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

Mathieu Desnoyers
2020-05-01T02:26:58Z

----- On Apr 30, 2020, at 9:13 PM, rostedt rostedt@goodmis.org wrote:

> [ Joerg, sending again this time not just to you. (hit reply to sender
> and not reply to all). Feel free to resend what you wrote before to this ]
>
> On Thu, 30 Apr 2020 21:14:34 +0200
> Joerg Roedel <jroedel@suse.de> wrote:
>
>> And alloc_percpu() calls down into pcpu_alloc(), which allocates new
>> percpu chunks using vmalloc() on x86. And there we are again in the
>> vmalloc area.
>
> So after a vmalloc() is made, should the page tables be synced?

Why should it ? Usually, the page fault handler is able to resolve the
resulting minor page faults lazily.

>
> This is a rather subtle bug, and I don't think it should be the caller of
> percpu_alloc() that needs to call vmalloc_sync_mappings().

Who said tracing was easy ? ;-)

> What's your suggestion for a fix?

I know the question is not addressed to me, but here are my 2 cents:

It's subtle because ftrace is tracing the page fault handler through
tracepoints. It would not make sense to slow down all vmalloc or
percpu_alloc() just because tracing recurses when tracing page faults.

I think the right approach to solve this is to call vmalloc_sync_mappings()
before any vmalloc'd memory ends up being observable by instrumentation.
This can be achieved by adding a vmalloc_sync_mappings call on tracepoint
registration like I proposed in my patchset a few week ago:

https://lore.kernel.org/r/20200409193543.18115-2-mathieu.desnoyers@efficios.com

The tracers just have to make sure they perform their vmalloc'd memory
allocation before registering the tracepoint which can touch it, else they
need to issue vmalloc_sync_mappings() on their own before making the
newly allocated memory observable by instrumentation.

This approach is not new: register_die_notifier() does exactly that today.

Thanks,

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.