shmem: shmem_get_partial_folio use filemap_get_entry

To avoid use of the FGP_ENTRY flag, adapt shmem_get_partial_folio() to use
filemap_get_entry() and folio_lock() instead of __filemap_get_folio(). 
Update "page" in the comments there to "folio".

Link: https://lkml.kernel.org/r/9d1aaa4-1337-fb81-6f37-74ebc96f9ef@google.com
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Andreas Gruenbacher <agruenba@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/shmem.c

@@ -885,14 +885,21 @@ static struct folio *shmem_get_partial_folio(struct inode *inode, pgoff_t index)
 
 	/*
 	 * At first avoid shmem_get_folio(,,,SGP_READ): that fails
-	 * beyond i_size, and reports fallocated pages as holes.
+	 * beyond i_size, and reports fallocated folios as holes.
 	 */
-	folio = __filemap_get_folio(inode->i_mapping, index,
-					FGP_ENTRY | FGP_LOCK, 0);
-	if (!xa_is_value(folio))
+	folio = filemap_get_entry(inode->i_mapping, index);
+	if (!folio)
 		return folio;
+	if (!xa_is_value(folio)) {
+		folio_lock(folio);
+		if (folio->mapping == inode->i_mapping)
+			return folio;
+		/* The folio has been swapped out */
+		folio_unlock(folio);
+		folio_put(folio);
+	}
 	/*
-	 * But read a page back from swap if any of it is within i_size
+	 * But read a folio back from swap if any of it is within i_size
 	 * (although in some cases this is just a waste of time).
 	 */
 	folio = NULL;