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From: Lokesh Gidra <lokeshgidra@google.com>
Date: Tue, 13 Feb 2024 10:14:38 -0800
Message-ID: <CA+EESO5URPpJj35-jQy+Lrp1EtKms8r1ri2ZY3ZOpsSJU+CScw@mail.gmail.com>
Subject: Re: [PATCH v5 3/3] userfaultfd: use per-vma locks in userfaultfd operations
To: "Liam R. Howlett" <Liam.Howlett@oracle.com>, Lokesh Gidra <lokeshgidra@google.com>, 
	akpm@linux-foundation.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, 
	linux-kernel@vger.kernel.org, selinux@vger.kernel.org, surenb@google.com, 
	kernel-team@android.com, aarcange@redhat.com, peterx@redhat.com, 
	david@redhat.com, axelrasmussen@google.com, bgeffon@google.com, 
	willy@infradead.org, jannh@google.com, kaleshsingh@google.com, 
	ngeoffray@google.com, timmurray@google.com, rppt@kernel.org
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On Tue, Feb 13, 2024 at 9:06=E2=80=AFAM Liam R. Howlett <Liam.Howlett@oracl=
e.com> wrote:
>
> * Lokesh Gidra <lokeshgidra@google.com> [240213 06:25]:
> > On Mon, Feb 12, 2024 at 7:33=E2=80=AFPM Liam R. Howlett <Liam.Howlett@o=
racle.com> wrote:
> > >
> > > * Lokesh Gidra <lokeshgidra@google.com> [240212 19:19]:
> > > > All userfaultfd operations, except write-protect, opportunistically=
 use
> > > > per-vma locks to lock vmas. On failure, attempt again inside mmap_l=
ock
> > > > critical section.
> > > >
> > > > Write-protect operation requires mmap_lock as it iterates over mult=
iple
> > > > vmas.
> > > >
> > > > Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
> > > > ---
> > > >  fs/userfaultfd.c              |  13 +-
> > > >  include/linux/userfaultfd_k.h |   5 +-
> > > >  mm/userfaultfd.c              | 392 ++++++++++++++++++++++++++----=
----
> > > >  3 files changed, 312 insertions(+), 98 deletions(-)
> > > >
> > > ...
>
> I just remembered an issue with the mmap tree that exists today that you
> needs to be accounted for in this change.
>
> If you hit a NULL VMA, you need to fall back to the mmap_lock() scenario
> today.

Unless I'm missing something, isn't that already handled in the patch?
We get the VMA outside mmap_lock critical section only via
lock_vma_under_rcu() (in lock_vma() and find_and_lock_vmas()) and in
both cases if we get NULL in return, we retry in mmap_lock critical
section with vma_lookup(). Wouldn't that suffice?
>
> This is a necessity to avoid a race of removal/replacement of a VMA in
> the mmap(MAP_FIXED) case.  In this case, we munmap() prior to mmap()'ing
> an area - which means you could see a NULL when there never should have
> been a null.
>
> Although this would be exceedingly rare, you need to handle this case.
>
> Sorry I missed this earlier,
> Liam