[LTP] [PATCH] madvise07: Increase probability of testing a supported page type

[Jan Stancek <jstancek@redhat.com>]

----- Original Message -----
> Hi!
> > > We were attempting to poison page types which do not support it (e.g. the
> > > zero
> > > page) due to our usage of mmap. Now we map some anonymous memory and
> > > write to
> > > it. Hopefully ensuring the underlying page is of a supported type.
> > > + * If the underlying page type of the memory we have mapped does not
> > > support
> > > + * poisoning then the test will fail. We try to map and write to the
> > > memory
> > > in
> > > + * such a way that by the time madvise is called the virtual memory
> > > address
> > > + * points to a supported page. However there may be some rare
> > > circumstances
> > > + * where the test produces the wrong result because we have somehow
> > > obtained
> > > + * an unsupported page.
> > 
> > Hi,
> > 
> > Can you elaborate please? If we always do mmap+touch anonymous memory,
> > how would we sometime end up with different page type?
> 
> Wouldn't this mean that the API is broken by desing?
> 
> One thing is that the call does not work on an unmapped page and fails
> to return an error. But if we cannot even guarantee that it will work
> if we make an effort to fault the page in advance its horribly broken by
> design.

I think Richard was talking about scenario where something happens
to page you just faulted in, e.g. it's swapped out for some reason.
That should be quite unlikely.

> 
> > I don't have objections to patch, but I'm thinking if we should go
> > further if there's possibility the test still won't be reliable.
> > We could relax the condition, for example by FAILing only if
> > child dies unexpectedly (signal != SIGBUS).
> 
> What would that mean, producing TCONF on any error from the madvise()
> call? Looking at manual pages the only error we may get running
> MADVISE_HWPOISON as a root on a mapped page is the EINVAL we handle
> anyway.

I'd stay with just "mmap+touch anonymous memory" for now and see if
that ever fails.

Regards,
Jan