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

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Message-ID: <CAJuCfpE+Nj9rQL=_pOtnAXSDnd9xEZt=r-j7guJnkXSy8wFNrw@mail.gmail.com>
Date: Wed, 16 Jul 2025 07:20:16 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org, Liam.Howlett@...cle.com, 
	vbabka@...e.cz, peterx@...hat.com, jannh@...gle.com, hannes@...xchg.org, 
	mhocko@...nel.org, paulmck@...nel.org, shuah@...nel.org, adobriyan@...il.com, 
	brauner@...nel.org, josef@...icpanda.com, yebin10@...wei.com, 
	linux@...ssschuh.net, willy@...radead.org, osalvador@...e.de, 
	andrii@...nel.org, ryan.roberts@....com, christophe.leroy@...roup.eu, 
	tjmercier@...gle.com, kaleshsingh@...gle.com, aha310510@...il.com, 
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	linux-mm@...ck.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v7 1/7] selftests/proc: add /proc/pid/maps tearing from
 vma split test

On Wed, Jul 16, 2025 at 3:50 AM Lorenzo Stoakes
<lorenzo.stoakes@...cle.com> wrote:
>
> 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@...gle.com>
> >
> > Why is this selftest not making use of any kselftest framework?
> >
> > I'm sure there is a very good reason :)

It used to be a part of proc-pid-vm.c and after the split I kept its
overall structure. I'll look into using the kselftest framework.
Thanks!

> >
> > 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.

Thanks for the pointers. I need to figure out a way to pass
command-line parameters to my test. Maybe I can use fixtures for
that... Let me read more about it.

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