ext4: don't zero the entire extent if EXT4_EXT_DATA_PARTIAL_VALID1

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@huawei.com>
Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Reviewed-by: Baokun Li <libaokun1@huawei.com>
Cc: stable@kernel.org
Message-ID: <20251129103247.686136-3-yi.zhang@huaweicloud.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 8d5ca450aa5d23b5d27d4d5c496442f8dd2ca5cb..1fee84ea20af1b1d01f6faf95ba909e43634c10b 100644 (file)
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -3310,6 +3310,15 @@ static struct ext4_ext_path *ext4_split_extent_at(handle_t *handle,
                }
 
                if (!err) {
+                       /*
+                        * 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);
                        ext4_ext_try_to_merge(handle, inode, path, ex);
@@ -3379,7 +3388,9 @@ 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))