Re: [PATCH] vfio/pci: take mmap write lock for io_remap_pfn_range

[Alex Williamson <alex.williamson@redhat.com>]

On Mon, 8 May 2023 13:48:30 -0300
Jason Gunthorpe <jgg@nvidia.com> wrote:

> On Mon, May 08, 2023 at 08:58:42PM +0800, Yan Zhao wrote:
> > In VFIO type1, vaddr_get_pfns() will try fault in MMIO PFNs after
> > pin_user_pages_remote() returns -EFAULT.
> > 
> > follow_fault_pfn
> >  fixup_user_fault
> >   handle_mm_fault
> >    handle_mm_fault
> >     do_fault
> >      do_shared_fault
> >       do_fault
> >        __do_fault
> >         vfio_pci_mmap_fault
> >          io_remap_pfn_range
> >           remap_pfn_range
> >            track_pfn_remap
> >             vm_flags_set         ==> mmap_assert_write_locked(vma->vm_mm)
> >            remap_pfn_range_notrack
> >             vm_flags_set         ==> mmap_assert_write_locked(vma->vm_mm)
> > 
> > As io_remap_pfn_range() will call vm_flags_set() to update vm_flags [1],
> > holding of mmap write lock is required.
> > So, update vfio_pci_mmap_fault() to drop mmap read lock and take mmap
> > write lock.
> > 
> > [1] https://lkml.kernel.org/r/20230126193752.297968-3-surenb@google.com
> > commit bc292ab00f6c ("mm: introduce vma->vm_flags wrapper functions")
> > commit 1c71222e5f23
> > ("mm: replace vma->vm_flags direct modifications with modifier calls")
> > 
> > Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
> > ---
> >  drivers/vfio/pci/vfio_pci_core.c | 17 +++++++++++++++++
> >  1 file changed, 17 insertions(+)
> > 
> > diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
> > index a5ab416cf476..5082f89152b3 100644
> > --- a/drivers/vfio/pci/vfio_pci_core.c
> > +++ b/drivers/vfio/pci/vfio_pci_core.c
> > @@ -1687,6 +1687,12 @@ static vm_fault_t vfio_pci_mmap_fault(struct vm_fault *vmf)
> >  	struct vfio_pci_mmap_vma *mmap_vma;
> >  	vm_fault_t ret = VM_FAULT_NOPAGE;
> >  
> > +	mmap_assert_locked(vma->vm_mm);
> > +	mmap_read_unlock(vma->vm_mm);
> > +
> > +	if (mmap_write_lock_killable(vma->vm_mm))
> > +		return VM_FAULT_RETRY;  
> 
> Certainly not..
> 
> I'm not sure how to resolve this properly, set the flags in advance?
> 
> The address space conversion?

We already try to set the flags in advance, but there are some
architectural flags like VM_PAT that make that tricky.  Cedric has been
looking at inserting individual pages with vmf_insert_pfn(), but that
incurs a lot more faults and therefore latency vs remapping the entire
vma on fault.  I'm not convinced that we shouldn't just attempt to
remove the fault handler entirely, but I haven't tried it yet to know
what gotchas are down that path.  Thanks,

Alex