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

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Message-ID: <c93110a4-19e4-4a1d-b044-6b7f521eaa0d@lucifer.local>
Date: Wed, 18 Jun 2025 15:07:11 +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 07:28:16PM +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.

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.