Re: [PATCH 10/11] xfs: make COW fork unwritten extent conversions more robust

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Mon, Dec 03, 2018 at 05:25:02PM -0500, Christoph Hellwig wrote:
> If we have racing buffered and direct I/O COW fork extents under
> writeback can have been moved to the data fork by the time we call
> xfs_reflink_convert_cow from xfs_submit_ioend.  This would be mostly
> harmless as the block numbers don't change by this move, except for
> the fact that xfs_bmapi_write will crash or trigger asserts when
> not finding existing extents, even despite trying to paper over this
> with the XFS_BMAPI_CONVERT_ONLY flag.
> 
> Instead of special casing non-transaction conversions in the already
> way too complicated xfs_bmapi_write just add a new helper for the much
> simpler non-transactional COW fork case, which simplify ignores not
> found extents.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/xfs/libxfs/xfs_bmap.c | 12 ++------
>  fs/xfs/libxfs/xfs_bmap.h |  8 +++---
>  fs/xfs/xfs_reflink.c     | 61 +++++++++++++++++++++++++---------------
>  3 files changed, 45 insertions(+), 36 deletions(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> index 1992ed8a60b0..fbed7ed34a7f 100644
> --- a/fs/xfs/libxfs/xfs_bmap.c
> +++ b/fs/xfs/libxfs/xfs_bmap.c
> @@ -2029,7 +2029,7 @@ xfs_bmap_add_extent_delay_real(
>  /*
>   * Convert an unwritten allocation to a real allocation or vice versa.
>   */
> -STATIC int				/* error */
> +int					/* error */
>  xfs_bmap_add_extent_unwritten_real(
>  	struct xfs_trans	*tp,
>  	xfs_inode_t		*ip,	/* incore inode pointer */
> @@ -4236,9 +4236,7 @@ xfs_bmapi_write(
>  
>  	ASSERT(*nmap >= 1);
>  	ASSERT(*nmap <= XFS_BMAP_MAX_NMAP);
> -	ASSERT(tp != NULL ||
> -	       (flags & (XFS_BMAPI_CONVERT | XFS_BMAPI_COWFORK)) ==
> -			(XFS_BMAPI_CONVERT | XFS_BMAPI_COWFORK));
> +	ASSERT(tp != NULL);
>  	ASSERT(len > 0);
>  	ASSERT(XFS_IFORK_FORMAT(ip, whichfork) != XFS_DINODE_FMT_LOCAL);
>  	ASSERT(xfs_isilocked(ip, XFS_ILOCK_EXCL));
> @@ -4316,9 +4314,6 @@ xfs_bmapi_write(
>  			 * locked and hence a truncate will block on them
>  			 * first.
>  			 */
> -			ASSERT(!((flags & XFS_BMAPI_CONVERT) &&
> -			         (flags & XFS_BMAPI_COWFORK)));
> -
>  			if (flags & XFS_BMAPI_DELALLOC) {
>  				if (eof || bno >= end)
>  					break;
> @@ -4333,8 +4328,7 @@ xfs_bmapi_write(
>  		 * First, deal with the hole before the allocated space
>  		 * that we found, if any.
>  		 */
> -		if ((need_alloc || wasdelay) &&
> -		    !(flags & XFS_BMAPI_CONVERT_ONLY)) {
> +		if (need_alloc || wasdelay) {
>  			bma.eof = eof;
>  			bma.conv = !!(flags & XFS_BMAPI_CONVERT);
>  			bma.wasdel = wasdelay;
> diff --git a/fs/xfs/libxfs/xfs_bmap.h b/fs/xfs/libxfs/xfs_bmap.h
> index f9a925caa70e..ee3848680684 100644
> --- a/fs/xfs/libxfs/xfs_bmap.h
> +++ b/fs/xfs/libxfs/xfs_bmap.h
> @@ -98,9 +98,6 @@ struct xfs_extent_free_item
>  /* Only convert delalloc space, don't allocate entirely new extents */
>  #define XFS_BMAPI_DELALLOC	0x400
>  
> -/* Only convert unwritten extents, don't allocate new blocks */
> -#define XFS_BMAPI_CONVERT_ONLY	0x800
> -
>  /* Skip online discard of freed extents */
>  #define XFS_BMAPI_NODISCARD	0x1000
>  
> @@ -118,7 +115,6 @@ struct xfs_extent_free_item
>  	{ XFS_BMAPI_REMAP,	"REMAP" }, \
>  	{ XFS_BMAPI_COWFORK,	"COWFORK" }, \
>  	{ XFS_BMAPI_DELALLOC,	"DELALLOC" }, \
> -	{ XFS_BMAPI_CONVERT_ONLY, "CONVERT_ONLY" }, \
>  	{ XFS_BMAPI_NODISCARD,	"NODISCARD" }, \
>  	{ XFS_BMAPI_NORMAP,	"NORMAP" }
>  
> @@ -227,6 +223,10 @@ int	xfs_bmapi_reserve_delalloc(struct xfs_inode *ip, int whichfork,
>  		xfs_fileoff_t off, xfs_filblks_t len, xfs_filblks_t prealloc,
