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

[Richard Palethorpe <rpalethorpe@suse.de>]

Hello Jan,

Jan Stancek writes:

> ----- Original Message -----
>> 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?

Possibly if the page is completely swapped out and it is not in the page
cache. I am not sure if that is even possible, but my point was that we
can't fully gaurantee that we are testing a valid page type because it
is not something you can fully control from user land (unless there is some
debug interface which allows it). I'm not sure if it is a helpful
comment or not.

>
> 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).

I think that if we see another failure and it is not a real bug, then we
do that.

--
Thank you,
Richard.