linux-ext4 - Re: [PATCH 03/11] ext4: Convert DAX reads to iomap infrastructure

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Date:   Fri, 11 Nov 2016 10:57:38 -0700
From:   Ross Zwisler <ross.zwisler@...ux.intel.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Ted Tso <tytso@....edu>, linux-ext4@...r.kernel.org,
        linux-fsdevel@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH 03/11] ext4: Convert DAX reads to iomap infrastructure

On Fri, Nov 11, 2016 at 11:17:51AM +0100, Jan Kara wrote:
> On Thu 10-11-16 14:54:31, Ross Zwisler wrote:
> > On Tue, Nov 08, 2016 at 12:08:09PM +0100, Jan Kara wrote:
> > > Implement basic iomap_begin function that handles reading and use it for
> > > DAX reads.
> > > 
> > > Signed-off-by: Jan Kara <jack@...e.cz>
> > > ---
> > >  fs/ext4/ext4.h  |  2 ++
> > >  fs/ext4/file.c  | 38 +++++++++++++++++++++++++++++++++++++-
> > >  fs/ext4/inode.c | 54 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
> > >  3 files changed, 93 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> > > index 282a51b07c57..098b39910001 100644
> > > --- a/fs/ext4/ext4.h
> > > +++ b/fs/ext4/ext4.h
> > > @@ -3271,6 +3271,8 @@ static inline bool ext4_aligned_io(struct inode *inode, loff_t off, loff_t len)
> > >  	return IS_ALIGNED(off, blksize) && IS_ALIGNED(len, blksize);
> > >  }
> > >  
> > > +extern struct iomap_ops ext4_iomap_ops;
> > > +
> > >  #endif	/* __KERNEL__ */
> > >  
> > >  #define EFSBADCRC	EBADMSG		/* Bad CRC detected */
> > > diff --git a/fs/ext4/file.c b/fs/ext4/file.c
> > > index 9facb4dc5c70..1f25c644cb12 100644
> > > --- a/fs/ext4/file.c
> > > +++ b/fs/ext4/file.c
> > > @@ -31,6 +31,42 @@
> > >  #include "xattr.h"
> > >  #include "acl.h"
> > >  
> > > +#ifdef CONFIG_FS_DAX
> > > +static ssize_t ext4_dax_read_iter(struct kiocb *iocb, struct iov_iter *to)
> > > +{
> > > +	struct inode *inode = file_inode(iocb->ki_filp);
> > > +	ssize_t ret;
> > > +
> > > +	inode_lock_shared(inode);
> > > +	/*
> > > +	 * Recheck under inode lock - at this point we are sure it cannot
> > > +	 * change anymore
> > > +	 */
> > > +	if (!IS_DAX(inode)) {
> > > +		inode_unlock_shared(inode);
> > > +		/* Fallback to buffered IO in case we cannot support DAX */
> > > +		return generic_file_read_iter(iocb, to);
> > 
> > Is this not also racy, since we've just dropped the inode lock?  What's to
> > prevent this sequence?
> > 
> > Thread 0				Thread 1
> > --------				--------
> > ext4_file_read_iter()
> >   IS_DAX() returns true
> >   					changes S_DAX to false
> >   ext4_dax_read_iter()
> >     inode_lock_shared()
> >     IS_DAX() returns false
> >     inode_unlock_shared()
> >   					changes S_DAX to true
> >     generic_file_read_iter() on a DAX inode
> > 
> > 
> > Or are we okay in this scenario?
> 
> Yup, I'm aware of this. The real problem is that there's no way to
> serialize with buffered reads for ext4 (they take only page locks) so
> currently you can have buffered reads in flight when inode gets switched to
> DAX mode. I agree there is a potential for breakage and it needs to be
> resolved eventually but the problem is not new and these patches don't make
> it really any worse so I just somewhat fixed it up by patch 2/11 and left
> full solution to a separate patch set.

Fair enough.  You can add:

Reviewed-by: Ross Zwisler <ross.zwisler@...ux.intel.com>
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