linux-ext4 - Re: [PATCH 2/3] jbd2: introduce jbd2

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Date:   Thu, 20 Jun 2019 09:09:11 -0600
From:   Ross Zwisler <zwisler@...gle.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Ross Zwisler <zwisler@...omium.org>, linux-kernel@...r.kernel.org,
        Theodore Ts'o <tytso@....edu>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jan Kara <jack@...e.com>, linux-ext4@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        Fletcher Woodruff <fletcherw@...gle.com>,
        Justin TerAvest <teravest@...gle.com>
Subject: Re: [PATCH 2/3] jbd2: introduce jbd2_inode dirty range scoping

On Thu, Jun 20, 2019 at 01:04:54PM +0200, Jan Kara wrote:
> On Wed 19-06-19 11:21:55, Ross Zwisler wrote:
> > Currently both journal_submit_inode_data_buffers() and
> > journal_finish_inode_data_buffers() operate on the entire address space
> > of each of the inodes associated with a given journal entry.  The
> > consequence of this is that if we have an inode where we are constantly
> > appending dirty pages we can end up waiting for an indefinite amount of
> > time in journal_finish_inode_data_buffers() while we wait for all the
> > pages under writeback to be written out.
> > 
> > The easiest way to cause this type of workload is do just dd from
> > /dev/zero to a file until it fills the entire filesystem.  This can
> > cause journal_finish_inode_data_buffers() to wait for the duration of
> > the entire dd operation.
> > 
> > We can improve this situation by scoping each of the inode dirty ranges
> > associated with a given transaction.  We do this via the jbd2_inode
> > structure so that the scoping is contained within jbd2 and so that it
> > follows the lifetime and locking rules for that structure.
> > 
> > This allows us to limit the writeback & wait in
> > journal_submit_inode_data_buffers() and
> > journal_finish_inode_data_buffers() respectively to the dirty range for
> > a given struct jdb2_inode, keeping us from waiting forever if the inode
> > in question is still being appended to.
> > 
> > Signed-off-by: Ross Zwisler <zwisler@...gle.com>
> 
> The patch looks good to me. I was thinking whether we should not have
> separate ranges for current and the next transaction but I guess it is not
> worth it at least for now. So just one nit below. With that applied feel free
> to add:
> 
> Reviewed-by: Jan Kara <jack@...e.cz>

We could definitely keep separate dirty ranges for each of the current and
next transaction.  I think the case where you would see a difference would be
if you had multiple transactions in a row which grew the dirty range for a
given jbd2_inode, and then had a random I/O workload which kept dirtying pages
inside that enlarged dirty range.

I'm not sure how often this type of workload would be a problem.  For the
workloads I've been testing which purely append to the inode, having a single
dirty range per jbd2_inode is sufficient.

I guess for now this single range seems simpler, but if later we find that
someone would benefit from separate tracking for each of the current and next
transactions, I'll take a shot at adding it.

Thank you for the review!

> > @@ -257,15 +262,24 @@ static int journal_finish_inode_data_buffers(journal_t *journal,
> >  	/* For locking, see the comment in journal_submit_data_buffers() */
> >  	spin_lock(&journal->j_list_lock);
> >  	list_for_each_entry(jinode, &commit_transaction->t_inode_list, i_list) {
> > +		loff_t dirty_start = jinode->i_dirty_start;
> > +		loff_t dirty_end = jinode->i_dirty_end;
> > +
> >  		if (!(jinode->i_flags & JI_WAIT_DATA))
> >  			continue;
> >  		jinode->i_flags |= JI_COMMIT_RUNNING;
> >  		spin_unlock(&journal->j_list_lock);
> > -		err = filemap_fdatawait_keep_errors(
> > -				jinode->i_vfs_inode->i_mapping);
> > +		err = filemap_fdatawait_range_keep_errors(
> > +				jinode->i_vfs_inode->i_mapping, dirty_start,
> > +				dirty_end);
> >  		if (!ret)
> >  			ret = err;
> >  		spin_lock(&journal->j_list_lock);
> > +
> > +		if (!jinode->i_next_transaction) {
> > +			jinode->i_dirty_start = 0;
> > +			jinode->i_dirty_end = 0;
> > +		}
> 
> This would be more logical in the next loop that moves jinode into the next
> transaction.

Yep, agreed, this is much better.  Fixed in v2.