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

[Matthew Wilcox <willy@infradead.org>]

On Thu, Nov 26, 2020 at 05:23:59PM -0500, Peter Xu wrote:
> 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.

But it might have been faulted into the cache by another task, so skipping
it is bad.

> 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.

Sounds like you have a missing page_mkwrite implementation.

> 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.

A measurable difference?