Re: [PATCH v6 07/10] ext4: add nolock mode to ext4_map_blocks()

[Jan Kara <jack@suse.cz>]

On Wed 29-05-24 01:20:00, Harshad Shirwadkar wrote:
> Add nolock flag to ext4_map_blocks() which skips grabbing
> i_data_sem in ext4_map_blocks. In FC commit path, we first
> mark the inode as committing and thereby prevent any mutations
> on it. Thus, it should be safe to call ext4_map_blocks()
> without i_data_sem in this case. This is a workaround to
> the problem mentioned in RFC V4 version cover letter[1] of this
> patch series which pointed out that there is in incosistency between
> ext4_map_blocks() behavior when EXT4_GET_BLOCKS_CACHED_NOWAIT is
> passed. This patch gets rid of the need to call ext4_map_blocks()
> with EXT4_GET_BLOCKS_CACHED_NOWAIT and instead call it with
> EXT4_GET_BLOCKS_NOLOCK. I verified that generic/311 which failed
> in cached_nowait mode passes with nolock mode.
> 
> [1] https://lwn.net/Articles/902022/
> 
> Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>

I'm sorry I forgot since last time - can you remind me why we cannot we
grab i_data_sem from ext4_fc_write_inode_data()? Because as you write
above, nobody should really be holding that lock while inode is
EXT4_STATE_FC_COMMITTING anyway...

								Honza

> ---
>  fs/ext4/ext4.h        |  1 +
>  fs/ext4/fast_commit.c | 16 ++++++++--------
>  fs/ext4/inode.c       | 14 ++++++++++++--
>  3 files changed, 21 insertions(+), 10 deletions(-)
> 
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index d802040e94df..196c513f82dd 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -720,6 +720,7 @@ enum {
>  #define EXT4_GET_BLOCKS_IO_SUBMIT		0x0400
>  	/* Caller is in the atomic contex, find extent if it has been cached */
>  #define EXT4_GET_BLOCKS_CACHED_NOWAIT		0x0800
> +#define EXT4_GET_BLOCKS_NOLOCK			0x1000
>  
>  /*
>   * The bit position of these flags must not overlap with any of the
> diff --git a/fs/ext4/fast_commit.c b/fs/ext4/fast_commit.c
> index b81b0292aa59..0b7064f8dfa5 100644
> --- a/fs/ext4/fast_commit.c
> +++ b/fs/ext4/fast_commit.c
> @@ -559,13 +559,6 @@ void ext4_fc_track_inode(handle_t *handle, struct inode *inode)
>  		!list_empty(&ei->i_fc_list))
>  		return;
>  
> -	/*
> -	 * If we come here, we may sleep while waiting for the inode to
> -	 * commit. We shouldn't be holding i_data_sem in write mode when we go
> -	 * to sleep since the commit path needs to grab the lock while
> -	 * committing the inode.
> -	 */
> -	WARN_ON(lockdep_is_held_type(&ei->i_data_sem, 1));
>  
>  	while (ext4_test_inode_state(inode, EXT4_STATE_FC_COMMITTING)) {
>  #if (BITS_PER_LONG < 64)
> @@ -898,7 +891,14 @@ static int ext4_fc_write_inode_data(struct inode *inode, u32 *crc)
>  	while (cur_lblk_off <= new_blk_size) {
>  		map.m_lblk = cur_lblk_off;
>  		map.m_len = new_blk_size - cur_lblk_off + 1;
> -		ret = ext4_map_blocks(NULL, inode, &map, 0);
> +		/*
> +		 * Given that this inode is being committed,
> +		 * EXT4_STATE_FC_COMMITTING is already set on this inode.
> +		 * Which means all the mutations on the inode are paused
> +		 * until the commit operation is complete. Thus it is safe
> +		 * call ext4_map_blocks() in no lock mode.
> +		 */
> +		ret = ext4_map_blocks(NULL, inode, &map, EXT4_GET_BLOCKS_NOLOCK);
>  		if (ret < 0)
>  			return -ECANCELED;
>  
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 61ffbdc2fb16..f00408017c7a 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -546,7 +546,8 @@ int ext4_map_blocks(handle_t *handle, struct inode *inode,
>  	 * Try to see if we can get the block without requesting a new
>  	 * file system block.
>  	 */
> -	down_read(&EXT4_I(inode)->i_data_sem);
> +	if (!(flags & EXT4_GET_BLOCKS_NOLOCK))
> +		down_read(&EXT4_I(inode)->i_data_sem);
>  	if (ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS)) {
>  		retval = ext4_ext_map_blocks(handle, inode, map, 0);
>  	} else {
> @@ -573,7 +574,15 @@ int ext4_map_blocks(handle_t *handle, struct inode *inode,
>  		ext4_es_insert_extent(inode, map->m_lblk, map->m_len,
>  				      map->m_pblk, status);
>  	}
> -	up_read((&EXT4_I(inode)->i_data_sem));
> +	/*
> +	 * We should never call ext4_map_blocks() in nolock mode outside
> +	 * of fast commit path.
> +	 */
> +	WARN_ON((flags & EXT4_GET_BLOCKS_NOLOCK) &&
> +		!ext4_test_inode_state(inode,
> +				       EXT4_STATE_FC_COMMITTING));
> +	if (!(flags & EXT4_GET_BLOCKS_NOLOCK))
> +		up_read((&EXT4_I(inode)->i_data_sem));
>  
>  found:
>  	if (retval > 0 && map->m_flags & EXT4_MAP_MAPPED) {
> @@ -614,6 +623,7 @@ int ext4_map_blocks(handle_t *handle, struct inode *inode,
>  	 * the write lock of i_data_sem, and call get_block()
>  	 * with create == 1 flag.
>  	 */
> +	WARN_ON((flags & EXT4_GET_BLOCKS_NOLOCK) != 0);
>  	down_write(&EXT4_I(inode)->i_data_sem);
>  
>  	/*
> -- 
> 2.45.1.288.g0e0cd299f1-goog
> 
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR