Re: [PATCH v4 04/12] mm/shmem: Support memfile_notifier

[Andy Lutomirski <luto@kernel.org>]

On 1/18/22 05:21, Chao Peng wrote:
> It maintains a memfile_notifier list in shmem_inode_info structure and
> implements memfile_pfn_ops callbacks defined by memfile_notifier. It
> then exposes them to memfile_notifier via
> shmem_get_memfile_notifier_info.
> 
> We use SGP_NOALLOC in shmem_get_lock_pfn since the pages should be
> allocated by userspace for private memory. If there is no pages
> allocated at the offset then error should be returned so KVM knows that
> the memory is not private memory.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>

>   static int memfile_get_notifier_info(struct inode *inode,
>   				     struct memfile_notifier_list **list,
>   				     struct memfile_pfn_ops **ops)
>   {
> -	return -EOPNOTSUPP;
> +	int ret = -EOPNOTSUPP;
> +#ifdef CONFIG_SHMEM
> +	ret = shmem_get_memfile_notifier_info(inode, list, ops);
> +#endif
> +	return ret;
>   }

> +int shmem_get_memfile_notifier_info(struct inode *inode,
> +				    struct memfile_notifier_list **list,
> +				    struct memfile_pfn_ops **ops)
> +{
> +	struct shmem_inode_info *info;
> +
> +	if (!shmem_mapping(inode->i_mapping))
> +		return -EINVAL;
> +
> +	info = SHMEM_I(inode);
> +	*list = &info->memfile_notifiers;
> +	if (ops)
> +		*ops = &shmem_pfn_ops;
> +
> +	return 0;

I can't wrap my head around exactly who is supposed to call these 
functions and when, but there appears to be a missing check that the 
inode is actually a shmem inode.

What is this code trying to do?  It's very abstract.