[PATCH 5.10 50/73] ext4: convert to exclusive lock while inserting delalloc extents

Sasha Levin
2024-03-13T16:47:37Z

From: Zhang Yi <yi.zhang@huawei.com>

[ Upstream commit acf795dc161f3cf481db20f05db4250714e375e5 ]

ext4_da_map_blocks() only hold i_data_sem in shared mode and i_rwsem
when inserting delalloc extents, it could be raced by another querying
path of ext4_map_blocks() without i_rwsem, .e.g buffered read path.
Suppose we buffered read a file containing just a hole, and without any
cached extents tree, then it is raced by another delayed buffered write
to the same area or the near area belongs to the same hole, and the new
delalloc extent could be overwritten to a hole extent.

pread() pwrite()
filemap_read_folio()
ext4_mpage_readpages()
ext4_map_blocks()
down_read(i_data_sem)
ext4_ext_determine_hole()
//find hole
ext4_ext_put_gap_in_cache()
ext4_es_find_extent_range()
//no delalloc extent
ext4_da_map_blocks()
down_read(i_data_sem)
ext4_insert_delayed_block()
//insert delalloc extent
ext4_es_insert_extent()
//overwrite delalloc extent to hole

This race could lead to inconsistent delalloc extents tree and
incorrect reserved space counter. Fix this by converting to hold
i_data_sem in exclusive mode when adding a new delalloc extent in
ext4_da_map_blocks().

Cc: stable@vger.kernel.org
Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Suggested-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20240127015825.1608160-3-yi.zhang@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/ext4/inode.c | 25 +++++++++++--------------
1 file changed, 11 insertions(+), 14 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 64162470a7e6c..8b48ed351c4b9 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -1728,10 +1728,8 @@ 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)) {
- if (ext4_es_is_hole(&es)) {
- down_read(&EXT4_I(inode)->i_data_sem);
+ if (ext4_es_is_hole(&es))
goto add_delayed;
- }

/*
* Delayed extent could be allocated by fallocate.
@@ -1773,8 +1771,10 @@ static int ext4_da_map_blocks(struct inode *inode, sector_t iblock,
retval = ext4_ext_map_blocks(NULL, inode, map, 0);
else
retval = ext4_ind_map_blocks(NULL, inode, map, 0);
- if (retval < 0)
- goto out_unlock;
+ if (retval < 0) {
+ up_read(&EXT4_I(inode)->i_data_sem);
+ return retval;
+ }
if (retval > 0) {
unsigned int status;

@@ -1790,24 +1790,21 @@ static int ext4_da_map_blocks(struct inode *inode, sector_t iblock,
EXTENT_STATUS_UNWRITTEN : EXTENT_STATUS_WRITTEN;
ext4_es_insert_extent(inode, map->m_lblk, map->m_len,
map->m_pblk, status);
- goto out_unlock;
+ up_read(&EXT4_I(inode)->i_data_sem);
+ return retval;
}
+ up_read(&EXT4_I(inode)->i_data_sem);

add_delayed:
- /*
- * XXX: __block_prepare_write() unmaps passed block,
- * is it OK?
- */
+ down_write(&EXT4_I(inode)->i_data_sem);
retval = ext4_insert_delayed_block(inode, map->m_lblk);
+ up_write(&EXT4_I(inode)->i_data_sem);
if (retval)
- goto out_unlock;
+ return retval;

map_bh(bh, inode->i_sb, invalid_block);
set_buffer_new(bh);
set_buffer_delay(bh);
-
-out_unlock:
- up_read((&EXT4_I(inode)->i_data_sem));
return retval;
}

--
2.43.0