Re: [PATCH v7 04/14] mm/shmem: Support memfile_notifier

[David Hildenbrand <david@redhat.com>]

On 06.07.22 10:20, Chao Peng wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
> 
> Implement shmem as a memfile_notifier backing store. Essentially it
> interacts with the memfile_notifier feature flags for userspace
> access/page migration/page reclaiming and implements the necessary
> memfile_backing_store callbacks.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> ---

[...]

> +#ifdef CONFIG_MEMFILE_NOTIFIER
> +static struct memfile_node *shmem_lookup_memfile_node(struct file *file)
> +{
> +	struct inode *inode = file_inode(file);
> +
> +	if (!shmem_mapping(inode->i_mapping))
> +		return NULL;
> +
> +	return  &SHMEM_I(inode)->memfile_node;
> +}
> +
> +
> +static int shmem_get_pfn(struct file *file, pgoff_t offset, pfn_t *pfn,
> +			 int *order)
> +{
> +	struct page *page;
> +	int ret;
> +
> +	ret = shmem_getpage(file_inode(file), offset, &page, SGP_WRITE);
> +	if (ret)
> +		return ret;
> +
> +	unlock_page(page);
> +	*pfn = page_to_pfn_t(page);
> +	*order = thp_order(compound_head(page));
> +	return 0;
> +}
> +
> +static void shmem_put_pfn(pfn_t pfn)
> +{
> +	struct page *page = pfn_t_to_page(pfn);
> +
> +	if (!page)
> +		return;
> +
> +	put_page(page);


Why do we export shmem_get_pfn/shmem_put_pfn and not simply

get_folio()

and let the caller deal with putting the folio? What's the reason to

a) Operate on PFNs and not folios
b) Have these get/put semantics?

> +}
> +
> +static struct memfile_backing_store shmem_backing_store = {
> +	.lookup_memfile_node = shmem_lookup_memfile_node,
> +	.get_pfn = shmem_get_pfn,
> +	.put_pfn = shmem_put_pfn,
> +};
> +#endif /* CONFIG_MEMFILE_NOTIFIER */
> +
>  void __init shmem_init(void)
>  {
>  	int error;
> @@ -3956,6 +4059,10 @@ void __init shmem_init(void)
>  	else
>  		shmem_huge = SHMEM_HUGE_NEVER; /* just in case it was patched */
>  #endif
> +
> +#ifdef CONFIG_MEMFILE_NOTIFIER
> +	memfile_register_backing_store(&shmem_backing_store);

Can we instead prove a dummy function that does nothing without
CONFIG_MEMFILE_NOTIFIER?

> +#endif
>  	return;
>  
>  out1:


-- 
Thanks,

David / dhildenb