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

[Michal Hocko <mhocko@suse.com>]

On Mon 20-04-26 14:47:04, Minchan Kim wrote:
> On Fri, Apr 17, 2026 at 09:04:31AM +0200, Michal Hocko wrote:
> > On Thu 16-04-26 23:30:09, Minchan Kim wrote:
> > > If I send the SIGKILL first to satisfy the process_mrelease() requirement,
> > > we immediately run into the scheduling race condition where the victim can
> > > enter the exit path before the reaper can set the flag.
> > 
> > Why don't you just grab the mm before you send the signal and then continue
> > with reaping? You just want to avoid a race where the victim manages to
> > process fatal signal, start its exit path and mrelease path losing that
> > race so you rely on the exit path, right?
> 
> The problem is that process_mrelease() operates on a task obtained from a pidfd.
> 
> Once the victim process receives the SIGKILL and enters the exit path (exit_mm),
> the kernel sets task->mm to NULL.
> 
> Even if we could somehow hold a reference to the mm_struct beforehand,
> process_mrelease() would still fail because mm_struct via task returns NULL
> after exit_mm() has been called.
> 
> Therefore, we cannot simply "grab the mm" before sending the signal and expect
> process_mrelease() to work after the victim starts exiting.

I do not follow. Why cannot you simply do this
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 5c6c95c169ee..b80a96f5460a 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -1241,9 +1241,14 @@ SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags)
 	if (task_will_free_mem(p))
 		reap = true;
 	else {
+		if (flags & PROCESS_MRELEASE_REAP_KILL) {
+		} else {
+			/* send SIGKILL */
+			reap = true;
 		/* Error only if the work has not been done already */
-		if (!mm_flags_test(MMF_OOM_SKIP, mm))
-			ret = -EINVAL;
+			if (!mm_flags_test(MMF_OOM_SKIP, mm))
+				ret = -EINVAL;
+		}
 	}
 	task_unlock(p);
 

-- 
Michal Hocko
SUSE Labs