Re: [PATCH 0/2] minor cleanups of include/linux/mm.h

[Matthew Wilcox]

On Mon, Apr 05, 2021 at 06:13:53PM +0300, Mike Rapoport wrote:
> I've also noticed that except page_mapping_file() we have somewhat similar
> page_file_mapping(), which seems superfluous, but I didn't dig further to
> see how one of them can be dropped. 

They can't ;-(  I looked at it, and they do different things.

+ * folio_file_mapping - Find the mapping this folio belongs to.
+ * @folio: The folio.
+ *
+ * For folios which are in the page cache, return the mapping that this
+ * page belongs to.  Folios in the swap cache return the mapping of the
+ * swap file or swap device where the data is stored.  This is different
+ * from the mapping returned by folio_mapping().  The only reason to
+ * use it is if, like NFS, you return 0 from ->activate_swapfile.

page_mapping_file() returns NULL for pages which are in the swap cache,
as they no longer need the dcache flushed.