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

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Feb 13, 2024 at 10:49 AM Liam R. Howlett
<Liam.Howlett@oracle.com> wrote:
>
> * Suren Baghdasaryan <surenb@google.com> [240213 13:25]:
> > On Tue, Feb 13, 2024 at 10:14 AM Lokesh Gidra <lokeshgidra@google.com> wrote:
> > >
> > > On Tue, Feb 13, 2024 at 9:06 AM Liam R. Howlett <Liam.Howlett@oracle.com> wrote:
> > > >
> > > > * Lokesh Gidra <lokeshgidra@google.com> [240213 06:25]:
> > > > > On Mon, Feb 12, 2024 at 7:33 PM Liam R. Howlett <Liam.Howlett@oracle.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_lock
> > > > > > > critical section.
> > > > > > >
> > > > > > > Write-protect operation requires mmap_lock as it iterates over multiple
> > > > > > > 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?
> >
> > I think that case is handled correctly by lock_vma().
>
> Yeah, it looks good.  I had a bit of a panic as I forgot to check that
> and I was thinking of a previous version.  I rechecked and v5 looks
> good.
>
> >
> > Sorry for coming back a bit late. The overall patch looks quite good
> > but the all these #ifdef CONFIG_PER_VMA_LOCK seem unnecessary to me.
> > Why find_and_lock_vmas() and lock_mm_and_find_vmas() be called the
> > same name (find_and_lock_vmas()) and in one case it would lock only
> > the VMA and in the other case it takes mmap_lock? Similarly
> > unlock_vma() would in one case unlock the VMA and in the other drop
> > the mmap_lock? That would remove all these #ifdefs from the code.
> > Maybe this was already discussed?
>
> Yes, I don't think we should be locking the mm in lock_vma(), as it
> makes things hard to follow.
>
> We could use something like uffd_prepare(), uffd_complete() but I
> thought of those names rather late in the cycle, but I've already caused
> many iterations of this patch set and that clean up didn't seem as vital
> as simplicity and clarity of the locking code.

Maybe lock_vma_for_uffd()/unlock_vma_for_uffd()? Whatever name is
better I'm fine with it but all these #ifdef's sprinkled around don't
contribute to the readability.
Anyway, I don't see this as a blocker, just nice to have.

>
> Thanks,
> Liam
>
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