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

[Mathieu Desnoyers]

----- On Apr 30, 2020, at 12:30 PM, rostedt rostedt@xxxxxxxxxxx wrote:

> 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.

Let me point you to:

pcpu_alloc()
  calling pcpu_create_chunk()

which is then responsible for calling the underlying 
pcpu_mem_zalloc() which then uses vmalloc. So batching
those allocations can be responsible for using vmalloc'd
memory rather than kmalloc'd even though the allocation
size is smaller than 4kB.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com