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SubjectRe: [PATCH v2] mm: Don't fault around userfaultfd-registered regions on reads
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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@google.com>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Reviewed-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>
> 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@redhat.com>

--
Thanks,

David / dhildenb

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