Re: [PATCH v2 02/25] mm: Optimise find_subpage for !THP

[Christoph Hellwig]

On Tue, Feb 11, 2020 at 08:18:22PM -0800, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
> 
> If THP is disabled, find_subpage can become a no-op.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> ---
>  include/linux/pagemap.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 75bdfec49710..0842622cca90 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -340,7 +340,7 @@ static inline struct page *find_subpage(struct page *page, pgoff_t offset)
>  
>  	VM_BUG_ON_PAGE(PageTail(page), page);
>  
> -	return page + (offset & (compound_nr(page) - 1));
> +	return page + (offset & (hpage_nr_pages(page) - 1));
>  }

So just above the quoted code is a

	if (PageHuge(page))
		return page;

So how can we get into a compound page that is not a huge page, but
only if THP is enabled?  (Yes, maybe I'm a little rusty on VM
internals).

Can you add comments describing the use case of this function and why
it does all these checks?  It looks like black magic to me.