Re: [PATCH v3 18/25] mm: Allow large pages to be added to the page cache

[Matthew Wilcox]

On Wed, Apr 29, 2020 at 06:36:50AM -0700, Matthew Wilcox wrote:
> @@ -886,7 +906,7 @@ static int __add_to_page_cache_locked(struct page *page,
>       /* Leave page->index set: truncation relies upon it */
>       if (!huge)
>               mem_cgroup_cancel_charge(page, memcg, false);
> -     put_page(page);
> +     page_ref_sub(page, nr);
>       return xas_error(&xas);
>  }
>  ALLOW_ERROR_INJECTION(__add_to_page_cache_locked, ERRNO);

This is wrong.  page_ref_sub() will not call __put_page() if the refcount
gets to zero.  What do people prefer?

-       put_page(page);

(a)
+       put_thp(page);

(b)
+       put_page_nr(page, nr);

(c)
+       if (page_ref_sub_return(page, nr) == 0)
+               __put_page(page);