linux-ext4 - Re: [PATCH v2 05/12] jbd2: fast-commit commit path new APIs

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Date:   Fri, 9 Aug 2019 14:20:09 -0700
From:   harshad shirwadkar <harshadshirwadkar@...il.com>
To:     Andreas Dilger <adilger@...ger.ca>
Cc:     Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH v2 05/12] jbd2: fast-commit commit path new APIs

On Fri, Aug 9, 2019 at 2:11 PM Andreas Dilger <adilger@...ger.ca> wrote:
>
>
> > On Aug 9, 2019, at 2:38 PM, Andreas Dilger <adilger@...ger.ca> wrote:
> >
> > On Aug 8, 2019, at 9:45 PM, Harshad Shirwadkar <harshadshirwadkar@...il.com> wrote:
> >>
> >> This patch adds new helper APIs that ext4 needs for fast
> >> commits. These new fast commit APIs are used by subsequent fast commit
> >> patches to implement fast commits. Following new APIs are added:
> >>
> >> /*
> >> * Returns when either a full commit or a fast commit
> >> * completes
> >> */
> >> int jbd2_fc_complete_commit(journal_tc *journal, tid_t tid,
> >>                          tid_t tid, tid_t subtid)
> >>
> >> /* Send all the data buffers related to an inode */
> >> int journal_submit_inode_data(journal_t *journal,
> >>                            struct jbd2_inode *jinode)
> >>
> >> /* Map one fast commit buffer for use by the file system */
> >> int jbd2_map_fc_buf(journal_t *journal, struct buffer_head **bh_out)
> >>
> >> /* Wait on fast commit buffers to complete IO */
> >> jbd2_wait_on_fc_bufs(journal_t *journal, int num_bufs)
> >>
> >> Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@...il.com>
> >>
> >> +int jbd2_map_fc_buf(journal_t *journal, struct buffer_head **bh_out)
> >> +{
> >> +    unsigned long long pblock;
> >> +    unsigned long blocknr;
> >> +    int ret = 0;
> >> +    struct buffer_head *bh;
> >> +    int fc_off;
> >> +    journal_header_t *jhdr;
> >> +
> >> +    write_lock(&journal->j_state_lock);
> >> +
> >> +    if (journal->j_fc_off + journal->j_first_fc < journal->j_last_fc) {
> >> +            fc_off = journal->j_fc_off;
> >> +            blocknr = journal->j_first_fc + fc_off;
> >> +            journal->j_fc_off++;
> >> +    } else {
> >> +            ret = -EINVAL;
> >> +    }
> >> +    write_unlock(&journal->j_state_lock);
> >> +
> >> +    if (ret)
> >> +            return ret;
> >> +
> >> +    ret = jbd2_journal_bmap(journal, blocknr, &pblock);
> >> +    if (ret)
> >> +            return ret;
> >> +
> >> +    bh = __getblk(journal->j_dev, pblock, journal->j_blocksize);
> >> +    if (!bh)
> >> +            return -ENOMEM;
> >> +
> >> +    lock_buffer(bh);
> >> +    jhdr = (journal_header_t *)bh->b_data;
> >> +    jhdr->h_magic = cpu_to_be32(JBD2_MAGIC_NUMBER);
> >> +    jhdr->h_blocktype = cpu_to_be32(JBD2_FC_BLOCK);
> >> +    jhdr->h_sequence = cpu_to_be32(journal->j_running_transaction->t_tid);
> >> +
> >> +    set_buffer_uptodate(bh);
> >> +    unlock_buffer(bh);
> >> +    journal->j_fc_wbuf[fc_off] = bh;
> >> +
> >> +    *bh_out = bh;
> >> +
> >> +    return 0;
> >> +}
> >> +EXPORT_SYMBOL(jbd2_map_fc_buf);
>
> One question about this function.  It seems that it is called for every
> commit by ext4_journal_fc_commit_cb().  Why does it need to map the fast
> journal commit blocks on every call?  It would make more sense to map the
> blocks once at initialization time and then just re-use them on each call.
>

The only reason why I did it this way is that this way JBD2 gets an
opportunity to set-up journal header at the beginning of the block
which contains TID information. But I guess we could have a separate
call for setting the journal header and ext4 could call that routine
instead of mapping buffers on every commit call. Thanks for pointing
this out. I'll fix this in V3.

> Cheers, Andreas
>
>
>
>
>