linux-kernel - Re: [PATCH v5 3/3] userfaultfd: use per-vma locks in userfaultfd operations

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Date: Tue, 13 Feb 2024 12:06:09 -0500
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Lokesh Gidra <lokeshgidra@...gle.com>
Cc: 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, rppt@...nel.org
Subject: Re: [PATCH v5 3/3] userfaultfd: use per-vma locks in userfaultfd
 operations

* Lokesh Gidra <lokeshgidra@...gle.com> [240213 06:25]:
> On Mon, Feb 12, 2024 at 7:33 PM Liam R. Howlett <Liam.Howlett@...cle.com> wrote:
> >
> > * Lokesh Gidra <lokeshgidra@...gle.com> [240212 19:19]:
> > > All userfaultfd operations, except write-protect, opportunistically use
> > > per-vma locks to lock vmas. On failure, attempt again inside mmap_lock
> > > critical section.
> > >
> > > Write-protect operation requires mmap_lock as it iterates over multiple
> > > vmas.
> > >
> > > Signed-off-by: Lokesh Gidra <lokeshgidra@...gle.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.

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