Re: [RESEND][PATCH 3/5] perf: Use current->flags & PF_KTHREAD|PF_USER_WORKER instead of current->mm == NULL

[Steven Rostedt <rostedt@kernel.org>]

On Wed, 12 Nov 2025 19:11:15 -0800
Guenter Roeck <linux@roeck-us.net> wrote:

> Hi Steven,

Hi Guenter,

Somehow this got filed away in my archive without me seeing it.

> 
> On Wed, Aug 20, 2025 at 02:03:41PM -0400, Steven Rostedt wrote:
> > From: Steven Rostedt <rostedt@goodmis.org>
> > 
> > To determine if a task is a kernel thread or not, it is more reliable to
> > use (current->flags & (PF_KTHREAD|PF_USER_WORKERi)) than to rely on
> > current->mm being NULL.  That is because some kernel tasks (io_uring
> > helpers) may have a mm field.
> > 
> > Link: https://lore.kernel.org/linux-trace-kernel/20250424163607.GE18306@noisy.programming.kicks-ass.net/
> > Link: https://lore.kernel.org/all/20250624130744.602c5b5f@batman.local.home/
> > 
> > Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> > ---
> >  kernel/events/callchain.c | 6 +++---
> >  kernel/events/core.c      | 4 ++--
> >  2 files changed, 5 insertions(+), 5 deletions(-)
> > 
> > diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
> > index cd0e3fc7ed05..5982d18f169b 100644
> > --- a/kernel/events/callchain.c
> > +++ b/kernel/events/callchain.c
> > @@ -246,10 +246,10 @@ get_perf_callchain(struct pt_regs *regs, bool kernel, bool user,
> >  
> >  	if (user && !crosstask) {
> >  		if (!user_mode(regs)) {
> > -			if  (current->mm)
> > -				regs = task_pt_regs(current);
> > -			else
> > +			if (current->flags & (PF_KTHREAD | PF_USER_WORKER))
> >  				regs = NULL;
> > +			else
> > +				regs = task_pt_regs(current);
> >  		}
> >  
> >  		if (regs) {
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index bade8e0fced7..f880cec0c980 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -7446,7 +7446,7 @@ static void perf_sample_regs_user(struct perf_regs *regs_user,
> >  	if (user_mode(regs)) {
> >  		regs_user->abi = perf_reg_abi(current);
> >  		regs_user->regs = regs;
> > -	} else if (!(current->flags & PF_KTHREAD)) {
> > +	} else if (!(current->flags & (PF_KTHREAD | PF_USER_WORKER))) {
> >  		perf_get_regs_user(regs_user, regs);
> >  	} else {
> >  		regs_user->abi = PERF_SAMPLE_REGS_ABI_NONE;
> > @@ -8086,7 +8086,7 @@ static u64 perf_virt_to_phys(u64 virt)
> >  		 * Try IRQ-safe get_user_page_fast_only first.
> >  		 * If failed, leave phys_addr as 0.
> >  		 */
> > -		if (current->mm != NULL) {
> > +		if (!(current->flags & (PF_KTHREAD | PF_USER_WORKER))) {  
> 
> Subsequent code uses current->mm. This triggers a crash when running a page
> table stress test. See below for details. I have seen the crash in 6.12.57
> and 6.18-rc5.

Hmm, that should not happen. But obvious it is. Can you add this:

diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
index 1f6589578703..ff201098e5e5 100644
--- a/kernel/events/callchain.c
+++ b/kernel/events/callchain.c
@@ -248,6 +248,8 @@ get_perf_callchain(struct pt_regs *regs, bool kernel, bool user,
 		if (!user_mode(regs)) {
 			if (current->flags & (PF_KTHREAD | PF_USER_WORKER))
 				goto exit_put;
+			if (WARN_ONCE(!current->mm, "Bad flags %x", current->flags))
+				goto exit_put;
 			regs = task_pt_regs(current);
 		}
 

I'd like to see what current->flags are when ->mm is NULL.

Thanks!

-- Steve