ext4: Properly initialize the buffer_head state

These struct buffer_heads are allocated on the stack (and hence are
initialized with stack garbage). They are only used to call a
get_blocks() function, so that's mostly OK, but b_state must be
initialized to be 0 so we don't have any unexpected BH_* flags set by
accident, such as BH_Unwritten or BH_Delay.

Signed-off-by: Aneesh Kumar K.V <[e-mail]>
Signed-off-by: "Theodore Ts'o" <[e-mail]>

fs/ext4/extents.c

···
 			ret = PTR_ERR(handle);
 			break;
 		}
+		map_bh.b_state = 0;
 		ret = ext4_get_blocks_wrap(handle, inode, block,
 					  max_blocks, &map_bh,
 					  EXT4_CREATE_UNINITIALIZED_EXT, 0, 0);

fs/ext4/inode.c

···
 	if ((mpd->b_state  & (1 << BH_Mapped)) &&
 	    !(mpd->b_state & (1 << BH_Delay)))
 		return 0;
-	new.b_state = mpd->b_state;
+	/*
+	 * We need to make sure the BH_Delay flag is passed down to
+	 * ext4_da_get_block_write(), since it calls
+	 * ext4_get_blocks_wrap() with the EXT4_DELALLOC_RSVED flag.
+	 * This flag causes ext4_get_blocks_wrap() to call
+	 * ext4_da_update_reserve_space() if the passed buffer head
+	 * has the BH_Delay flag set.  In the future, once we clean up
+	 * the interfaces to ext4_get_blocks_wrap(), we should pass in
+	 * a separate flag which requests that the delayed allocation
+	 * statistics should be updated, instead of depending on the
+	 * state information getting passed down via the map_bh's
+	 * state bitmasks plus the magic EXT4_DELALLOC_RSVED flag.
+	 */
+	new.b_state = mpd->b_state & (1 << BH_Delay);
 	new.b_blocknr = 0;
 	new.b_size = mpd->b_size;
 	next = mpd->b_blocknr;

fs/mpage.c

···
 	struct buffer_head map_bh;
 	unsigned long first_logical_block = 0;
 
-	clear_buffer_mapped(&map_bh);
+	map_bh.b_state = 0;
+	map_bh.b_size = 0;
 	for (page_idx = 0; page_idx < nr_pages; page_idx++) {
 		struct page *page = list_entry(pages->prev, struct page, lru);
 
···
 	struct buffer_head map_bh;
 	unsigned long first_logical_block = 0;
 
-	clear_buffer_mapped(&map_bh);
+	map_bh.b_state = 0;
+	map_bh.b_size = 0;
 	bio = do_mpage_readpage(bio, page, 1, &last_block_in_bio,
 			&map_bh, &first_logical_block, get_block);
 	if (bio)