linux-kernel - Re: [PATCH 1/6] mm/selftests: Fix virtual_address

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Message-ID: <fc5c8193-2642-49f7-9f2a-00ad33353773@arm.com>
Date: Wed, 18 Jun 2025 19:28:16 +0530
From: Dev Jain <dev.jain@....com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Aboorva Devarajan <aboorvad@...ux.ibm.com>, akpm@...ux-foundation.org,
 Liam.Howlett@...cle.com, shuah@...nel.org, pfalcato@...e.de,
 david@...hat.com, ziy@...dia.com, baolin.wang@...ux.alibaba.com,
 npache@...hat.com, ryan.roberts@....com, baohua@...nel.org,
 linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
 linux-kernel@...r.kernel.org, donettom@...ux.ibm.com, ritesh.list@...il.com
Subject: Re: [PATCH 1/6] mm/selftests: Fix virtual_address_range test issues.

On 18/06/25 5:27 pm, Lorenzo Stoakes wrote:
> On Wed, Jun 18, 2025 at 05:15:50PM +0530, Dev Jain wrote:
>> On 18/06/25 5:07 pm, Lorenzo Stoakes wrote:
>>> On Wed, Jun 18, 2025 at 04:58:56PM +0530, Dev Jain wrote:
>>>> MAP_CHUNK_SIZE was chosen randomly. Good to see it translates into something logical : )
>>>>
>>>> So I guess I am correct, if we can find two VMAs (except at the edge of the high addr boundary)
>>>> with a gap of greater than MAP_CHUNK_SIZE then there is a bug in mmap().
>>> No haha, not at all!! Firstly fixed addressed override a lot of this, secondly
>>> the 256 page gap (which is configurable btw) is only applicable for mappings
>>> below a stack (in stack grow down arch).
>> Sorry, I was making that assertion w.r.t this specific selftest. What the test
>> is doing is exhausting VA space without passing a hint or MAP_FIXED. With this
>> context, where does this assertion fail? One of them will be if the stack guard
>> gap is more than 256 pages.
> Are you accounting for sys.max_map_count? If not, then you'll be hitting that
> first.

run_vmtests.sh will run the test in overcommit mode so that won't be an issue.

>
>> Also, note that the test hasn't reported frequent failures post my change, so
>> in general settings, w.r.t this test, the assertion experimentally seems to
>> be true : )
> I don't really have time to dig into the test in detail sorry too much else on
> at the moment.
>
> But it isn't a big problem even if it happened to turn out that this test isn't
> really testing quite what you expected :)