linux-kernel - Re: [PATCH v2 2/3] userfaultfd: protect mmap_changing with rw_sem in userfaulfd

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Message-ID: <ZcnTZiyHRf4Dj3P2@kernel.org>
Date: Mon, 12 Feb 2024 10:14:30 +0200
From: Mike Rapoport <rppt@...nel.org>
To: Lokesh Gidra <lokeshgidra@...gle.com>
Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>, akpm@...ux-foundation.org,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, selinux@...r.kernel.org,
	surenb@...gle.com, kernel-team@...roid.com, aarcange@...hat.com,
	peterx@...hat.com, david@...hat.com, axelrasmussen@...gle.com,
	bgeffon@...gle.com, willy@...radead.org, jannh@...gle.com,
	kaleshsingh@...gle.com, ngeoffray@...gle.com, timmurray@...gle.com
Subject: Re: [PATCH v2 2/3] userfaultfd: protect mmap_changing with rw_sem in
 userfaulfd_ctx

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@...nel.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.