Re: [REGRESSION] Re: [PATCH v8 15/19] mm: don't allow huge faults for files with pre content watches

Peter Xu
2025-02-01T01:19:27Z

On Fri, Jan 31, 2025 at 11:59:56AM -0800, Linus Torvalds wrote:
> On Fri, 31 Jan 2025 at 11:17, Alex Williamson
> <alex.williamson@redhat.com> wrote:
> >
> > 20bf82a898b6 ("mm: don't allow huge faults for files with pre content watches")
> >
> > This breaks huge_fault support for PFNMAPs that was recently added in
> > v6.12 and is used by vfio-pci to fault device memory using PMD and PUD
> > order mappings.
>
> Surely only for content watches?
>
> Which shouldn't be a valid situation *anyway*.
>
> IOW, there must be some unrelated bug somewhere: either somebody is
> allowed to set a pre-content match on a special device.
>
> That should be disabled by the whole
>
> /*
> * If there are permission event watchers but no pre-content event
> * watchers, set FMODE_NONOTIFY | FMODE_NONOTIFY_PERM to indicate that.
> */
>
> thing in file_set_fsnotify_mode() which only allows regular files and
> directories to be notified on.
>
> Or, alternatively, that check for huge-fault disabling is just
> checking the wrong bits.
>
> Or - quite possibly - I am missing something obvious?

Is it possible that we have some paths got overlooked in setting up the
fsnotify bits in f_mode? Meanwhile since the default is "no bit set" on
those bits, I think it means FMODE_FSNOTIFY_HSM() can always return true on
those if overlooked..

One thing to mention is, /dev/vfio/* are chardevs, however the PCI bars are
not mmap()ed from these fds - whatever under /dev/vfio/* represents IOMMU
groups rather than the device fd itself.

The app normally needs to first open the IOMMU group fd under /dev/vfio/*,
then using VFIO ioctl(VFIO_GROUP_GET_DEVICE_FD) to get the device fd, which
will be the mmap() target, instead of the ones under /dev.

I checked, those device fds were allocated from vfio_device_open_file()
within the ioctl, which internally uses anon_inode_getfile(). I don't see
anywhere in that path that will set the fanotify bits..

Further, I'm not sure whether some callers of alloc_file() can also suffer
from similar issue, because at least memfd_create() syscall also uses the
API, which (hopefully?) would used to allow THPs for shmem backed memfds on
aligned mmap()s, but not sure whether it'll also wrongly trigger the
FALLBACK path similarly in create_huge_pmd() just like vfio's VMAs. I
didn't verify it though, nor did I yet check more users.

So I wonder whether we should setup the fanotify bits in at least
alloc_file() too (to FMODE_NONOTIFY?).

I'm totally not familiar with fanotify, and it's a bit late to try verify
anything (I cannot quickly find my previous huge pfnmap setup, so setup
those will also take time..). but maybe above can provide some clues for
others..

Thanks,

--
Peter Xu