[PATCH v2 03/13] ext4: don't zero the entire extent if EXT4_EXT_DATA_PARTIAL_VALID1

[Zhang Yi]

From: Zhang Yi <yi.zhang@xxxxxxxxxx>

When allocating initialized blocks from a large unwritten extent, or
when splitting an unwritten extent during end I/O and converting it to
initialized, there is currently a potential issue of stale data if the
extent needs to be split in the middle.

       0  A      B  N
       [UUUUUUUUUUUU]    U: unwritten extent
       [--DDDDDDDD--]    D: valid data
          |<-  ->| ----> this range needs to be initialized

ext4_split_extent() first try to split this extent at B with
EXT4_EXT_DATA_ENTIRE_VALID1 and EXT4_EXT_MAY_ZEROOUT flag set, but
ext4_split_extent_at() failed to split this extent due to temporary lack
of space. It zeroout B to N and mark the entire extent from 0 to N
as written.

       0  A      B  N
       [WWWWWWWWWWWW]    W: written extent
       [SSDDDDDDDDZZ]    Z: zeroed, S: stale data

ext4_split_extent() then try to split this extent at A with
EXT4_EXT_DATA_VALID2 flag set. This time, it split successfully and left
a stale written extent from 0 to A.

       0  A      B   N
       [WW|WWWWWWWWWW]
       [SS|DDDDDDDDZZ]

Fix this by pass EXT4_EXT_DATA_PARTIAL_VALID1 to ext4_split_extent_at()
when splitting at B, don't convert the entire extent to written and left
it as unwritten after zeroing out B to N. The remaining work is just
like the standard two-part split. ext4_split_extent() will pass the
EXT4_EXT_DATA_VALID2 flag when it calls ext4_split_extent_at() for the
second time, allowing it to properly handle the split. If the split is
successful, it will keep extent from 0 to A as unwritten.

Signed-off-by: Zhang Yi <yi.zhang@xxxxxxxxxx>
---
 fs/ext4/extents.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index f7aa497e5d6c..cafe66cb562f 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -3294,6 +3294,13 @@ static struct ext4_ext_path *ext4_split_extent_at(handle_t *handle,
 		err = ext4_ext_zeroout(inode, &zero_ex);
 		if (err)
 			goto fix_extent_len;
+		/*
+		 * The first half contains partially valid data, the splitting
+		 * of this extent has not been completed, fix extent length
+		 * and ext4_split_extent() split will the first half again.
+		 */
+		if (split_flag & EXT4_EXT_DATA_PARTIAL_VALID1)
+			goto fix_extent_len;
 
 		/* update the extent length and mark as initialized */
 		ex->ee_len = cpu_to_le16(ee_len);
@@ -3364,7 +3371,9 @@ static struct ext4_ext_path *ext4_split_extent(handle_t *handle,
 			split_flag1 |= EXT4_EXT_MARK_UNWRIT1 |
 				       EXT4_EXT_MARK_UNWRIT2;
 		if (split_flag & EXT4_EXT_DATA_VALID2)
-			split_flag1 |= EXT4_EXT_DATA_ENTIRE_VALID1;
+			split_flag1 |= map->m_lblk > ee_block ?
+				       EXT4_EXT_DATA_PARTIAL_VALID1 :
+				       EXT4_EXT_DATA_ENTIRE_VALID1;
 		path = ext4_split_extent_at(handle, inode, path,
 				map->m_lblk + map->m_len, split_flag1, flags1);
 		if (IS_ERR(path))
-- 
2.46.1