Re: [PATCH v3 9/9] userfaultfd/selftests: add test exercising minor fault handling

[Peter Xu <peterx@redhat.com>]

On Thu, Jan 28, 2021 at 02:48:19PM -0800, Axel Rasmussen wrote:
> Fix a dormant bug in userfaultfd_events_test(), where we did
> `return faulting_process(0)` instead of `exit(faulting_process(0))`.
> This caused the forked process to keep running, trying to execute any
> further test cases after the events test in parallel with the "real"
> process.
> 
> Add a simple test case which exercises minor faults. In short, it does
> the following:
> 
> 1. "Sets up" an area (area_dst) and a second shared mapping to the same
>    underlying pages (area_dst_alias).
> 
> 2. Register one of these areas with userfaultfd, in minor fault mode.
> 
> 3. Start a second thread to handle any minor faults.
> 
> 4. Populate the underlying pages with the non-UFFD-registered side of
>    the mapping. Basically, memset() each page with some arbitrary
>    contents.
> 
> 5. Then, using the UFFD-registered mapping, read all of the page
>    contents, asserting that the contents match expectations (we expect
>    the minor fault handling thread can modify the page contents before
>    resolving the fault).
> 
> The minor fault handling thread, upon receiving an event, flips all the
> bits (~) in that page, just to prove that it can modify it in some
> arbitrary way. Then it issues a UFFDIO_CONTINUE ioctl, to setup the
> mapping and resolve the fault. The reading thread should wake up and see
> this modification.
> 
> Currently the minor fault test is only enabled in hugetlb_shared mode,
> as this is the only configuration the kernel feature supports.
> 
> Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>

Reviewed-by: Peter Xu <peterx@redhat.com>

-- 
Peter Xu