Re: [PATCH] xfs: update i_size after unwritten conversion in dio completion

[Eryu Guan <eguan@redhat.com>]

On Thu, Sep 21, 2017 at 08:07:10AM -0400, Brian Foster wrote:
> On Thu, Sep 21, 2017 at 06:38:28PM +0800, Eryu Guan wrote:
> > Since commit d531d91d6990 ("xfs: always use unwritten extents for
> > direct I/O writes"), we start allocating unwritten extents for all
> > direct writes to allow appending aio in XFS.
> > 
> > But for dio writes that could extend file size we update the in-core
> > inode size first, then convert the unwritten extents to real
> > allocations at dio completion time in xfs_dio_write_end_io(). Thus a
> > racing direct read could see the new i_size and find the unwritten
> > extents first and read zeros instead of actual data, if the direct
> > writer also takes a shared iolock.
> > 
> > Fix it by updating the in-core inode size after the unwritten extent
> > conversion. To do this, introduce a new boolean argument to
> > xfs_iomap_write_unwritten() to tell if we want to update in-core
> > i_size or not.
> > 
> > Suggested-by: Brian Foster <bfoster@redhat.com>
> > Signed-off-by: Eryu Guan <eguan@redhat.com>
> > ---
> > 
> > Patch passed the test posted by Eric[1] and a locally modified aio
> > version of the test.
> > 
> > I also ran fstests with config xfs_4k_crc, xfs_2k_reflink, xfs_1k_rmap
> > and xfs_512, and aio-dio tests from ltp, I don't see any new failures
> > introduced.
> > 
> > [1] http://www.spinics.net/lists/fstests/msg06978.html
> > 
> 
> Looks pretty good, just a few comment/whitespace nits...
> 
> >  fs/xfs/xfs_aops.c  |  9 ++++++++-
> >  fs/xfs/xfs_file.c  | 34 ++++++++++++++++++++--------------
> >  fs/xfs/xfs_iomap.c |  7 ++++++-
> >  fs/xfs/xfs_iomap.h |  2 +-
> >  fs/xfs/xfs_pnfs.c  |  2 +-
> >  5 files changed, 36 insertions(+), 18 deletions(-)
> > 
> > diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
> > index 29172609f2a3..f937968e9515 100644
> > --- a/fs/xfs/xfs_aops.c
> > +++ b/fs/xfs/xfs_aops.c
> > @@ -343,7 +343,14 @@ xfs_end_io(
> >  		error = xfs_reflink_end_cow(ip, offset, size);
> >  		break;
> >  	case XFS_IO_UNWRITTEN:
> > -		error = xfs_iomap_write_unwritten(ip, offset, size);
> > +		/*
> > +		 * The correct in-core inode size should have been updated by
> > +		 * generic_write_end, and the 'size' here is buffer head
> > +		 * granularity size of the ioend, which could be larger than
> > +		 * the actual bytes written. So skip in-core i_size update in
> > +		 * xfs_iomap_write_unwritten()
> > +		 */
> 
> The more I think about this, the less applicable the granularity seems
> to be (sorry :P). It seems rather broken if writeback were to affect
> isize in general. Perhaps just the following?
> 
> 		/* writeback should never update isize */

Sure :)

> 
> > +		error = xfs_iomap_write_unwritten(ip, offset, size, false);
> >  		break;
> >  	default:
> >  		ASSERT(!xfs_ioend_is_append(ioend) || ioend->io_append_trans);
> > diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> > index 350b6d43ba23..d4796c5a88fe 100644
> > --- a/fs/xfs/xfs_file.c
> > +++ b/fs/xfs/xfs_file.c
> > @@ -434,7 +434,6 @@ xfs_dio_write_end_io(
> >  	struct inode		*inode = file_inode(iocb->ki_filp);
> >  	struct xfs_inode	*ip = XFS_I(inode);
> >  	loff_t			offset = iocb->ki_pos;
> > -	bool			update_size = false;
> >  	int			error = 0;
> >  
> >  	trace_xfs_end_io_direct_write(ip, offset, size);
> > @@ -445,6 +444,22 @@ xfs_dio_write_end_io(
> >  	if (size <= 0)
> >  		return size;
> >  
> > +	if (flags & IOMAP_DIO_COW) {
> > +		error = xfs_reflink_end_cow(ip, offset, size);
> > +		if (error)
> > +			return error;
> > +	}
> > +
> > +	 /*
> > +	 * Deal with IOMAP_DIO_UNWRITTEN case before check & update in-core
> > +	 * inode size, we do it only after converting the unwritten extents to
> > +	 * real allocations in xfs_iomap_write_write_unwritten() (by passing
> > +	 * 'true' as the last argument to it), otherwise a racing direct read
> > +	 * could see the new size thus read zeros because of unwritten extents.
> > +	 */
> 
> /*
>  * Unwritten conversion updates the in-core isize after extent
>  * conversion but before updating the on-disk size. Updating isize any
>  * earlier allows a racing dio read to find unwritten extents before
>  * they are converted.
>  */

