Re: [RFC PATCH 6/6] selftests/mm: add nommu mmap and mremap behavior tests

[Hajime Tazaki <thehajime@gmail.com>]

On Fri, 14 Aug 2026 22:28:53 +0900,
Lorenzo Stoakes (ARM) wrote:
> 
> On Thu, Aug 13, 2026 at 03:34:01PM +0900, Hajime Tazaki wrote:
> > Introduce a kselftest utility to validate memory mapping capabilities
> > under nommu kernels, aligned with
> > Documentation/admin-guide/mm/nommu-mmap.rst.
> >
> > The test implements basic checks into a generic architecture-agnostic
> > test matrix applicable across nommu targets. It evaluates:
> >
> > 1. MAP_FIXED allocation rejections.
> > 2. Standard MAP_PRIVATE and MAP_ANONYMOUS allocation resilience.
> > 3. MAP_UNINITIALIZED allocations via optional kernel configurations.
> > 4. Regular file mappings via standard filesystem storage.
> > 5. Memory-backed file mapping with /dev/zero
> > 6. Block device subsystem mappings (gracefully skipping if node is
> >    missing).
> > 7. Shared vs Private backing discrepancies under nommu conditions.
> > 8. mremap limits, ensuring non-expandable restrictions behave properly.
> 
> Thanks for adding these.
> 
> It's horrible to not have the kselftest harness for these, but as established,
> fork() doesn't work for nommu (!)
> 
> A lot of these tests seem to relate to functionality I'm not really happy with
> in the rest of the series so let's see how things pan out with the series
> overall I guess!

(snip)

> > diff --git a/tools/testing/selftests/mm/nommu_mremap_test.c b/tools/testing/selftests/mm/nommu_mremap_test.c
> > new file mode 100644
> > index 000000000000..b1b8a4e64b55
> > --- /dev/null
> > +++ b/tools/testing/selftests/mm/nommu_mremap_test.c
> > @@ -0,0 +1,583 @@
(snip)
> > +
> > +static int get_shared_writable_file_expected_error(const char *path)
> > +{
> > +	if (get_fs_type(path) == RAMFS_MAGIC)
> > +		return EPERM; /* ramfs failed */
> > +
> > +	return 0;
> > +}
> > +
> > +static int pre_conf_trim_page(void)
> > +{
> > +	return set_nr_trim_pages("0\n");
> > +}
> > +
> > +static int post_conf_trim_page(int value)
> > +{
> > +	char buf[32];
> > +
> > +	if (snprintf(buf, sizeof(buf), "%d\n", value) < 0)
> > +		return -EIO;
> > +
> > +	return set_nr_trim_pages(buf);
> > +}
> > +
> > +struct mremap_case_t {
> > +	const char *name;
> > +	const char *pathname;
> > +	int open_flags;
> > +	int mmap_prot;
> > +	int mmap_flags;
> > +	int exp_err;
> > +	int (*resolve_exp_err)(const char *path);
> > +	unsigned int old_pages;
> > +	unsigned int new_pages;
> > +	int (*pre_hook)(void);
> > +	int (*post_hook)(int value);
> > +};
> > +
> > +static struct mremap_case_t mremap_cases[] = {
> > +	{
> > +		.name = "anonymous shrink (r--)",
> > +		.pathname = NULL,
> > +		.open_flags = O_CREAT | O_RDWR | O_EXCL,
> > +		.mmap_prot = PROT_READ,
> > +		.mmap_flags = MAP_ANONYMOUS | MAP_PRIVATE,
> > +		.exp_err = 0,
> > +		.resolve_exp_err = 0,
> > +	},
> > +	{
> > +		.name = "shared file shrink (r--)",
> > +		.pathname = "/tmp/ksft.nommu-remap-XXXXXX",
> > +		.open_flags = O_CREAT | O_RDWR | O_EXCL,
> > +		.mmap_prot = PROT_READ,
> > +		.mmap_flags = MAP_SHARED,
> > +		.exp_err = 0,
> > +#ifdef CONFIG_NOMMU
> > +		.resolve_exp_err = get_shared_writable_file_expected_error,
> > +#else
> 
> Hmm aren't these all nommu tests anyway?

those tests are indeed for nommu but also compare with mmu systems.
actually I was trying to implement test behaviors which are described
in Documentation/admin-guide/mm/nommu-mmap.rst.

this particular case ("shared file shrink (r--)") is not described in
the doc, but test with a conditional error on nommu, and expects
success on mmu kernel, which is interesting (at least for nommu folks)
to validate future regressions.

I think now if I split into several series of this patchset, I would
drop this particular chunk which is needed for other series, and keep
only basic tests for nommu kernel which is described in the doc.

-- Hajime