Re: [PATCH 09/39] drivers/usb/mon: update to use mmap_prepare + map kernel pages

[Greg Kroah-Hartman]

On Tue, Sep 08, 2026 at 09:01:13PM +0100, Lorenzo Stoakes (ARM) wrote:
> Replace the deprecated .mmap hook with its replacement .mmap_prepare. As
> part of this change, additionally take the approach of mapping pages upon
> mmap rather than providing a fault handler.
> 
> The page span cannot be mutated when an mmap mapping is in place, so this
> is safe to do in advance (the MON_IOCT_RING_SIZE ioctl operation exits
> -EBUSY if it's attempted, gated by the rp->mmap_active reference count).
> 
> Utilise the newly introduced mmap_action_map_discontig_kernel_pages() to do
> this, which allows for iteration over pages in mon_bin_discontig_get().
> 
> mon_bin_discontig_init() increments the rp->mmap_active reference count to
> stabilise page spans. Should an error arise the core unmaps the VMA and
> mon_bin_vma_close() drops the reference again.
> 
> The vm_ops->close hook implemented in mon_bin_vma_close() will ensure
> correct reference count arithmetic upon unmap (with mon_bin_vma_open()
> accounting for splitting).
> 
> The existing semantics are all retained, including not mapping past the
> range of available pages, with a SIGBUS being raised in a userland process
> that attempts to access past this point.
> 
> Ultimately insert_page() is invoked to insert each page, which increments
> the reference count on each mapped page. This mimics what was being done
> previously, only we pre-map the entire range rather than doing so on
> demand.
> 
> The existing fault handler did nothing that required demand paging, and was
> presumably implemented this way due for historic reasons.
> 
> One behavioural difference: pages are no longer faulted in on demand, so a
> page discarded with MADV_DONTNEED is not repopulated and a subsequent
> access raises SIGBUS, as with other pre-populated kernel mappings.
> 
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>

Acked-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>