>  		struct xfs_bmbt_irec *got, struct xfs_iext_cursor *cur,
>  		int eof);
> +int	xfs_bmap_add_extent_unwritten_real(struct xfs_trans *tp,
> +		struct xfs_inode *ip, int whichfork,
> +		struct xfs_iext_cursor *icur, struct xfs_btree_cur **curp,
> +		struct xfs_bmbt_irec *new, int *logflagsp);
>  
>  static inline void
>  xfs_bmap_add_free(
> diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
> index d59b556d42cb..0cf13cb1b2fe 100644
> --- a/fs/xfs/xfs_reflink.c
> +++ b/fs/xfs/xfs_reflink.c
> @@ -234,26 +234,42 @@ xfs_reflink_trim_around_shared(
>  	}
>  }
>  
> -/* Convert part of an unwritten CoW extent to a real one. */
> -STATIC int
> -xfs_reflink_convert_cow_extent(
> -	struct xfs_inode		*ip,
> -	struct xfs_bmbt_irec		*imap,
> -	xfs_fileoff_t			offset_fsb,
> -	xfs_filblks_t			count_fsb)
> +static int
> +xfs_reflink_convert_cow_locked(
> +	struct xfs_inode	*ip,
> +	xfs_fileoff_t		offset_fsb,
> +	xfs_filblks_t		count_fsb)
>  {
> -	int				nimaps = 1;
> +	struct xfs_iext_cursor	icur;
> +	struct xfs_bmbt_irec	got;
> +	struct xfs_btree_cur	*dummy_cur = NULL;
> +	int			dummy_logflags;
> +	int			error;
>  
> -	if (imap->br_state == XFS_EXT_NORM)
> +	if (!xfs_iext_lookup_extent(ip, ip->i_cowfp, offset_fsb, &icur, &got))
>  		return 0;
>  
> -	xfs_trim_extent(imap, offset_fsb, count_fsb);
> -	trace_xfs_reflink_convert_cow(ip, imap);
> -	if (imap->br_blockcount == 0)
> -		return 0;
> -	return xfs_bmapi_write(NULL, ip, imap->br_startoff, imap->br_blockcount,
> -			XFS_BMAPI_COWFORK | XFS_BMAPI_CONVERT, 0, imap,
> -			&nimaps);
> +	do {
> +		if (got.br_startoff >= offset_fsb + count_fsb)
> +			break;
> +		if (got.br_state == XFS_EXT_NORM)
> +			continue;
> +		if (WARN_ON_ONCE(isnullstartblock(got.br_startblock)))
> +			return -EIO;
> +
> +		xfs_trim_extent(&got, offset_fsb, count_fsb);
> +		if (!got.br_blockcount)
> +			continue;
> +
> +		got.br_state = XFS_EXT_NORM;
> +		error = xfs_bmap_add_extent_unwritten_real(NULL, ip,
> +				XFS_COW_FORK, &icur, &dummy_cur, &got,
> +				&dummy_logflags);
> +		if (error)
> +			return error;
> +	} while (xfs_iext_next_extent(ip->i_cowfp, &icur, &got));
> +
> +	return error;
>  }
>  
>  /* Convert all of the unwritten CoW extents in a file's range to real ones. */
> @@ -267,15 +283,12 @@ xfs_reflink_convert_cow(
>  	xfs_fileoff_t		offset_fsb = XFS_B_TO_FSBT(mp, offset);
>  	xfs_fileoff_t		end_fsb = XFS_B_TO_FSB(mp, offset + count);
>  	xfs_filblks_t		count_fsb = end_fsb - offset_fsb;
> -	struct xfs_bmbt_irec	imap;
> -	int			nimaps = 1, error = 0;
> +	int			error;
>  
>  	ASSERT(count != 0);
>  
>  	xfs_ilock(ip, XFS_ILOCK_EXCL);
> -	error = xfs_bmapi_write(NULL, ip, offset_fsb, count_fsb,
> -			XFS_BMAPI_COWFORK | XFS_BMAPI_CONVERT |
> -			XFS_BMAPI_CONVERT_ONLY, 0, &imap, &nimaps);
> +	error = xfs_reflink_convert_cow_locked(ip, offset_fsb, count_fsb);

At this point you might as well convert the one remaining caller of
xfs_reflink_convert_cow to take and drop the ILOCK around the
reflink_convert_cow call...

--D

>  	xfs_iunlock(ip, XFS_ILOCK_EXCL);
>  	return error;
>  }
> @@ -405,9 +418,11 @@ xfs_reflink_allocate_cow(
>  	if (nimaps == 0)
>  		return -ENOSPC;
>  convert:
> -	if (!(flags & IOMAP_DIRECT))
> +	xfs_trim_extent(imap, offset_fsb, count_fsb);
> +	if (!(flags & IOMAP_DIRECT) || imap->br_state == XFS_EXT_NORM)
>  		return 0;
> -	return xfs_reflink_convert_cow_extent(ip, imap, offset_fsb, count_fsb);
> +	trace_xfs_reflink_convert_cow(ip, imap);
> +	return xfs_reflink_convert_cow_locked(ip, offset_fsb, count_fsb);
>  
>  out_unreserve:
>  	xfs_trans_unreserve_quota_nblks(tp, ip, (long)resblks, 0,
> -- 
> 2.19.1
>