buffer: make block_write_full_page() handle large folios correctly

Keep the interface as struct page, but work entirely on the folio
internally.  Removes several PAGE_SIZE assumptions and removes some
references to page->index and page->mapping.

Link: https://lkml.kernel.org/r/20230612210141.730128-7-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Tested-by: Bob Peterson <rpeterso@redhat.com>
Reviewed-by: Bob Peterson <rpeterso@redhat.com>
Cc: Andreas Gruenbacher <agruenba@redhat.com>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/buffer.c

@@ -2678,33 +2678,31 @@ int block_write_full_page(struct page *page, get_block_t *get_block,
 			struct writeback_control *wbc)
 {
 	struct folio *folio = page_folio(page);
-	struct inode * const inode = page->mapping->host;
+	struct inode * const inode = folio->mapping->host;
 	loff_t i_size = i_size_read(inode);
-	const pgoff_t end_index = i_size >> PAGE_SHIFT;
-	unsigned offset;
 
-	/* Is the page fully inside i_size? */
-	if (page->index < end_index)
+	/* Is the folio fully inside i_size? */
+	if (folio_pos(folio) + folio_size(folio) <= i_size)
 		return __block_write_full_folio(inode, folio, get_block, wbc,
 					       end_buffer_async_write);
 
-	/* Is the page fully outside i_size? (truncate in progress) */
-	offset = i_size & (PAGE_SIZE-1);
-	if (page->index >= end_index+1 || !offset) {
+	/* Is the folio fully outside i_size? (truncate in progress) */
+	if (folio_pos(folio) >= i_size) {
 		folio_unlock(folio);
 		return 0; /* don't care */
 	}
 
 	/*
-	 * The page straddles i_size.  It must be zeroed out on each and every
+	 * The folio straddles i_size.  It must be zeroed out on each and every
 	 * writepage invocation because it may be mmapped.  "A file is mapped
 	 * in multiples of the page size.  For a file that is not a multiple of
-	 * the  page size, the remaining memory is zeroed when mapped, and
+	 * the page size, the remaining memory is zeroed when mapped, and
 	 * writes to that region are not written out to the file."
 	 */
-	zero_user_segment(page, offset, PAGE_SIZE);
+	folio_zero_segment(folio, offset_in_folio(folio, i_size),
+			folio_size(folio));
 	return __block_write_full_folio(inode, folio, get_block, wbc,
-							end_buffer_async_write);
+			end_buffer_async_write);
 }
 EXPORT_SYMBOL(block_write_full_page);