ext4: trim delalloc extent

In ext4_da_map_blocks(), we could find four kind of extents in the
extent status tree: hole, unwritten, written and delayed extent. Now we
only trim the map len if we found an unwritten extent or a written
extent. This is okay now since map->m_len is always set to one and we
always insert one delayed block at a time. But this will become isn't
okay for other two cases if ext4_insert_delayed_block() and
ext4_da_map_blocks() support inserting multiple map->len blocks later.

1. If we found a hole in the extent status tree which es->es_len is
   shorter than the length we want to write, we should trim the
   map->m_len to prevent adding extra delay more blocks than we
   expected. For example, assume we write data [A, C) to a file that
   contains a hole extent [A, B) and a written extent [B, D) in the
   cache.

                         A     B  C  D
   before da write:   ...hhhhhh|wwwwww....

   Then we will get extent [A, B), we should trim map->m_len to B-A
   before inserting new delalloc blocks, if not, the range [B, C) will
   be duplicated.

2. If we found a delayed extent in the extent status tree which
   es->es_len is shorter than the length we want to write, we should
   trim the map->m_len to es->es_len and return directly since the front
   part of this map has been delayed, we can't insert the delalloc
   extent that contains the latter part in this round, we should return
   the delayed length and the caller should increase the position and
   call ext4_da_map_blocks() again. For example, assume we write data
   [A, C) to a file that contains a delayed extent [A, B) in the cache.

                         A     B  C
   before da write:   ...dddddd|hhh....

   Then we will get delayed extent [A, B), we should also trim map->m_len
   to B-A and return, if not, we will incorrectly assume that the write
   is complete and won't insert [B, C).

So we need to always trim the map->m_len if the found es->es_len in the
extent status tree is shorter than the map->m_len, prearing for
inserting a extent with multiple delalloc blocks. This patch only does a
pre-fix, the handle is crude and ext4_da_map_blocks() deserve a cleanup,
we will do that later.

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20240517124005.347221-5-yi.zhang@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

1 files changed, 10 insertions, 4 deletions

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 4b0d64a76e88..200bbe89ded2 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -1734,6 +1734,11 @@ static int ext4_da_map_blocks(struct inode *inode, sector_t iblock,
 
 	/* Lookup extent status tree firstly */
 	if (ext4_es_lookup_extent(inode, iblock, NULL, &es)) {
+		retval = es.es_len - (iblock - es.es_lblk);
+		if (retval > map->m_len)
+			retval = map->m_len;
+		map->m_len = retval;
+
 		if (ext4_es_is_hole(&es))
 			goto add_delayed;
 
@@ -1750,10 +1755,6 @@ found:
 		}
 
 		map->m_pblk = ext4_es_pblock(&es) + iblock - es.es_lblk;
-		retval = es.es_len - (iblock - es.es_lblk);
-		if (retval > map->m_len)
-			retval = map->m_len;
-		map->m_len = retval;
 		if (ext4_es_is_written(&es))
 			map->m_flags |= EXT4_MAP_MAPPED;
 		else if (ext4_es_is_unwritten(&es))
@@ -1790,6 +1791,11 @@ add_delayed:
 	 * the extent status tree.
 	 */
 	if (ext4_es_lookup_extent(inode, iblock, NULL, &es)) {
+		retval = es.es_len - (iblock - es.es_lblk);
+		if (retval > map->m_len)
+			retval = map->m_len;
+		map->m_len = retval;
+
 		if (!ext4_es_is_hole(&es)) {
 			up_write(&EXT4_I(inode)->i_data_sem);
 			goto found;