Re: [PATCH] selftests/mm: Fix soft-dirty kselftest supported check

[Audra Mitchell <audra@redhat.com>]

Sorry! I missed this email so never responded!

On Tue, Feb 24, 2026 at 05:15:14PM +0100, David Hildenbrand (Arm) wrote:
> On 2/18/26 19:42, Audra Mitchell wrote:
> > On architectures with separate user address space, such as s390 or
> > those without an MMU, the call to __access_ok will return true.
> 
> Where is this __access_ok() you mention here? Somewhere in
> fs/proc/task_mmu.c?
>
> Where in the soft-dirty test is that triggered?
> 
> I'm wondering whether the soft-dirty test should be adjusted, but I did
> not yet understand from where this behavior is triggered.

The problem arises when we are checking to see what features/categories are
supported. The call chain for the soft-dirty program goes:

  main()
    ->test_simple()
      ->pagemap_is_softdirty()
        ->page_entry_is()
          ->pagemap_scan_supported()
            ->__pagemap_scan_get_categories()
              ->ioctl()
  
We enter the kernel with an ioctl, expecting to have an EFAULT returned (see
the comment from pagemap_scan_get_categories():
    
          /* Provide an invalid address in order to trigger EFAULT. */
        ret = __pagemap_scan_get_categories(fd, start, (struct page_region *) ~0UL);

Once we enter the kernel, we will check the arguments passed which includes the
call to access_ok: 

  do_pagemap_cmd()
    ->do_pagemap_scan()
      ->pagemap_scan_get_args()
        ->access_ok()

Here is the path within pagemap_scan_get_args where we expect to fail return
the EFAULT:

        if (arg->vec && !access_ok((void __user *)(long)arg->vec,
                                   size_mul(arg->vec_len, sizeof(struct page_region))))
                return -EFAULT;
                
However, if CONFIG_ALTERNATE_USER_ADDRESS_SPACE is enabled or if CONFIG_MMU is
NOT enabled, then we just return true:

        if (IS_ENABLED(CONFIG_ALTERNATE_USER_ADDRESS_SPACE) ||
            !IS_ENABLED(CONFIG_MMU))
                return true;

The intent appears to be just getting the categories available to us and
verifying that we have the feature available for testing. However, this corner
case means the soft-dirty test will fail with the following:

  # --------------------
  # running ./soft-dirty
  # --------------------
  # TAP version 13
  # 1..15
  # Bail out! PAGEMAP_SCAN succeeded unexpectedly
  # # Totals: pass:0 fail:0 xfail:0 xpass:0 skip:0 error:0
  # [FAIL]
  not ok 1 soft-dirty # exit=1
  # SUMMARY: PASS=0 SKIP=0 FAIL=1
  1..1
  
Since the intent is just to validate that the features are available to us for
testing, I think we can just modify the check so that we don't fail if we
return 0.
  
Let me know what you think, or if you have more questions!

> Do we have a Fixes: tag?

I always hesistate to add a Fixes tag on situations like this since this is a
corner case that was not considered by the original author. If we need a
fixes tag, then it would be:

Fixes: 600bca580579 ("selftests/mm: check that PAGEMAP_SCAN returns correct categories")

Thanks a bunch!
-- Audra Mitchell