Re: [PATCH v2] mm: Don't fault around userfaultfd-registered regions on reads

[David Hildenbrand]

On 01.12.20 00:06, 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>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Reviewed-by: Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> ---
> 
> v2:
> - use userfaultfd_armed() directly [Mike]
> 
> 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.
> ---
>  mm/memory.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index eeae590e526a..59b2be22565e 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(userfaultfd_armed(vmf->vma)))
> +		return 0;
> +
>  	nr_pages = READ_ONCE(fault_around_bytes) >> PAGE_SHIFT;
>  	mask = ~(nr_pages * PAGE_SIZE - 1) & PAGE_MASK;
>  
> 

Thanks for the clarifying comment.

Acked-by: David Hildenbrand <david@xxxxxxxxxx>

-- 
Thanks,

David / dhildenb