+	partial_end = ((lend + 1) % PAGE_SIZE) > 0;

The > 0 reads a little odd as we can never get a negative value here.

+	shmem_get_folio(inode, lstart >> PAGE_SHIFT, &folio, SGP_READ);
+	if (folio) {
+		bool same_page;
+
+		same_page = lend < folio_pos(folio) + folio_size(folio);
+		if (same_page)
+			partial_end = false;
+		folio_mark_dirty(folio);
+		if (!truncate_inode_partial_folio(folio, lstart, lend)) {
+			start = folio->index + folio_nr_pages(folio);
+			if (same_page)
+				end = folio->index;
 		}
+		folio_unlock(folio);
+		folio_put(folio);
+		folio = NULL;

The entire logic after the shmem_get_folio call is duplicated in
truncate_inode_pages_range.  Shouldn't we ty to combine that by moving it
into a helper?

If we can't merge it I think same_page shoud be renamed to same_folio as in
truncate_inode_pages_range.

 	pgoff_t		start;		/* inclusive */
 	pgoff_t		end;		/* exclusive */
-	unsigned int	partial_start;	/* inclusive */
-	unsigned int	partial_end;	/* exclusive */
 	struct folio_batch fbatch;
 	pgoff_t		indices[PAGEVEC_SIZE];
 	pgoff_t		index;
 	int		i;
+	struct folio *	folio;

Weird indentation after the star.

+	bool partial_end;

And the style here in general does not seem to match the existing code.