Re: [PATCH 6.9 0/3] Backport VFIO refactor to fix fork ordering bug

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

On Wed, 17 Jul 2024 14:33:36 -0700
Axel Rasmussen <axelrasmussen@google.com> wrote:

> 35e351780fa9 ("fork: defer linking file vma until vma is fully initialized")
> switched the ordering of vm_ops->open() and copy_page_range() on fork. This is a
> bug for VFIO, because it causes two problems:
> 
> 1. Because open() is called before copy_page_range(), the range can conceivably
>    have unmapped 'holes' in it. This causes the code underneath untrack_pfn() to
>    WARN.
> 
> 2. More seriously, open() is trying to guarantee that the entire range is
>    zapped, so any future accesses in the child will result in the VFIO fault
>    handler being called. Because we copy_page_range() *after* open() (and
>    therefore after zapping), this guarantee is violatd.
> 
> We can't revert 35e351780fa9, because it fixes a real bug for hugetlbfs. The fix
> is also not as simple as just reodering open() and copy_page_range(), as Miaohe
> points out in [1]. So, although these patches are kind of large for stable, just
> backport this refactoring which completely sidesteps the issue.
> 
> Note that patch 2 is the key one here which fixes the issue. Patch 1 is a
> prerequisite required for patch 2 to build / work. This would almost be enough,
> but we might see significantly regressed performance. Patch 3 fixes that up,
> putting performance back on par with what it was before.
> 
> Note [1] also has a more full discussion justifying taking these backports.
> 
> [1]: https://lore.kernel.org/all/20240702042948.2629267-1-leah.rumancik@gmail.com/T/
> 
> Alex Williamson (3):
>   vfio: Create vfio_fs_type with inode per device
>   vfio/pci: Use unmap_mapping_range()
>   vfio/pci: Insert full vma on mmap'd MMIO fault
> 
>  drivers/vfio/device_cdev.c       |   7 +
>  drivers/vfio/group.c             |   7 +
>  drivers/vfio/pci/vfio_pci_core.c | 271 ++++++++-----------------------
>  drivers/vfio/vfio_main.c         |  44 +++++
>  include/linux/vfio.h             |   1 +
>  include/linux/vfio_pci_core.h    |   2 -
>  6 files changed, 125 insertions(+), 207 deletions(-)
> 
> --
> 2.45.2.993.g49e7a77208-goog
> 

LGTM

Reviewed-by: Alex Williamson <alex.williamson@redhat.com>

Thanks,
Alex