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

Christoph Hellwig
2020-02-12T07:41:07Z

On Tue, Feb 11, 2020 at 08:18:22PM -0800, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
>
> If THP is disabled, find_subpage can become a no-op.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
> 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.