Re: double fault -> PAGE_KERNEL flagged memory

[Mathieu Desnoyers <compudj at krystal dot dyndns dot org>]

* Tom Zanussi (zanussi@us.ibm.com) wrote:
> Mathieu Desnoyers writes:
>  > Generally speaking, what would happen if :
>  > 
>  > - A page fault handler has a trace point that logs an event in the RelayFS
>  >   buffer.
>  > - This exact write causes a (minor) page fault.
>  > 
>  > It may not happen often, but I think it would lead to a double fault. Is this
>  > case possible ?
>  > 
> 
> Yes, I'm sure it is possible, in fact I'm sure it happens all the time
> in LTT and it's never been a problem.  Have you verified that it is a
> problem (e.g. tracing only page faults and seeing a double fault).
> 
> 

I have just done this, and nothing bad happened and I discovered why :

The LTT instrumentation of do_page_fault comes right after the
"We fault-in kernel-space virtual memory on-demand." section. So, minor page
faults are not instrumented.

Maybe this isn't so interesting to instrument it, but if we would like to, I
suggest to add an optionnal support for kmalloc'd memory areas in RelayFS so
the client can ask for a channel which consists of a small buffer that is
reentrant for the memory subsystem. This should be documented with a big
warning about the small size the client should ask for.


Mathieu

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