Re: [PATCH v2 3/3] userfaultfd: use per-vma locks in userfaultfd operations

[Liam R. Howlett]

* Lokesh Gidra <lokeshgidra@xxxxxxxxxx> [240130 21:49]:
> On Mon, Jan 29, 2024 at 6:58 PM Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> wrote:
> >
> > * Lokesh Gidra <lokeshgidra@xxxxxxxxxx> [240129 19:28]:
> > > On Mon, Jan 29, 2024 at 12:53 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
> > > >
> >

..

> >
> > > Your suggestion is definitely simpler and easier to follow, but due to
> > > the overflow situation that Suren pointed out, I would still need to
> > > keep the locking/boolean dance, no? IIUC, even if I were to return
> > > EAGAIN to the userspace, there is no guarantee that subsequent ioctls
> > > on the same vma will succeed due to the same overflow, until someone
> > > acquires and releases mmap_lock in write-mode.
> > > Also, sometimes it seems insufficient whether we managed to lock vma
> > > or not. For instance, lock_vma_under_rcu() checks if anon_vma (for
> > > anonymous vma) exists. If not then it bails out.
> > > So it seems to me that we have to provide some fall back in
> > > userfaultfd operations which executes with mmap_lock in read-mode.
> >
> > Fair enough, what if we didn't use the sequence number and just locked
> > the vma directly?
> 
> Looks good to me, unless someone else has any objections.
> >
> > /* This will wait on the vma lock, so once we return it's locked */
> > void vma_aquire_read_lock(struct vm_area_struct *vma)
> > {
> >         mmap_assert_locked(vma->vm_mm);
> >         down_read(&vma->vm_lock->lock);
> > }
> >
> > struct vm_area_struct *lock_vma(struct mm_struct *mm,
> >         unsigned long addr))    /* or some better name.. */
> > {
> >         struct vm_area_struct *vma;
> >
> >         vma = lock_vma_under_rcu(mm, addr);
> >         if (vma)
> >                 return vma;
> >
> >         mmap_read_lock(mm);
> >         /* mm sequence cannot change, no mm writers anyways.
> >          * find_mergeable_anon_vma is only a concern in the page fault
> >          * path
> >          * start/end won't change under the mmap_lock
> >          * vma won't become detached as we have the mmap_lock in read
> >          * We are now sure no writes will change the VMA
> >          * So let's make sure no other context is isolating the vma
> >          */
> >         vma = lookup_vma(mm, addr);
> >         if (vma)
> We can take care of anon_vma as well here right? I can take a bool
> parameter ('prepare_anon' or something) and then:
> 
>            if (vma) {
>                     if (prepare_anon && vma_is_anonymous(vma)) &&
> !anon_vma_prepare(vma)) {
>                                       vma = ERR_PTR(-ENOMEM);
>                                       goto out_unlock;
>                    }
> >                 vma_aquire_read_lock(vma);
>            }
> out_unlock:
> >         mmap_read_unlock(mm);
> >         return vma;
> > }

Do you need this?  I didn't think this was happening in the code as
written?  If you need it I would suggest making it happen always and
ditch the flag until a user needs this variant, but document what's
going on in here or even have a better name.

Thanks,
Liam