[PATCH 5/7] ext4: Add a callback to convert unwritten extents

[Matthew Wilcox]

A different bug was masking the problem that unwritten extents need to
be converted to written extents once we've faulted them into existence.
Following the XFS example, add a b_end_io callback.  We "borrow" a few
additional fields in the buffer_head, but there aren't any big enough
for a sector_t.  Fortunately, we only use this callback for DAX, and
ext4 already requires a 4k block size for using DAX, which puts the
limit at 16TB.  The page cache already limits file sizes to 16TB on
32-bit systems, so we don't need to grow any fields.
---
 fs/ext4/inode.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 5edd903..eaa293a 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -676,6 +676,18 @@ has_zeroout:
 	return retval;
 }
 
+static void ext4_end_io_unwritten(struct buffer_head *bh, int uptodate)
+{
+	struct inode *inode = bh->b_assoc_map->host;
+	/* XXX: breaks on 32-bit > 16GB. Is that even supported? */
+	loff_t offset = (loff_t)(uintptr_t)bh->b_private << inode->i_blkbits;
+	int err;
+	if (!uptodate)
+		return;
+	WARN_ON(!buffer_unwritten(bh));
+	err = ext4_convert_unwritten_extents(NULL, inode, offset, bh->b_size);
+}
+
 /* Maximum number of blocks we map for direct IO at once. */
 #define DIO_MAX_BLOCKS 4096
 
@@ -713,6 +725,11 @@ static int _ext4_get_block(struct inode *inode, sector_t iblock,
 
 		map_bh(bh, inode->i_sb, map.m_pblk);
 		bh->b_state = (bh->b_state & ~EXT4_MAP_FLAGS) | map.m_flags;
+		if (IS_DAX(inode) && buffer_unwritten(bh) && !io_end) {
+			bh->b_assoc_map = inode->i_mapping;
+			bh->b_private = (void *)(unsigned long)iblock;
+			bh->b_end_io = ext4_end_io_unwritten;
+		}
 		if (io_end && io_end->flag & EXT4_IO_END_UNWRITTEN)
 			set_buffer_defer_completion(bh);
 		bh->b_size = inode->i_sb->s_blocksize * map.m_len;
-- 
2.1.0


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