Re: [PATCH v4 17/21] mm: allow handling of stacked mmap_prepare hooks in more drivers

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 3/20/26 23:39, Lorenzo Stoakes (Oracle) wrote:
> While the conversion of mmap hooks to mmap_prepare is underway, we will
> encounter situations where mmap hooks need to invoke nested mmap_prepare
> hooks.
> 
> The nesting of mmap hooks is termed 'stacking'.  In order to flexibly
> facilitate the conversion of custom mmap hooks in drivers which stack, we
> must split up the existing __compat_vma_mmap() function into two separate
> functions:
> 
> * compat_set_desc_from_vma() - This allows the setting of a vm_area_desc
>   object's fields to the relevant fields of a VMA.
> 
> * __compat_vma_mmap() - Once an mmap_prepare hook has been executed upon a
>   vm_area_desc object, this function performs any mmap actions specified by
>   the mmap_prepare hook and then invokes its vm_ops->mapped() hook if any
>   were specified.
> 
> In ordinary cases, where a file's f_op->mmap_prepare() hook simply needs
> to be invoked in a stacked mmap() hook, compat_vma_mmap() can be used.
> 
> However some drivers define their own nested hooks, which are invoked in
> turn by another hook.
> 
> A concrete example is vmbus_channel->mmap_ring_buffer(), which is invoked
> in turn by bin_attribute->mmap():
> 
> vmbus_channel->mmap_ring_buffer() has a signature of:
> 
> int (*mmap_ring_buffer)(struct vmbus_channel *channel,
> 			struct vm_area_struct *vma);
> 
> And bin_attribute->mmap() has a signature of:
> 
> 	int (*mmap)(struct file *, struct kobject *,
> 		    const struct bin_attribute *attr,
> 		    struct vm_area_struct *vma);
> 
> And so compat_vma_mmap() cannot be used here for incremental conversion of
> hooks from mmap() to mmap_prepare().
> 
> There are many such instances like this, where conversion to mmap_prepare
> would otherwise cascade to a huge change set due to nesting of this kind.
> 
> The changes in this patch mean we could now instead convert
> vmbus_channel->mmap_ring_buffer() to
> vmbus_channel->mmap_prepare_ring_buffer(), and implement something like:
> 
> 	struct vm_area_desc desc;
> 	int err;
> 
> 	compat_set_desc_from_vma(&desc, file, vma);
> 	err = channel->mmap_prepare_ring_buffer(channel, &desc);
> 	if (err)
> 		return err;
> 
> 	return __compat_vma_mmap(&desc, vma);
> 
> Allowing us to incrementally update this logic, and other logic like it.
> 
> Unfortunately, as part of this change, we need to be able to flexibly
> assign to the VMA descriptor, so have to remove some of the const
> declarations within the structure.
> 
> Also update the VMA tests to reflect the changes.
> 
> Signed-off-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>

Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>