Message-ID: <c978c50f-16b0-4514-8169-96b69bc7af33@arm.com>
Date: Mon, 19 Jan 2026 14:36:05 +0530
From: Dev Jain <dev.jain@....com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 David Hildenbrand <david@...nel.org>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
 Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 linux-kselftest@...r.kernel.org, Mark Brown <broonie@...nel.org>,
 aneesh.kumar@...nel.org, Anshuman Khandual <anshuman.khandual@....com>
Subject: Re: [PATCH] selftests/mm: remove virtual_address_range test


On 19/01/26 2:29 pm, Lorenzo Stoakes wrote:
> On Mon, Jan 19, 2026 at 11:51:47AM +0530, Dev Jain wrote:
>>> Well no, you're asserting gap lengths repeatedly, you are making assertions
>>> about get_unmapped_area() behaviour that are totally inappropriate in a
>>> self-test.
>> Apologies - so I discussed with Aneesh and Anshuman (CCed) and it turns out that the objective
>> of the test was to test the switch boundary. Upon exhaustion of the lower VA space, kernel
>> must not start giving out VMAs in the higher VA space, if the hint address is not given. The
>> original commit is 4e5ce33ceb32 ("selftests/vm: add a test for virtual address range mapping").
> This doesn't change anything, this is still testing get_unmapped_area() which by
> definition is what is returning this.
>
> Also exhausting VA space is an inherently silly thing for a test to do, you're
> making assumptions about existing VMA layout which is absolutely an
> implementation detail and may even be influence by libc...
>
>> I cannot find this API requirement on the man page (because no one bothered to update it),
>> but it is mentioned in Documentation/arch/arm64/memory.rst:
>>
>> "To maintain compatibility with software that relies on the ARMv8.0 VA space maximum size
>> of 48-bits, the kernel will, by default, return virtual addresses to userspace from
>> a 48-bit range.
>>
>> Software can "opt-in" to receiving VAs from a 52-bit space by specifying an mmap hint
>> parameter that is larger than 48-bit."
>>
>> So this is a thing that needs to be tested on arm64, and on ppc64 (for which the test
>> was originally added). Not sure about x86.
> Well 'needs' is strong here...
>
> It would be far more efficient to implement this as a kunit test and wouldn't
> require a extremely slow test that makes assumptions about VMA layout.
>
>> About internal impl details, how is this test any different from merge.c, cow.c,
>> etc - which consistently test/depend on whether the VMA splits/merges?
> This is not a hugely civil/productive way of responding here to be honest, it's
> what-about-ery and implying something that isn't very kind...

Sorry if I have offended you, I did not mean to imply "two wrongs make a right", I
meant to understand how the two tests differ...

>
> But since I am a reasonable if grumpy maintainer, let me indulge you a second
> here.
>
> I thought I'd been clear BUT for avoidance of doubt, I want to remove this test
> because of the COMBINATION of:
>
> 1. It is completely broken and has been broken for some time and nobody noticed.
> 2. It is asserting kernel implementation details.
> 3. It is poorly implemented and breaks often.
> 4. It takes a very long time to run even on fast machines and is a timeout risk.
>
> So even if you had a point, it wouldn't argue against removal.
>
> But you do not - both VMA merge and CoW impact API. Re: merging certain
> user-facing functions, most notably mremap(), have API requirements that the
> user must not cross VMA boundaries. It is therefore ENTIRELY a user-facing and
> kernel/user API thing that has to be tested from this perspective.
>
> CoW is equally a documented and expected behaviour and also affects merging.
>
> Anyway.
>
> Practically speaking I think there are two ways forward here (not mutually
> exclusive):
>
> 1. Implement something in kunit or similar that explicitly tests
>    get_unmapped_area().
>
> 2. Add a _new_ selftest, named something sensible like mmap_hint.c or something,
>    that runs only on relevant arches, and does NOT try to do crazy stuff like
>    mapping the entire VA space, but instead simply tries some trial unhinted
>    mappings some hints in 48-bit space, and some hints in 52-bit space and
>    asserts things are as expected.
>
> If you do point 2, please please use a. use the kselftest_harness.h to write the
> tests in a nice way (see e.g. guard-regions.c for an example of how it's used)
> and b. use the procmap helpers in vm_util.h to check on VMA ranges, you can see
> how they're used in... the merge.c tests you so deride :)
>
> If you or others do both/either I promise to dedicate review resource to the
> series(es). That fair enough?
>
> Thanks, Lorenzo

Powered by blists - more mailing lists