linux-kernel - Re: [PATCH v2 3/3] Turn off test_uffdio_wp if CONFIG_PTE_MARKER_UFFD

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Date: Mon, 24 Jun 2024 09:53:57 -0400
From: Audra Mitchell <audra@...hat.com>
To: Peter Xu <peterx@...hat.com>
Cc: viro@...iv.linux.org.uk, brauner@...nel.org, jack@...e.cz,
	aarcange@...hat.com, akpm@...ux-foundation.org,
	rppt@...ux.vnet.ibm.com, shli@...com, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, shuah@...nel.org,
	linux-kselftest@...r.kernel.org, raquini@...hat.com
Subject: Re: [PATCH v2 3/3] Turn off test_uffdio_wp if
 CONFIG_PTE_MARKER_UFFD_WP is not configured.

On Fri, Jun 21, 2024 at 05:27:43PM -0400, Peter Xu wrote:
> On Fri, Jun 21, 2024 at 02:12:24PM -0400, Audra Mitchell wrote:
> > If CONFIG_PTE_MARKER_UFFD_WP is disabled, then testing with test_uffdio_up
> 
> Here you're talking about pte markers, then..
> 
> > enables calling uffdio_regsiter with the flag UFFDIO_REGISTER_MODE_WP. The
> > kernel ensures in vma_can_userfault() that if CONFIG_PTE_MARKER_UFFD_WP
> > is disabled, only allow the VM_UFFD_WP on anonymous vmas.
> > 
> > Signed-off-by: Audra Mitchell <audra@...hat.com>
> > ---
> >  tools/testing/selftests/mm/uffd-stress.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/tools/testing/selftests/mm/uffd-stress.c b/tools/testing/selftests/mm/uffd-stress.c
> > index b9b6d858eab8..2601c9dfadd6 100644
> > --- a/tools/testing/selftests/mm/uffd-stress.c
> > +++ b/tools/testing/selftests/mm/uffd-stress.c
> > @@ -419,6 +419,9 @@ static void parse_test_type_arg(const char *raw_type)
> >  	test_uffdio_wp = test_uffdio_wp &&
> >  		(features & UFFD_FEATURE_PAGEFAULT_FLAG_WP);
> >  
> > +	if (test_type != TEST_ANON && !(features & UFFD_FEATURE_WP_UNPOPULATED))
> > +		test_uffdio_wp = false;
> 
> ... here you're checking against wp_unpopulated.  I'm slightly confused.
> 
> Are you running this test over shmem/hugetlb when the WP feature isn't
> supported?
>
> I'm wondering whether you're looking for UFFD_FEATURE_WP_HUGETLBFS_SHMEM
> instead.

I can confirm, its all really confusing... So in userfaultfd_api, we disable
three features if CONFIG_PTE_MARKER_UFFD_WP is not enabled- including 
UFFD_FEATURE_WP_UNPOPULATED:

#ifndef CONFIG_PTE_MARKER_UFFD_WP
        uffdio_api.features &= ~UFFD_FEATURE_WP_HUGETLBFS_SHMEM;
        uffdio_api.features &= ~UFFD_FEATURE_WP_UNPOPULATED;
        uffdio_api.features &= ~UFFD_FEATURE_WP_ASYNC;
#endif

If you run the userfaultfd selftests with the run_vmtests script we get
several failures stemming from trying to call uffdio_regsiter with the flag 
UFFDIO_REGISTER_MODE_WP. However, the kernel ensures in vma_can_userfault() 
that if CONFIG_PTE_MARKER_UFFD_WP is disabled, only allow the VM_UFFD_WP -
which is set when you pass the UFFDIO_REGISTER_MODE_WP flag - on 
anonymous vmas.

In parse_test_type_arg() I added the features check against 
UFFD_FEATURE_WP_UNPOPULATED as it seemed the most well know feature/flag. I'm 
more than happy to take any suggestions and adapt them if you have any! 

Thanks in advance and happy Monday!

-- Audra


 
> Thanks,
> 
> > +
> >  	close(uffd);
> >  	uffd = -1;
> >  }
> > -- 
> > 2.44.0
> > 
> 
> -- 
> Peter Xu
>