Re: [PATCH v2 2/9] ext4: check the extent status again before inserting delalloc block

Jan Kara
2024-04-24T20:05:34Z

On Wed 10-04-24 11:41:56, Zhang Yi wrote:
> From: Zhang Yi <yi.zhang@huawei.com>
>
> Now we lookup extent status entry without holding the i_data_sem before
> inserting delalloc block, it works fine in buffered write path and
> because it holds i_rwsem and folio lock, and the mmap path holds folio
> lock, so the found extent locklessly couldn't be modified concurrently.
> But it could be raced by fallocate since it allocate block whitout
> holding i_rwsem and folio lock.
>
> ext4_page_mkwrite() ext4_fallocate()
> block_page_mkwrite()
> ext4_da_map_blocks()
> //find hole in extent status tree
> ext4_alloc_file_blocks()
> ext4_map_blocks()
> //allocate block and unwritten extent
> ext4_insert_delayed_block()
> ext4_da_reserve_space()
> //reserve one more block
> ext4_es_insert_delayed_block()
> //drop unwritten extent and add delayed extent by mistake
>
> Then, the delalloc extent is wrong until writeback, the one more
> reserved block can't be release any more and trigger below warning:
>
> EXT4-fs (pmem2): Inode 13 (00000000bbbd4d23): i_reserved_data_blocks(1) not cleared!
>
> Hold i_data_sem in write mode directly can fix the problem, but it's
> expansive, we should keep the lockless check and check the extent again
> once we need to add an new delalloc block.
>
> Cc: stable@vger.kernel.org
> Signed-off-by: Zhang Yi <yi.zhang@huawei.com>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

Honza

> ---
> fs/ext4/inode.c | 19 +++++++++++++++++++
> 1 file changed, 19 insertions(+)
>
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 6a41172c06e1..118b0497a954 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -1737,6 +1737,7 @@ static int ext4_da_map_blocks(struct inode *inode, sector_t iblock,
> if (ext4_es_is_hole(&es))
> goto add_delayed;
>
> +found:
> /*
> * Delayed extent could be allocated by fallocate.
> * So we need to check it.
> @@ -1781,6 +1782,24 @@ static int ext4_da_map_blocks(struct inode *inode, sector_t iblock,
>
> add_delayed:
> down_write(&EXT4_I(inode)->i_data_sem);
> + /*
> + * Lookup extents tree again under i_data_sem, make sure this
> + * inserting delalloc range haven't been delayed or allocated
> + * whitout holding i_rwsem and folio lock.
> + */
> + if (ext4_es_lookup_extent(inode, iblock, NULL, &es)) {
> + if (!ext4_es_is_hole(&es)) {
> + up_write(&EXT4_I(inode)->i_data_sem);
> + goto found;
> + }
> + } else if (!ext4_has_inline_data(inode)) {
> + retval = ext4_map_query_blocks(NULL, inode, map);
> + if (retval) {
> + up_write(&EXT4_I(inode)->i_data_sem);
> + return retval;
> + }
> + }
> +
> retval = ext4_insert_delayed_block(inode, map->m_lblk);
> up_write(&EXT4_I(inode)->i_data_sem);
> if (retval)
> --
> 2.39.2
>
--
Jan Kara <jack@suse.com>
SUSE Labs, CR