Re: [PATCH v3] userfaultfd/shmem: fix MCOPY_ATOMIC_CONTNUE behavior

[Peter Xu]

Axel,

On Tue, Mar 30, 2021 at 04:30:13PM -0700, Axel Rasmussen wrote:
> Yes, a refactor like that is promising. It's hard to say for certain
> without actually looking at the result - I'll spend some time tomorrow
> on a few options, and send along the cleanest version I come up with.

Before you move onto a new version...  See this commit:

5b51072e97d5 ("userfaultfd: shmem: allocate anonymous memory for MAP_PRIVATE shmem", 2018-11-30)

I found it when I was thinking why not move the whole continue logic directly
into mfill_atomic_pte(), if we can have the pte installation helper, because
that's all we need.

So previously I got the semantics a bit mixed up: for private shmem mappings,
UFFDIO_COPY won't fill in page cache at all, but it's all private.  We keep the
page cache empty even after UFFDIO_COPY for a private mapping.

UFFDIO_CONTINUE is slightly different, since we _know_ the page cache is
there..  So I'm thinking maybe you need to handle the continue request in
mfill_atomic_pte() before the VM_SHARED check so as to cover both cases.

-- 
Peter Xu