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Date:   Thu, 7 Jan 2021 19:26:11 -0700
From:   Andreas Dilger <adilger@...ger.ca>
To:     daejun7.park@...sung.com,
        harshad shirwadkar <harshadshirwadkar@...il.com>
Cc:     "tytso@....edu" <tytso@....edu>,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ext4: Remove expensive flush on fast commit

On Tue, Jan 5, 2021 at 5:32 PM Daejun Park <daejun7.park@...sung.com> 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.

Does the tail block *always* contain a CRC, or is that dependent on
EXT4_FEATURE_RO_COMPAT_METADATA_CSUM, JBD2_FEATURE_INCOMPAT_CSUM_V2,
or JBD2_FEATURE_INCOMPAT_CSUM_V3 being enabled?

If one of those features is *required* before the FAST_COMMIT feature
can be used, then this patch looks OK.  Otherwise, the CSUM feature
should be checked before the FUA is skipped for non-tail blocks.

Cheers, Andreas

> Signed-off-by: Daejun Park <daejun7.park@...sung.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


Cheers, Andreas






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