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Message-ID: <20141127133227.GA11872@quack.suse.cz>
Date: Thu, 27 Nov 2014 14:32:27 +0100
From: Jan Kara <jack@...e.cz>
To: Dave Chinner <david@...morbit.com>
Cc: Andreas Dilger <adilger@...ger.ca>, Theodore Ts'o <tytso@....edu>,
Linux Filesystem Development List
<linux-fsdevel@...r.kernel.org>,
Ext4 Developers List <linux-ext4@...r.kernel.org>,
Linux btrfs Developers List <linux-btrfs@...r.kernel.org>,
XFS Developers <xfs@....sgi.com>
Subject: Re: [PATCH-v4 6/7] ext4: add support for a lazytime mount option
On Thu 27-11-14 14:27:52, Jan Kara wrote:
> On Thu 27-11-14 10:35:37, Dave Chinner wrote:
> > On Wed, Nov 26, 2014 at 04:10:44PM -0700, Andreas Dilger wrote:
> > > On Nov 26, 2014, at 3:48 PM, Dave Chinner <david@...morbit.com> wrote:
> > > >
> > > > On Wed, Nov 26, 2014 at 05:23:56AM -0500, Theodore Ts'o wrote:
> > > >> Add an optimization for the MS_LAZYTIME mount option so that we will
> > > >> opportunistically write out any inodes with the I_DIRTY_TIME flag set
> > > >> in a particular inode table block when we need to update some inode
> > > >> in that inode table block anyway.
> > > >>
> > > >> Also add some temporary code so that we can set the lazytime mount
> > > >> option without needing a modified /sbin/mount program which can set
> > > >> MS_LAZYTIME. We can eventually make this go away once util-linux has
> > > >> added support.
> > > >>
> > > >> Google-Bug-Id: 18297052
> > > >>
> > > >> Signed-off-by: Theodore Ts'o <tytso@....edu>
> > > >> ---
> > > >> fs/ext4/inode.c | 49 ++++++++++++++++++++++++++++++++++++++++++---
> > > >> fs/ext4/super.c | 9 +++++++++
> > > >> include/trace/events/ext4.h | 30 +++++++++++++++++++++++++++
> > > >> 3 files changed, 85 insertions(+), 3 deletions(-)
> > > >>
> > > >> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> > > >> index 5653fa4..8308c82 100644
> > > >> --- a/fs/ext4/inode.c
> > > >> +++ b/fs/ext4/inode.c
> > > >> @@ -4140,6 +4140,51 @@ static int ext4_inode_blocks_set(handle_t *handle,
> > > >> }
> > > >>
> > > >> /*
> > > >> + * Opportunistically update the other time fields for other inodes in
> > > >> + * the same inode table block.
> > > >> + */
> > > >> +static void ext4_update_other_inodes_time(struct super_block *sb,
> > > >> + unsigned long orig_ino, char *buf)
> > > >> +{
> > > >> + struct ext4_inode_info *ei;
> > > >> + struct ext4_inode *raw_inode;
> > > >> + unsigned long ino;
> > > >> + struct inode *inode;
> > > >> + int i, inodes_per_block = EXT4_SB(sb)->s_inodes_per_block;
> > > >> + int inode_size = EXT4_INODE_SIZE(sb);
> > > >> +
> > > >> + ino = orig_ino & ~(inodes_per_block - 1);
> > > >> + for (i = 0; i < inodes_per_block; i++, ino++, buf += inode_size) {
> > > >> + if (ino == orig_ino)
> > > >> + continue;
> > > >> + inode = find_active_inode_nowait(sb, ino);
> > > >> + if (!inode ||
> > > >> + (inode->i_state & I_DIRTY_TIME) == 0 ||
> > > >> + !spin_trylock(&inode->i_lock)) {
> > > >> + iput(inode);
> > > >> + continue;
> > > >> + }
> > > >> + inode->i_state &= ~I_DIRTY_TIME;
> > > >> + inode->i_ts_dirty_day = 0;
> > > >> + spin_unlock(&inode->i_lock);
> > > >> + inode_requeue_dirtytime(inode);
> > > >> +
> > > >> + ei = EXT4_I(inode);
> > > >> + raw_inode = (struct ext4_inode *) buf;
> > > >> +
> > > >> + spin_lock(&ei->i_raw_lock);
> > > >> + EXT4_INODE_SET_XTIME(i_ctime, inode, raw_inode);
> > > >> + EXT4_INODE_SET_XTIME(i_mtime, inode, raw_inode);
> > > >> + EXT4_INODE_SET_XTIME(i_atime, inode, raw_inode);
> > > >> + ext4_inode_csum_set(inode, raw_inode, ei);
> > > >> + spin_unlock(&ei->i_raw_lock);
> > > >> + trace_ext4_other_inode_update_time(inode, orig_ino);
> > > >> + iput(inode);
> > > >> + }
> > > >> +}
> > > >
> > > > Am I right in that this now does unlogged timestamp updates of
> > > > inodes? What happens when that buffer gets overwritten by log
> > > > recover after a crash? The timestamp updates get lost?
> > > >
> > > > FYI, XFS has had all sorts of nasty log recovery corner cases
> > > > caused by log recovery overwriting non-logged inode updates like
> > > > this. In the past few years we've removed every single non-logged
> > > > inode update "optimisation" so that all changes (including timestamps)
> > > > are transactional so inode state on disk not matching what log
> > > > recovery wrote to disk for all the other inode metadata...
> > > >
> > > > Optimistic unlogged inode updates are a slippery slope, and history
> > > > tells me that it doesn't lead to a nice place....
> > >
> > > Since ext4/jbd2 is logging the whole block, unlike XFS which is doing
> > > logical journaling, this isn't an unlogged update. It is just taking
> > > advantage of the fact that the whole block is going to be logged and
> > > written to the disk anyway.
> >
> > Urk - that's worse, isn't it? i.e the code above calls iput() from
> > within a current transaction context? What happens if that drops
> > the last reference to the inode and it gets evicted due to racing
> > with an unlink? Won't that try to start another transaction to free
> > the inode (i.e. through ext4_evict_inode())?
> Yeah, the patch looks buggy (and racy wrt concurrent updates of time
> stamps as well). I think if we want to do this optimization, we would need
> a function like "clear inode dirty bits for this range of inode numbers".
> That is doable atomically within VFS and although it looks somewhat ugly,
> the performance
Sorry, I sent this too early (did send instead of postpone). So the patch
looks buggy because of iput() but it isn't racy wrt time updates as I
checked now. So it would be enough to move calling of this outside of the
transaction and start new handle for each inode.
Honza
--
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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