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

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Message-ID: <2756fa2b-e8bf-4c66-bf9b-c85dc63dfc33@lucifer.local>
Date: Wed, 18 Jun 2025 15:35:57 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Dev Jain <dev.jain@....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 Wed, Jun 18, 2025 at 07:47:18PM +0530, Dev Jain wrote:
>
> On 18/06/25 7:37 pm, Lorenzo Stoakes wrote:
> > On Wed, Jun 18, 2025 at 07:28:16PM +0530, Dev Jain wrote:
> > > On 18/06/25 5:27 pm, Lorenzo Stoakes wrote:
> > > > On Wed, Jun 18, 2025 at 05:15:50PM +0530, Dev Jain wrote:
> > > > 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.
> > Umm, what? You mean overcommit all mode, and that has no bearing on the max
> > mapping count check.
> >
> > In do_mmap():
> >
> > 	/* Too many mappings? */
> > 	if (mm->map_count > sysctl_max_map_count)
> > 		return -ENOMEM;
> >
> >
> > As well as numerous other checks in mm/vma.c.
>
> Ah sorry, didn't look at the code properly just assumed that overcommit_always meant overriding
> this.

No problem! It's hard to be aware of everything in mm :)

>
> >
> > I'm not sure why an overcommit toggle is even necessary when you could use
> > MAP_NORESERVE or simply map PROT_NONE to avoid the OVERCOMMIT_GUESS limits?
> >
> > I'm pretty confused as to what this test is really achieving honestly. This
> > isn't a useful way of asserting mmap() behaviour as far as I can tell.
>
> Well, seems like a useful way to me at least : ) Not sure if you are in the mood
> to discuss that but if you'd like me to explain from start to end what the test
> is doing, I can do that : )
>

I just don't have time right now, I guess I'll have to come back to it
later... it's not the end of the world for it to be iffy in my view as long as
it passes, but it might just not be of great value.

Philosophically I'd rather we didn't assert internal implementation details like
where we place mappings in userland memory. At no point do we promise to not
leave larger gaps if we feel like it :)

I'm guessing, reading more, the _real_ test here is some mathematical assertion
about layout from HIGH_ADDR_SHIFT -> end of address space when using hints.

But again I'm not sure that achieves much and again also is asserting internal
implementation details.

Correct behaviour of this kind of thing probably better belongs to tests in the
userland VMA testing I'd say.

Sorry I don't mean to do down work you've done before, just giving an honest
technical appraisal!

Anyway don't let this block work to fix the test if it's failing. We can revisit
this later.