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From:   harshad shirwadkar <harshadshirwadkar@gmail.com>
Date:   Tue, 8 Mar 2022 05:06:51 -0800
Message-ID: <CAD+ocbzwGW91MdnwBS2hZ_W+kum-cSpUfAWYJ0jU0xjnt3Y_SQ@mail.gmail.com>
Subject: Re: [PATCH 3/5] ext4: for committing inode, make ext4_fc_track_inode wait
To:     Jan Kara <jack@suse.cz>
Cc:     Ext4 Developers List <linux-ext4@vger.kernel.org>,
        Ritesh Harjani <riteshh@linux.ibm.com>,
        "Theodore Y. Ts'o" <tytso@mit.edu>
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On Tue, 8 Mar 2022 at 04:30, Jan Kara <jack@suse.cz> wrote:
>
> On Tue 08-03-22 02:51:10, Harshad Shirwadkar wrote:
> > From: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
> >
> > If the inode that's being requested to track using ext4_fc_track_inode
> > is being committed, then wait until the inode finishes the commit.
> >
> > Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
>
> Looks mostly good. Just some notes below.
>
> > diff --git a/fs/ext4/ext4_jbd2.c b/fs/ext4/ext4_jbd2.c
> > index 3477a16d08ae..7fa301b0a35a 100644
> > --- a/fs/ext4/ext4_jbd2.c
> > +++ b/fs/ext4/ext4_jbd2.c
> > @@ -106,6 +106,18 @@ handle_t *__ext4_journal_start_sb(struct super_block *sb, unsigned int line,
> >                                  GFP_NOFS, type, line);
> >  }
> >
> > +handle_t *__ext4_journal_start(struct inode *inode, unsigned int line,
> > +                               int type, int blocks, int rsv_blocks,
> > +                               int revoke_creds)
> > +{
> > +     handle_t *handle = __ext4_journal_start_sb(inode->i_sb, line,
> > +                                                type, blocks, rsv_blocks,
> > +                                                revoke_creds);
> > +     if (ext4_handle_valid(handle) && !IS_ERR(handle))
> > +             ext4_fc_track_inode(handle, inode);
>
> Why do you need to call ext4_fc_track_inode() here? Calls in
> ext4_map_blocks() and ext4_mark_iloc_dirty() should be enough, shouldn't
> they?
This is just a precautionary call. ext4_fc_track_inode is an
idempotent function, so it doesn't matter if it gets called multiple
times. This check just covers cases (such as the ones in inline.c)
where ext4_reserve_inode_write() doesn't get called. I saw a few
failures in the log group when I remove this call. The right way to
fix this though is to ensure that ext4_reserve_inode_write() gets
called before every inode update.

>
> > +     return handle;
> > +}
> > +
> >  int __ext4_journal_stop(const char *where, unsigned int line, handle_t *handle)
> >  {
> >       struct super_block *sb;
>
> ...
>
> > @@ -519,6 +525,33 @@ void ext4_fc_track_inode(handle_t *handle, struct inode *inode)
> >               return;
> >       }
> >
> > +     if (!test_opt2(inode->i_sb, JOURNAL_FAST_COMMIT) ||
> > +         (EXT4_SB(inode->i_sb)->s_mount_state & EXT4_FC_REPLAY))
> > +             return;
> > +
> > +     spin_lock(&ei->i_fc_lock);
> > +     while (ext4_test_inode_state(inode, EXT4_STATE_FC_COMMITTING)) {
> > +#if (BITS_PER_LONG < 64)
> > +             DEFINE_WAIT_BIT(wait, &ei->i_state_flags,
> > +                             EXT4_STATE_FC_COMMITTING);
> > +             wq = bit_waitqueue(&ei->i_state_flags,
> > +                                EXT4_STATE_FC_COMMITTING);
> > +#else
> > +             DEFINE_WAIT_BIT(wait, &ei->i_flags,
> > +                             EXT4_STATE_FC_COMMITTING);
> > +             wq = bit_waitqueue(&ei->i_flags,
> > +                                EXT4_STATE_FC_COMMITTING);
> > +#endif
> > +
> > +             prepare_to_wait(wq, &wait.wq_entry, TASK_UNINTERRUPTIBLE);
> > +             spin_unlock(&ei->i_fc_lock);
> > +
> > +             schedule();
> > +             finish_wait(wq, &wait.wq_entry);
> > +             spin_lock(&ei->i_fc_lock);
> > +     }
> > +     spin_unlock(&ei->i_fc_lock);
>
> Hum, we operate inode state with atomic bitops. So I think there's no real
> need for ei->i_fc_lock here. You just need to be careful and check inode
> state again after prepare_to_wait() call.
Okay that makes sense, I'll do this in V2.

- Harshad
>
>                                                                 Honza
> --
> Jan Kara <jack@suse.com>
> SUSE Labs, CR