Re: [PATCH v7 1/7] selftests/proc: add /proc/pid/maps tearing from vma split test

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Wed, Jul 16, 2025 at 12:44:23PM +0200, David Hildenbrand wrote:
> On 16.07.25 05:05, Suren Baghdasaryan wrote:
> > The /proc/pid/maps file is generated page by page, with the mmap_lock
> > released between pages.  This can lead to inconsistent reads if the
> > underlying vmas are concurrently modified. For instance, if a vma split
> > or merge occurs at a page boundary while /proc/pid/maps is being read,
> > the same vma might be seen twice: once before and once after the change.
> > This duplication is considered acceptable for userspace handling.
> > However, observing a "hole" where a vma should be (e.g., due to a vma
> > being replaced and the space temporarily being empty) is unacceptable.
> >
> > Implement a test that:
> > 1. Forks a child process which continuously modifies its address space,
> > specifically targeting a vma at the boundary between two pages.
> > 2. The parent process repeatedly reads the child's /proc/pid/maps.
> > 3. The parent process checks the last vma of the first page and
> > the first vma of the second page for consistency, looking for the
> > effects of vma splits or merges.
> >
> > The test duration is configurable via the -d command-line parameter
> > in seconds to increase the likelihood of catching the race condition.
> > The default test duration is 5 seconds.
> >
> > Example Command: proc-maps-race -d 10
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
>
> Why is this selftest not making use of any kselftest framework?
>
> I'm sure there is a very good reason :)
>
> Reading assert() feels very weird compared to other selftests.

Sorry to meta-review via your review again David :P

But just to say tools/testing/selftests/kselftest_harness.h is really good, and
makes life simple. See tools/testing/selftests/mm/guard-regions.c for an example
of how they can be used - pretty straightforward and avoids a lot of kselftest
boilerplate.

>
> --
> Cheers,
>
> David / dhildenb
>
>