Re: double fault -> PAGE_KERNEL flagged memory

[Tom Zanussi <zanussi at us dot ibm dot com>]

Mathieu Desnoyers writes:
 > * 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.
 > 

That would be one option, but how about instrumenting the vmalloc
fault case to check whether the faulting address lies inside the
buffer and if so, don't log an event in that case.  You wouldn't see
those vmalloc faults touching the relayfs buffers, but I'm not sure
that's interesting anyway...

Tom