linux-ext4 - Re: [PATCH] writeback: avoid double-writing the inode on a lazytime expiration

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Date:   Thu, 12 Mar 2020 07:34:45 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     "Theodore Y. Ts'o" <tytso@....edu>,
        Eric Biggers <ebiggers@...nel.org>,
        Linux Filesystem Development List 
        <linux-fsdevel@...r.kernel.org>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        linux-f2fs-devel@...ts.sourceforge.net, linux-xfs@...r.kernel.org
Subject: Re: [PATCH] writeback: avoid double-writing the inode on a lazytime
 expiration

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 time stamps that may be 24 hours out of date in memory, and they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, 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.