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

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Message-ID: <a49ee718-eaa1-4f0a-8c3a-0312e40b1d40@lucifer.local>
Date: Wed, 18 Jun 2025 12:50:02 +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 12:37:29PM +0100, 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 : )
> >

To correct myself for being an idiot before, 256 x 4 KB is 1 MB not 1 GB
sorry :)

> > 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).
>
> This assumption is totally incorrect, sorry. I'd suggest making assertions about
> this is really not all that useful, as things vary by arch and kernel
> configuration.

You can play with this program to see what happens in reality.

On my system the mappings of first two VMAs are immediately adjacent, then
the other is >1MB below:

#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>

int main()
{
	char *ptr;

	ptr = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_ANON | MAP_PRIVATE, -1, 0);
	if (ptr == MAP_FAILED) {
		perror("mmap 1");
		return EXIT_FAILURE;
	}
	printf("ptr1 = %p\n", ptr);

	ptr = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_ANON | MAP_PRIVATE | MAP_GROWSDOWN, -1, 0);
	if (ptr == MAP_FAILED) {
		perror("mmap 2");
		return EXIT_FAILURE;
	}
	printf("ptr2 = %p\n", ptr);

	ptr = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_ANON | MAP_PRIVATE, -1, 0);
	if (ptr == MAP_FAILED) {
		perror("mmap 3");
		return EXIT_FAILURE;
	}
	printf("ptr3 = %p\n", ptr);


	return EXIT_SUCCESS;
}

The definitive answers are in the get unmapped area logic.

But again not very useful to test imo beyond hand-wavey basics (and you
have to check that against all arches to be sure your hand waving is always
true :)