Re: [PATCH] ext4: Remove expensive flush on fast commit

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Wed, Jan 06, 2021 at 10:32:42AM +0900, Daejun Park wrote:
> In the fast commit, it adds REQ_FUA and REQ_PREFLUSH on each fast commit
> block when barrier is enabled. However, in recovery phase, ext4 compares
> CRC value in the tail. So it is sufficient adds REQ_FUA and REQ_PREFLUSH
> on the block that has tail.

Just out of curiosity, how does the new fastcommit code handle tests
like generic/475 that simulate sudden disk failure?

--D

> Signed-off-by: Daejun Park <daejun7.park@samsung.com>
> ---
>  fs/ext4/fast_commit.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/ext4/fast_commit.c b/fs/ext4/fast_commit.c
> index 4fcc21c25e79..e66507be334c 100644
> --- a/fs/ext4/fast_commit.c
> +++ b/fs/ext4/fast_commit.c
> @@ -604,13 +604,13 @@ void ext4_fc_track_range(handle_t *handle, struct inode *inode, ext4_lblk_t star
>  	trace_ext4_fc_track_range(inode, start, end, ret);
>  }
>  
> -static void ext4_fc_submit_bh(struct super_block *sb)
> +static void ext4_fc_submit_bh(struct super_block *sb, bool is_tail)
>  {
>  	int write_flags = REQ_SYNC;
>  	struct buffer_head *bh = EXT4_SB(sb)->s_fc_bh;
>  
> -	/* TODO: REQ_FUA | REQ_PREFLUSH is unnecessarily expensive. */
> -	if (test_opt(sb, BARRIER))
> +	/* Add REQ_FUA | REQ_PREFLUSH only its tail */
> +	if (test_opt(sb, BARRIER) && is_tail)
>  		write_flags |= REQ_FUA | REQ_PREFLUSH;
>  	lock_buffer(bh);
>  	set_buffer_dirty(bh);
> @@ -684,7 +684,7 @@ static u8 *ext4_fc_reserve_space(struct super_block *sb, int len, u32 *crc)
>  		*crc = ext4_chksum(sbi, *crc, tl, sizeof(*tl));
>  	if (pad_len > 0)
>  		ext4_fc_memzero(sb, tl + 1, pad_len, crc);
> -	ext4_fc_submit_bh(sb);
> +	ext4_fc_submit_bh(sb, false);
>  
>  	ret = jbd2_fc_get_buf(EXT4_SB(sb)->s_journal, &bh);
>  	if (ret)
> @@ -741,7 +741,7 @@ static int ext4_fc_write_tail(struct super_block *sb, u32 crc)
>  	tail.fc_crc = cpu_to_le32(crc);
>  	ext4_fc_memcpy(sb, dst, &tail.fc_crc, sizeof(tail.fc_crc), NULL);
>  
> -	ext4_fc_submit_bh(sb);
> +	ext4_fc_submit_bh(sb, true);
>  
>  	return 0;
>  }
> -- 
> 2.25.1
>