Re: [PATCH v10 05/14] unwind_user/deferred: Add unwind cache

[Steven Rostedt <rostedt@goodmis.org>]

On Wed, 18 Jun 2025 16:13:45 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> > diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h
> > index f94f3fdf15fc..6e850c9d3f0c 100644
> > --- a/include/linux/entry-common.h
> > +++ b/include/linux/entry-common.h
> > @@ -12,6 +12,7 @@
> >  #include <linux/resume_user_mode.h>
> >  #include <linux/tick.h>
> >  #include <linux/kmsan.h>
> > +#include <linux/unwind_deferred.h>
> >  
> >  #include <asm/entry-common.h>
> >  #include <asm/syscall.h>
> > @@ -362,6 +363,7 @@ static __always_inline void exit_to_user_mode(void)
> >  	lockdep_hardirqs_on_prepare();
> >  	instrumentation_end();
> >  
> > +	unwind_exit_to_user_mode();  
> 
> So I was expecting this to do the actual unwind, and was about to go
> yell this is the wrong place for that.
> 
> But this is not that. Perhaps find a better name like:
> unwind_clear_cache() or so?

Sure.

How about unwind_reset_info()?

As it's not going to just clear the cache but also reset the trace info
(like the timestamp and such).


> 
> >  	user_enter_irqoff();
> >  	arch_exit_to_user_mode();
> >  	lockdep_hardirqs_on(CALLER_ADDR0);  
> 
> 
> > diff --git a/include/linux/unwind_deferred_types.h b/include/linux/unwind_deferred_types.h
> > index aa32db574e43..db5b54b18828 100644
> > --- a/include/linux/unwind_deferred_types.h
> > +++ b/include/linux/unwind_deferred_types.h
> > @@ -2,8 +2,13 @@
> >  #ifndef _LINUX_UNWIND_USER_DEFERRED_TYPES_H
> >  #define _LINUX_UNWIND_USER_DEFERRED_TYPES_H
> >  
> > +struct unwind_cache {
> > +	unsigned int		nr_entries;
> > +	unsigned long		entries[];
> > +};
> > +
> >  struct unwind_task_info {
> > -	unsigned long		*entries;
> > +	struct unwind_cache	*cache;
> >  };
> >  
> >  #endif /* _LINUX_UNWIND_USER_DEFERRED_TYPES_H */
> > diff --git a/kernel/unwind/deferred.c b/kernel/unwind/deferred.c
> > index 0bafb95e6336..e3913781c8c6 100644
> > --- a/kernel/unwind/deferred.c
> > +++ b/kernel/unwind/deferred.c
> > @@ -24,6 +24,7 @@
> >  int unwind_deferred_trace(struct unwind_stacktrace *trace)
> >  {
> >  	struct unwind_task_info *info = &current->unwind_info;
> > +	struct unwind_cache *cache;
> >  
> >  	/* Should always be called from faultable context */
> >  	might_fault();
> > @@ -31,17 +32,30 @@ int unwind_deferred_trace(struct unwind_stacktrace *trace)
> >  	if (current->flags & PF_EXITING)
> >  		return -EINVAL;
> >  
> > -	if (!info->entries) {
> > -		info->entries = kmalloc_array(UNWIND_MAX_ENTRIES, sizeof(long),
> > -					      GFP_KERNEL);
> > -		if (!info->entries)
> > +	if (!info->cache) {
> > +		info->cache = kzalloc(struct_size(cache, entries, UNWIND_MAX_ENTRIES),
> > +				      GFP_KERNEL);  
> 
> And now you're one 'long' larger than a page. Surely that's a crap size
> for an allocator?

Bah, Ingo suggested to put the counter in the allocation and I didn't think
about the size going over the page. Good catch!

Since it can make one per task, it may be good to make this into a
kmemcache.

-- Steve