linux-kernel - Re: [RFC PATCH V2 08/12] fs/xfs: Add lock/unlock mode to xfs

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Date:   Thu, 16 Jan 2020 11:12:10 -0800
From:   Ira Weiny <ira.weiny@...el.com>
To:     Jan Kara <jack@...e.cz>
Cc:     linux-kernel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Christoph Hellwig <hch@....de>,
        "Theodore Y. Ts'o" <tytso@....edu>, linux-ext4@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [RFC PATCH V2 08/12] fs/xfs: Add lock/unlock mode to xfs

On Thu, Jan 16, 2020 at 10:24:46AM +0100, Jan Kara wrote:
> On Fri 10-01-20 11:29:38, ira.weiny@...el.com wrote:
> > From: Ira Weiny <ira.weiny@...el.com>
> > 
> > XFS requires regular files to be locked while changing to/from DAX mode.
> > 
> > Define a new DAX lock type and implement the [un]lock_mode() inode
> > operation callbacks.
> > 
> > We define a new XFS_DAX_* lock type to carry the lock through the
> > transaction because we don't want to use IOLOCK as that would cause
> > performance issues with locking of the inode itself.
> > 
> > Signed-off-by: Ira Weiny <ira.weiny@...el.com>
> ...
> > diff --git a/fs/xfs/xfs_inode.h b/fs/xfs/xfs_inode.h
> > index 492e53992fa9..693ca66bd89b 100644
> > --- a/fs/xfs/xfs_inode.h
> > +++ b/fs/xfs/xfs_inode.h
> > @@ -67,6 +67,9 @@ typedef struct xfs_inode {
> >  	spinlock_t		i_ioend_lock;
> >  	struct work_struct	i_ioend_work;
> >  	struct list_head	i_ioend_list;
> > +
> > +	/* protect changing the mode to/from DAX */
> > +	struct percpu_rw_semaphore i_dax_sem;
> >  } xfs_inode_t;
> 
> This adds overhead of ~32k per inode for typical distro kernel.

Wow!

> That's not
> going to fly.

Probably not...

> That's why ext4 has similar kind of lock in the superblock
> shared by all inodes. For read side it does not matter because that's
> per-cpu and shared lock. For write side we don't care as changing inode
> access mode should be rare.

Sounds reasonable to me.  I'll convert it.

Thanks for pointing this out, that would have been bad indeed.
Ira