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From: Dave Chinner <david@fromorbit.com>
To: Christoph Hellwig <hch@infradead.org>
Cc: "Theodore Y. Ts'o" <tytso@mit.edu>,
	Eric Biggers <ebiggers@kernel.org>,
	Linux Filesystem Development List 
	<linux-fsdevel@vger.kernel.org>,
	Ext4 Developers List <linux-ext4@vger.kernel.org>,
	linux-f2fs-devel@lists.sourceforge.net,
	linux-xfs@vger.kernel.org
Subject: Re: [PATCH] writeback: avoid double-writing the inode on a lazytime expiration
Date: Fri, 13 Mar 2020 09:39:13 +1100	[thread overview]
Message-ID: <20200312223913.GL10776@dread.disaster.area> (raw)
In-Reply-To: <20200312143445.GA19160@infradead.org>

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 time stamps that may be 24 hours out of date in memory, and they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

WARNING: multiple messages have this Message-ID
From: Dave Chinner <david@fromorbit.com>
To: Christoph Hellwig <hch@infradead.org>
Cc: linux-xfs@vger.kernel.org, "Theodore Y. Ts'o" <tytso@mit.edu>,
	linux-f2fs-devel@lists.sourceforge.net,
	Eric Biggers <ebiggers@kernel.org>,
	Linux Filesystem Development List <linux-fsdevel@vger.kernel.org>,
	Ext4 Developers List <linux-ext4@vger.kernel.org>
Subject: Re: [f2fs-dev] [PATCH] writeback: avoid double-writing the inode on a lazytime expiration
Date: Fri, 13 Mar 2020 09:39:13 +1100	[thread overview]
Message-ID: <20200312223913.GL10776@dread.disaster.area> (raw)
In-Reply-To: <20200312143445.GA19160@infradead.org>

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


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Linux-f2fs-devel@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel

  reply	other threads:[~2020-03-12 22:39 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-06  0:45 lazytime causing inodes to remain dirty after sync? Eric Biggers
2020-03-06  0:45 ` [f2fs-dev] " Eric Biggers
2020-03-07  2:00 ` [PATCH] writeback: avoid double-writing the inode on a lazytime expiration Theodore Ts'o
2020-03-07  2:00   ` [f2fs-dev] " Theodore Ts'o
2020-03-11  3:20   ` Eric Biggers
2020-03-11  3:20     ` [f2fs-dev] " Eric Biggers
2020-03-11 12:57     ` Theodore Y. Ts'o
2020-03-11 12:57       ` [f2fs-dev] " Theodore Y. Ts'o
2020-03-12  0:07       ` Dave Chinner
2020-03-12  0:07         ` [f2fs-dev] " Dave Chinner
2020-03-12 14:34         ` Christoph Hellwig
2020-03-12 14:34           ` [f2fs-dev] " Christoph Hellwig
2020-03-12 22:39           ` Dave Chinner [this message]
2020-03-12 22:39             ` Dave Chinner
2020-03-20  2:46           ` Theodore Y. Ts'o
2020-03-20  2:46             ` [f2fs-dev] " Theodore Y. Ts'o
2020-03-20  2:52             ` [PATCH 1/2] " Theodore Ts'o
2020-03-20  2:52               ` [f2fs-dev] " Theodore Ts'o
2020-03-20  2:52               ` [PATCH 2/2] writeback, xfs: call dirty_inode() with I_DIRTY_TIME_EXPIRED when appropriate Theodore Ts'o
2020-03-20  2:52                 ` [f2fs-dev] " Theodore Ts'o
2020-03-23 17:58                 ` Theodore Y. Ts'o
2020-03-23 17:58                   ` [f2fs-dev] " Theodore Y. Ts'o
2020-03-24  8:37                   ` Christoph Hellwig
2020-03-24  8:37                     ` [f2fs-dev] " Christoph Hellwig
2020-03-24 18:43                     ` Theodore Y. Ts'o
2020-03-24 18:43                       ` [f2fs-dev] " Theodore Y. Ts'o
2020-03-25  9:20               ` [PATCH 1/2] writeback: avoid double-writing the inode on a lazytime expiration Christoph Hellwig
2020-03-25  9:20                 ` [f2fs-dev] " Christoph Hellwig
2020-03-25 15:21                 ` Theodore Y. Ts'o
2020-03-25 15:21                   ` [f2fs-dev] " Theodore Y. Ts'o
2020-03-25 15:47                   ` Darrick J. Wong
2020-03-25 15:47                     ` [f2fs-dev] " Darrick J. Wong
2020-03-11 23:54     ` [PATCH] " Dave Chinner
2020-03-11 23:54       ` [f2fs-dev] " Dave Chinner

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