Re: [PATCH v2 2/3] userfaultfd: protect mmap_changing with rw_sem in userfaulfd_ctx

[Mike Rapoport <rppt@kernel.org>]

On Wed, Feb 07, 2024 at 12:24:52PM -0800, Lokesh Gidra wrote:
> On Wed, Feb 7, 2024 at 7:27 AM Mike Rapoport <rppt@kernel.org> wrote:
> >
> > > > The write-lock is not a requirement here for correctness and I don't see
> > > > why we would need userfaultfd_remove_prep().
> > > >
> > > > As I've said earlier, having a write-lock here will let CRIU to run
> > > > background copy in parallel with processing of uffd events, but I don't
> > > > feel strongly about doing it.
> > > >
> > > Got it. Anyways, such a change needn't be part of this patch, so I'm
> > > going to keep it unchanged.
> >
> > You mean with a read lock?
> 
> No, I think write lock is good as it enables parallel background copy.
> Also because it brings consistency in blocking userfaultfd operations.
> 
> I meant encapsulating remove operations within
> userfaultfd_remove_prep() and userfaultfd_remove_complete(). I
> couldn't figure out any need for that.

I don't think there is a need for that. With fork/mremap prep is required
to ensure there's uffd context for new vmas.
 
-- 
Sincerely yours,
Mike.