Re: [PATCH] mm: Don't fault around userfaultfd-registered regions on reads

[Mike Rapoport]

On Thu, Nov 26, 2020 at 05:23:59PM -0500, Peter Xu wrote:
> Faulting around for reads are in most cases helpful for the performance so that
> continuous memory accesses may avoid another trip of page fault.  However it
> may not always work as expected.
> 
> For example, userfaultfd registered regions may not be the best candidate for
> pre-faults around the reads.
> 
> For missing mode uffds, fault around does not help because if the page cache
> existed, then the page should be there already.  If the page cache is not
> there, nothing else we can do, either.  If the fault-around code is destined to
> be helpless for userfault-missing vmas, then ideally we can skip it.
> 
> For wr-protected mode uffds, errornously fault in those pages around could lead
> to threads accessing the pages without uffd server's awareness.  For example,
> when punching holes on uffd-wp registered shmem regions, we'll first try to
> unmap all the pages before evicting the page cache but without locking the
> page (please refer to shmem_fallocate(), where unmap_mapping_range() is called
> before shmem_truncate_range()).  When fault-around happens near a hole being
> punched, we might errornously fault in the "holes" right before it will be
> punched.  Then there's a small window before the page cache was finally
> dropped, and after the page will be writable again (NOTE: the uffd-wp protect
> information is totally lost due to the pre-unmap in shmem_fallocate(), so the
> page can be writable within the small window).  That's severe data loss.
> 
> Let's grant the userspace full control of the uffd-registered ranges, rather
> than trying to do the tricks.
> 
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>

One nit below, except that

Reviewed-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>

> ---
> 
> Note that since no file-backed uffd-wp support is there yet upstream, so the
> uffd-wp check is actually not really functioning.  However since we have all
> the necessary uffd-wp concepts already upstream, maybe it's better to do it
> once and for all.
> 
> This patch comes from debugging a data loss issue when working on the uffd-wp
> support on shmem/hugetlbfs.  I posted this out for early review and comments,
> but also because it should already start to benefit missing mode userfaultfd to
> avoid trying to fault around on reads.
> ---
>  include/linux/userfaultfd_k.h |  5 +++++
>  mm/memory.c                   | 17 +++++++++++++++++
>  2 files changed, 22 insertions(+)
> 
> diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> index a8e5f3ea9bb2..451d99bb3a1a 100644
> --- a/include/linux/userfaultfd_k.h
> +++ b/include/linux/userfaultfd_k.h
> @@ -62,6 +62,11 @@ static inline bool userfaultfd_wp(struct vm_area_struct *vma)
>  	return vma->vm_flags & VM_UFFD_WP;
>  }
> 
> +static inline bool vma_registered_userfaultfd(struct vm_area_struct *vma)
> +{
> +	return userfaultfd_missing(vma) || userfaultfd_wp(vma);
> +}

We have userfaultfd_armed() that does exectly this, don't we?

> +
>  static inline bool userfaultfd_pte_wp(struct vm_area_struct *vma,
>  				      pte_t pte)
>  {
> diff --git a/mm/memory.c b/mm/memory.c
> index eeae590e526a..ca58ada94c96 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3933,6 +3933,23 @@ static vm_fault_t do_fault_around(struct vm_fault *vmf)
>  	int off;
>  	vm_fault_t ret = 0;
> 
> +	/*
> +	 * Be extremely careful with uffd-armed regions.
> +	 *
> +	 * For missing mode uffds, fault around does not help because if the
> +	 * page cache existed, then the page should be there already.  If the
> +	 * page cache is not there, nothing else we can do either.
> +	 *
> +	 * For wr-protected mode uffds, errornously fault in those pages around
> +	 * could lead to threads accessing the pages without uffd server's
> +	 * awareness, finally it could cause ghostly data corruption.
> +	 *
> +	 * The idea is that, every single page of uffd regions should be
> +	 * governed by the userspace on which page to fault in.
> +	 */
> +	if (unlikely(vma_registered_userfaultfd(vmf->vma)))
> +		return 0;
> +
>  	nr_pages = READ_ONCE(fault_around_bytes) >> PAGE_SHIFT;
>  	mask = ~(nr_pages * PAGE_SIZE - 1) & PAGE_MASK;
> 
> -- 
> 2.26.2
> 

-- 
Sincerely yours,
Mike.