Re: [PATCH] writeback: avoid double-writing the inode on a lazytime expiration

[Dave Chinner <david@fromorbit.com>]

On Thu, Mar 12, 2020 at 07:34:45AM -0700, Christoph Hellwig wrote:
> On Thu, Mar 12, 2020 at 11:07:17AM +1100, Dave Chinner wrote:
> > > That's true, but when the timestamps were originally modified,
> > > dirty_inode() will be called with flag == I_DIRTY_TIME, which will
> > > *not* be a no-op; which is to say, XFS will force the timestamps to be
> > > updated on disk when the timestamps are first dirtied, because it
> > > doesn't support I_DIRTY_TIME.
> > 
> > We log the initial timestamp change, and then ignore timestamp
> > updates until the dirty time expires and the inode is set
> > I_DIRTY_SYNC via __mark_inode_dirty_sync(). IOWs, on expiry, we have
> > time stamps that may be 24 hours out of date in memory, and they
> > still need to be flushed to the journal.
> > 
> > However, your change does not mark the inode dirtying on expiry
> > anymore, so...
> > 
> > > So I think we're fine.
> > 
> > ... we're not fine. This breaks XFS and any other filesystem that
> > relies on a I_DIRTY_SYNC notification to handle dirty time expiry
> > correctly.
> 
> I haven't seen the original mail this replies to,

The original problem was calling mark_inode_dirty_sync() on expiry
during inode writeback was causing the inode to be put back on the
dirty inode list and so ext4 was flushing it twice - once on expiry
and once 5 seconds later on the next background writeback pass.

This is a problem that XFS does not have because it does not
implement ->write_inode...

> but if we could
> get the lazytime expirty by some other means (e.g. an explicit
> callback), XFS could opt out of all the VFS inode tracking again,
> which would simplify a few things.

Yes, that would definitely make things simpler for XFS, and it would
also solve the problem that the generic lazytime expiry code has....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com