folio: Add a function to get the host inode for a folio

Add a convenience function, folio_inode() that will get the host inode from
a folio's mapping.

Changes:
 ver #3:
  - Fix mistake in function description[2].
 ver #2:
  - Fix contradiction between doc and implementation by disallowing use
    with swap caches[1].
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Reviewed-by: NMatthew Wilcox (Oracle) <willy@infradead.org>
Tested-by: NJeff Layton <jlayton@kernel.org>
Tested-by: NDominique Martinet <asmadeus@codewreck.org>
Tested-by: kafs-testing@auristor.com
Link: https://lore.kernel.org/r/YST8OcVNy02Rivbm@casper.infradead.org/ [1]
Link: https://lore.kernel.org/r/YYKLkBwQdtn4ja+i@casper.infradead.org/ [2]
Link: https://lore.kernel.org/r/162880453171.3369675.3704943108660112470.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/162981151155.1901565.7010079316994382707.stgit@warthog.procyon.org.uk/
Link: https://lore.kernel.org/r/163005744370.2472992.18324470937328925723.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163584184628.4023316.9386282630968981869.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/163649325519.309189.15072332908703129455.stgit@warthog.procyon.org.uk/ # v4
Link: https://lore.kernel.org/r/163657850401.834781.1031963517399283294.stgit@warthog.procyon.org.uk/ # v5

include/linux/pagemap.h

@@ -253,6 +253,20 @@ static inline struct address_space *page_mapping_file(struct page *page)
 	return folio_mapping(folio);
 }
 
+/**
+ * folio_inode - Get the host inode for this folio.
+ * @folio: The folio.
+ *
+ * For folios which are in the page cache, return the inode that this folio
+ * belongs to.
+ *
+ * Do not call this for folios which aren't in the page cache.
+ */
+static inline struct inode *folio_inode(struct folio *folio)
+{
+	return folio->mapping->host;
+}
+
 static inline bool page_cache_add_speculative(struct page *page, int count)
 {
 	VM_BUG_ON_PAGE(PageTail(page), page);

mm/page-writeback.c

@@ -2967,7 +2967,7 @@ EXPORT_SYMBOL_GPL(folio_wait_writeback_killable);
  */
 void folio_wait_stable(struct folio *folio)
 {
-	if (folio->mapping->host->i_sb->s_iflags & SB_I_STABLE_WRITES)
+	if (folio_inode(folio)->i_sb->s_iflags & SB_I_STABLE_WRITES)
 		folio_wait_writeback(folio);
 }
 EXPORT_SYMBOL_GPL(folio_wait_stable);