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

[Lokesh Gidra <lokeshgidra@google.com>]

On Tue, Feb 13, 2024 at 10:57 AM Suren Baghdasaryan <surenb@google.com> wrote:
>
> 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.

I anyway have to send another version to fix the error handling that
you reported earlier. I can take care of this in that version.

mfill_atomic...() functions (annoyingly) have to sometimes unlock and
relock. Using prepare/complete in that context seems incompatible.

>
> 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.

I'll wait for an agreement on this because I too don't like using so
many ifdef's either.

Since these functions are supposed to have prototype depending on
mfill/move, how about the following names:

uffd_lock_mfill_vma()/uffd_unlock_mfill_vma()
uffd_lock_move_vmas()/uffd_unlock_move_vmas()

Of course, I'm open to other suggestions as well.

> Anyway, I don't see this as a blocker, just nice to have.
>
> >
> > Thanks,
> > Liam
> >
> > --
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