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

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Message-ID: <f0344770-fd17-4f7d-b802-ba2f3570ac57@lucifer.local>
Date: Tue, 22 Apr 2025 12:07:47 +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 12:03:08PM +0100, Ryan Roberts wrote:
> On 22/04/2025 11:47, Lorenzo Stoakes wrote:
> > On Tue, Apr 22, 2025 at 11:37:57AM +0100, Ryan Roberts wrote:

[snip]

> >>
> >> 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?
>
> Yep, and the anon version of the test is passing, as are all the uffd tests.
>
> >
> > 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.
>
> Ahh that's probably it. I'm on Ubuntu and it looks like /tmp is just a dir on
> the rootfs (XFS in my case).
>
> Forcing a tmpfs to /tmp solved it.
>
> Looks like uffd-unit-tests (see shmem_allocate_area()) is just using memfd.
> Would it be reasonable to take that approach? Or just use anon+shared via mmap?

Yeah could be either memfd or MAP_ANON | MAP_SHARED. I think I did it this
way to make the code a little easier given all the variant stuff.

Unless you felt especially keen, I can add fixing this to my todos :)

Sorry about this! Entirely my fault, knew it wasn't ideal when I wrote it,
but with test code sometimes you (perhaps incorrectly) tolerate things you
wouldn't elsewhere...

Cheers, Lorenzo

>
> Thanks,
> Ryan
>

[snip]