/3] mm: process_mrelease: introduce PROCESS_MRELEASE_REAP_KILL flag

[Minchan Kim <minchan@kernel.org>]

Currently, process_mrelease() requires userspace to send a SIGKILL signal
prior to invocation. This separation introduces a race window where the
victim task may receive the signal and enter the exit path before the
reaper can invoke process_mrelease().

In this case, the victim task frees its memory via the standard, unoptimized
exit path, bypassing the expedited clean file folio reclamation optimization
introduced in the previous patch (which relies on the MMF_UNSTABLE flag).

This patch introduces the PROCESS_MRELEASE_REAP_KILL UAPI flag to support
an integrated auto-kill mode. When specified, process_mrelease() directly
injects a SIGKILL into the target task.

Crucially, this patch utilizes a dedicated signal code (KILL_MRELEASE)
during signal injection, belonging to a new SIGKILL si_codes section.
This special code ensures that the kernel's signal delivery path reliably
intercepts the request and marks the target address space as unstable
(MMF_UNSTABLE). This mechanism guarantees that the MMF_UNSTABLE flag is set
before either the victim task or the reaper proceeds, ensuring that the
expedited reclamation optimization is utilized regardless of scheduling
order.

Signed-off-by: Minchan Kim <minchan@kernel.org>
---
 include/uapi/asm-generic/siginfo.h |  6 ++++++
 include/uapi/linux/mman.h          |  4 ++++
 kernel/signal.c                    |  4 ++++
 mm/oom_kill.c                      | 20 +++++++++++++++++++-
 4 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/include/uapi/asm-generic/siginfo.h b/include/uapi/asm-generic/siginfo.h
index 5a1ca43b5fc6..0f59b791dab4 100644
--- a/include/uapi/asm-generic/siginfo.h
+++ b/include/uapi/asm-generic/siginfo.h
@@ -252,6 +252,12 @@ typedef struct siginfo {
 #define BUS_MCEERR_AO	5
 #define NSIGBUS		5
 
+/*
+ * SIGKILL si_codes
+ */
+#define KILL_MRELEASE	1	/* sent by process_mrelease */
+#define NSIGKILL	1
+
 /*
  * SIGTRAP si_codes
  */
diff --git a/include/uapi/linux/mman.h b/include/uapi/linux/mman.h
index e89d00528f2f..4266976b45ad 100644
--- a/include/uapi/linux/mman.h
+++ b/include/uapi/linux/mman.h
@@ -56,4 +56,8 @@ struct cachestat {
 	__u64 nr_recently_evicted;
 };
 
+/* Flags for process_mrelease */
+#define PROCESS_MRELEASE_REAP_KILL	(1 << 0)
+#define PROCESS_MRELEASE_VALID_FLAGS	(PROCESS_MRELEASE_REAP_KILL)
+
 #endif /* _UAPI_LINUX_MMAN_H */
diff --git a/kernel/signal.c b/kernel/signal.c
index d65d0fe24bfb..c21b2176dc5e 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1134,6 +1134,10 @@ static int __send_signal_locked(int sig, struct kernel_siginfo *info,
 
 out_set:
 	signalfd_notify(t, sig);
+
+	if (sig == SIGKILL && !is_si_special(info) &&
+	    info->si_code == KILL_MRELEASE && t->mm)
+		mm_flags_set(MMF_UNSTABLE, t->mm);
 	sigaddset(&pending->signal, sig);
 
 	/* Let multiprocess signals appear after on-going forks */
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 5c6c95c169ee..0b5da5208707 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -20,6 +20,8 @@
 
 #include <linux/oom.h>
 #include <linux/mm.h>
+#include <uapi/linux/mman.h>
+#include <linux/capability.h>
 #include <linux/err.h>
 #include <linux/gfp.h>
 #include <linux/sched.h>
@@ -1218,13 +1220,29 @@ SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags)
 	bool reap = false;
 	long ret = 0;
 
-	if (flags)
+	if (flags & ~PROCESS_MRELEASE_VALID_FLAGS)
 		return -EINVAL;
 
 	task = pidfd_get_task(pidfd, &f_flags);
 	if (IS_ERR(task))
 		return PTR_ERR(task);
 
+	if (flags & PROCESS_MRELEASE_REAP_KILL) {
+		struct kernel_siginfo info;
+
+		if (!capable(CAP_KILL)) {
+			ret = -EPERM;
+			goto put_task;
+		}
+		clear_siginfo(&info);
+		info.si_signo = SIGKILL;
+		info.si_code = KILL_MRELEASE;
+		info.si_pid = task_tgid_vnr(current);
+		info.si_uid = from_kuid_munged(current_user_ns(), current_uid());
+
+		do_send_sig_info(SIGKILL, &info, task, PIDTYPE_TGID);
+	}
+
 	/*
 	 * Make sure to choose a thread which still has a reference to mm
 	 * during the group exit
-- 
2.54.0.rc0.605.g598a273b03-goog