linux-kernel - Re: [PATCH 3/4] tools/selftests: expand all guard region tests to file-backed

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Message-ID: <7d5e0f61-66d9-471c-b6ef-bf68dbffa614@lucifer.local>
Date: Tue, 22 Apr 2025 11:47:17 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Matthew Wilcox <willy@...radead.org>, Vlastimil Babka <vbabka@...e.cz>,
        "Paul E . McKenney" <paulmck@...nel.org>, Jann Horn <jannh@...gle.com>,
        David Hildenbrand <david@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Shuah Khan <shuah@...nel.org>,
        linux-kselftest@...r.kernel.org, linux-api@...r.kernel.org,
        John Hubbard <jhubbard@...dia.com>, Juan Yescas <jyescas@...gle.com>,
        Kalesh Singh <kaleshsingh@...gle.com>
Subject: Re: [PATCH 3/4] tools/selftests: expand all guard region tests to
 file-backed

On Tue, Apr 22, 2025 at 11:37:57AM +0100, Ryan Roberts wrote:
> On 13/02/2025 18:17, Lorenzo Stoakes wrote:
> > Extend the guard region tests to allow for test fixture variants for anon,
> > shmem, and local file files.
> >
> > This allows us to assert that each of the expected behaviours of anonymous
> > memory also applies correctly to file-backed (both shmem and an a file
> > created locally in the current working directory) and thus asserts the same
> > correctness guarantees as all the remaining tests do.
> >
> > The fixture teardown is now performed in the parent process rather than
> > child forked ones, meaning cleanup is always performed, including unlinking
> > any generated temporary files.
> >
> > Additionally the variant fixture data type now contains an enum value
> > indicating the type of backing store and the mmap() invocation is
> > abstracted to allow for the mapping of whichever backing store the variant
> > is testing.
> >
> > We adjust tests as necessary to account for the fact they may now reference
> > files rather than anonymous memory.
>
> Hi Lorenzo,
>
> I'm getting a test failure in v6.15-rc3 on arm64:
>
> ----8<----
> #  RUN           guard_regions.shmem.uffd ...
> # guard-regions.c:1467:uffd:Expected ioctl(uffd, UFFDIO_REGISTER, &reg) (-1) ==
> 0 (0)
> # uffd: Test terminated by assertion
> #          FAIL  guard_regions.shmem.uffd
> not ok 45 guard_regions.shmem.uffd
> ----8<----
>
> The ioctl is returning EINVAL.

Hm strange, that works fine <resists urge to say 'on my machine'> on x86-64. Is
userfaultfd enabled in your config, to ask a silly question?

It'd be odd for this to vary depending upon arch.

So a factor here is a _stupidity_ in the testing - does your system mount /tmp
as tmpfs or an actual file system? As the test code unconditionally assumes /tmp
is indeed going to get you a shmem file.

It's shameful to be honest. But actually I suspect this more than anything
else...

>
> [...]
>
> > @@ -1281,6 +1398,9 @@ TEST_F(guard_regions, uffd)
> >  	struct uffdio_register reg;
> >  	struct uffdio_range range;
> >
> > +	if (!is_anon_backed(variant))
>
> Perhaps this should be filtering out shmem too? Although the manual for
> userfaultfd implies that shmem is supported:

Yeah it should work with it fine.

>
> """
> Up to Linux 4.11, userfaultfd can be used only with anonymous private memory
> mappings.  Since Linux 4.11, userfaultfd can be also used with hugetlbfs and
> shared memory mappings.
> """
>
> But I'm not sure if that's referring specifically to UFFDIO_REGISTER_MODE_MISSING?
>
> Any ideas before I start debugging further?
>
> Thanks,
> Ryan
>
> > +		SKIP(return, "uffd only works on anon backing");
> > +
> >  	/* Set up uffd. */
> >  	uffd = userfaultfd(0);
> >  	if (uffd == -1 && errno == EPERM)
> > @@ -1290,8 +1410,8 @@ TEST_F(guard_regions, uffd)
> >  	ASSERT_EQ(ioctl(uffd, UFFDIO_API, &api), 0);
> >
> >  	/* Map 10 pages. */
> > -	ptr = mmap(NULL, 10 * page_size, PROT_READ | PROT_WRITE,
> > -		   MAP_ANON | MAP_PRIVATE, -1, 0);
> > +	ptr = mmap_(self, variant, NULL, 10 * page_size,
> > +		    PROT_READ | PROT_WRITE, 0, 0);
> >  	ASSERT_NE(ptr, MAP_FAILED);
> >
> >  	/* Register the range with uffd. */
>