I'll just copy native English speaker's comments :)

> 
> > +	if (flags & IOMAP_DIO_UNWRITTEN)
> > +		return xfs_iomap_write_unwritten(ip, offset, size, true);
> > +
> >  	/*
> >  	 * We need to update the in-core inode size here so that we don't end up
> >  	 * with the on-disk inode size being outside the in-core inode size. We
> > @@ -459,20 +474,11 @@ xfs_dio_write_end_io(
> >  	spin_lock(&ip->i_flags_lock);
> >  	if (offset + size > i_size_read(inode)) {
> >  		i_size_write(inode, offset + size);
> > -		update_size = true;
> > -	}
> > -	spin_unlock(&ip->i_flags_lock);
> > -
> > -	if (flags & IOMAP_DIO_COW) {
> > -		error = xfs_reflink_end_cow(ip, offset, size);
> > -		if (error)
> > -			return error;
> > -	}
> > -
> > -	if (flags & IOMAP_DIO_UNWRITTEN)
> > -		error = xfs_iomap_write_unwritten(ip, offset, size);
> > -	else if (update_size)
> > +		spin_unlock(&ip->i_flags_lock);
> >  		error = xfs_setfilesize(ip, offset, size);
> > +	} else {
> > +		spin_unlock(&ip->i_flags_lock);
> > +	}
> >  
> >  	return error;
> >  }
> > diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> > index a1909bc064e9..90dc551a8621 100644
> > --- a/fs/xfs/xfs_iomap.c
> > +++ b/fs/xfs/xfs_iomap.c
> > @@ -829,7 +829,8 @@ int
> >  xfs_iomap_write_unwritten(
> >  	xfs_inode_t	*ip,
> >  	xfs_off_t	offset,
> > -	xfs_off_t	count)
> > +	xfs_off_t	count,
> > +	bool		update_isize)
> >  {
> >  	xfs_mount_t	*mp = ip->i_mount;
> >  	xfs_fileoff_t	offset_fsb;
> > @@ -840,6 +841,7 @@ xfs_iomap_write_unwritten(
> >  	xfs_trans_t	*tp;
> >  	xfs_bmbt_irec_t imap;
> >  	struct xfs_defer_ops dfops;
> > +	struct inode	*inode = VFS_I(ip);
> >  	xfs_fsize_t	i_size;
> >  	uint		resblks;
> >  	int		error;
> > @@ -900,6 +902,9 @@ xfs_iomap_write_unwritten(
> >  		if (i_size > offset + count)
> >  			i_size = offset + count;
> >  
> > +		if (update_isize && (i_size > i_size_read(inode)))
> > +			i_size_write(inode, i_size);
> > +
> 
> We could probably kill the blank lines around the above hunk.

Will do.

> 
> With those minor fixups:
> 
> Reviewed-by: Brian Foster <bfoster@redhat.com>
> 
> Thanks for taking care of this!

Thanks a lot for your suggestions and reviews!

Eryu

> 
> >  		i_size = xfs_new_eof(ip, i_size);
> >  		if (i_size) {
> >  			ip->i_d.di_size = i_size;
> > diff --git a/fs/xfs/xfs_iomap.h b/fs/xfs/xfs_iomap.h
> > index 00db3ecea084..ee535065c5d0 100644
> > --- a/fs/xfs/xfs_iomap.h
> > +++ b/fs/xfs/xfs_iomap.h
> > @@ -27,7 +27,7 @@ int xfs_iomap_write_direct(struct xfs_inode *, xfs_off_t, size_t,
> >  			struct xfs_bmbt_irec *, int);
> >  int xfs_iomap_write_allocate(struct xfs_inode *, int, xfs_off_t,
> >  			struct xfs_bmbt_irec *);
> > -int xfs_iomap_write_unwritten(struct xfs_inode *, xfs_off_t, xfs_off_t);
> > +int xfs_iomap_write_unwritten(struct xfs_inode *, xfs_off_t, xfs_off_t, bool);
> >  
> >  void xfs_bmbt_to_iomap(struct xfs_inode *, struct iomap *,
> >  		struct xfs_bmbt_irec *);
> > diff --git a/fs/xfs/xfs_pnfs.c b/fs/xfs/xfs_pnfs.c
> > index 2f2dc3c09ad0..4246876df7b7 100644
> > --- a/fs/xfs/xfs_pnfs.c
> > +++ b/fs/xfs/xfs_pnfs.c
> > @@ -274,7 +274,7 @@ xfs_fs_commit_blocks(
> >  					(end - 1) >> PAGE_SHIFT);
> >  		WARN_ON_ONCE(error);
> >  
> > -		error = xfs_iomap_write_unwritten(ip, start, length);
> > +		error = xfs_iomap_write_unwritten(ip, start, length, false);
> >  		if (error)
> >  			goto out_drop_iolock;
> >  	}
> > -- 
> > 2.13.5
> > 
> > --
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