Re: [RFC][PATCH] x86/mm: Sync all vmalloc mappings before text_poke()

[Steven Rostedt]

On Thu, 30 Apr 2020 12:18:22 -0400 (EDT)
Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote:

> ----- On Apr 30, 2020, at 12:16 PM, rostedt rostedt@xxxxxxxxxxx wrote:
> 
> > On Thu, 30 Apr 2020 11:20:15 -0400 (EDT)
> > Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
> >   
> >> > The right fix is to call vmalloc_sync_mappings() right after allocating
> >> > tracing or perf buffers via v[zm]alloc().  
> >> 
> >> Either right after allocation, or right before making the vmalloc'd data
> >> structure visible to the instrumentation. In the case of the pid filter,
> >> that would be the rcu_assign_pointer() which publishes the new pid filter
> >> table.
> >> 
> >> As long as vmalloc_sync_mappings() is performed somewhere *between* allocation
> >> and publishing the pointer for instrumentation, it's fine.
> >> 
> >> I'll let Steven decide on which approach works best for him.  
> > 
> > As stated in the other email, I don't see it having anything to do with
> > vmalloc, but with the per_cpu() allocation. I'll test this theory out by
> > not even allocating the pid masks and touching the per cpu data at every
> > event to see if it crashes.  
> 
> As pointed out in my other email, per-cpu allocation uses vmalloc when
> size > PAGE_SIZE.

And as I replied:

	buf->data = alloc_percpu(struct trace_array_cpu);

struct trace_array_cpu {
	atomic_t		disabled;
	void			*buffer_page;	/* ring buffer spare */

	unsigned long		entries;
	unsigned long		saved_latency;
	unsigned long		critical_start;
	unsigned long		critical_end;
	unsigned long		critical_sequence;
	unsigned long		nice;
	unsigned long		policy;
	unsigned long		rt_priority;
	unsigned long		skipped_entries;
	u64			preempt_timestamp;
	pid_t			pid;
	kuid_t			uid;
	char			comm[TASK_COMM_LEN];

	bool			ignore_pid;
#ifdef CONFIG_FUNCTION_TRACER
	bool			ftrace_ignore_pid;
#endif
};

That doesn't look bigger than PAGE_SIZE to me.

-- Steve