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

[Linus Torvalds]

On Fri, 31 Jan 2025 at 11:17, Alex Williamson
<alex.williamson@xxxxxxxxxx> 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?

             Linus