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

[Lokesh Gidra]

On Tue, Feb 13, 2024 at 9:06 AM Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> wrote:
>
> * Lokesh Gidra <lokeshgidra@xxxxxxxxxx> [240213 06:25]:
> > On Mon, Feb 12, 2024 at 7:33 PM Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> wrote:
> > >
> > > * Lokesh Gidra <lokeshgidra@xxxxxxxxxx> [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@xxxxxxxxxx>
> > > > ---
> > > >  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