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

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Date: Tue, 25 Jun 2024 16:05:58 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Peter Xu <peterx@...hat.com>
Cc: Audra Mitchell <audra@...hat.com>, viro@...iv.linux.org.uk,
 brauner@...nel.org, jack@...e.cz, aarcange@...hat.com,
 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 Mon, 24 Jun 2024 10:42:00 -0400 Peter Xu <peterx@...hat.com> wrote:

> >         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! 
> 
> There're documents for these features in the headers:
> 
> 	 * UFFD_FEATURE_WP_HUGETLBFS_SHMEM indicates that userfaultfd
> 	 * write-protection mode is supported on both shmem and hugetlbfs.
> 	 *
> 	 * UFFD_FEATURE_WP_UNPOPULATED indicates that userfaultfd
> 	 * write-protection mode will always apply to unpopulated pages
> 	 * (i.e. empty ptes).  This will be the default behavior for shmem
> 	 * & hugetlbfs, so this flag only affects anonymous memory behavior
> 	 * when userfault write-protection mode is registered.
> 
> While in this context ("test_type != TEST_ANON") IIUC the accurate feature
> to check is UFFD_FEATURE_WP_HUGETLBFS_SHMEM.
> 
> In most kernels they should behave the same indeed, but note that since
> UNPOPULATED was introduced later than shmem/hugetlb support, it means on
> some kernel the result of checking these two features will be different.

I'm unsure what to do with this series.  Peter, your review comments
are unclear - do you request updates?

Also, the patches weren't cc:linux-mm which limits the audience.  I'll
drop this version.  Audra, please continue to move this forward.