Re: [PATCH 08/11] xfs: merge COW handling into xfs_file_iomap_begin_delay

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

On Mon, Dec 03, 2018 at 05:25:00PM -0500, Christoph Hellwig wrote:
> Besides simplifying the code a bit this allows to actually implement
> the behavior of using COW preallocation for non-COW data mentioned
> in the current comments.
> 
> Note that this breaks the current version of xfs/420, but that is
> because the test is broken.  A separate fix will be sent for it.

(Still waiting for this...)

> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/xfs/xfs_iomap.c   | 132 ++++++++++++++++++++++++++++++-------------
>  fs/xfs/xfs_reflink.c |  67 ----------------------
>  fs/xfs/xfs_trace.h   |   1 -
>  3 files changed, 93 insertions(+), 107 deletions(-)
> 
> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> index d851abac16a9..d19f99e5476a 100644
> --- a/fs/xfs/xfs_iomap.c
> +++ b/fs/xfs/xfs_iomap.c
> @@ -534,15 +534,16 @@ xfs_file_iomap_begin_delay(
>  {
>  	struct xfs_inode	*ip = XFS_I(inode);
>  	struct xfs_mount	*mp = ip->i_mount;
> -	struct xfs_ifork	*ifp = XFS_IFORK_PTR(ip, XFS_DATA_FORK);
>  	xfs_fileoff_t		offset_fsb = XFS_B_TO_FSBT(mp, offset);
>  	xfs_fileoff_t		maxbytes_fsb =
>  		XFS_B_TO_FSB(mp, mp->m_super->s_maxbytes);
>  	xfs_fileoff_t		end_fsb;
> -	int			error = 0, eof = 0;
> -	struct xfs_bmbt_irec	got;
> -	struct xfs_iext_cursor	icur;
> +	struct xfs_bmbt_irec	imap, cmap;
> +	struct xfs_iext_cursor	icur, ccur;
>  	xfs_fsblock_t		prealloc_blocks = 0;
> +	bool			eof = false, cow_eof = false, shared;
> +	int			whichfork = XFS_DATA_FORK;
> +	int			error = 0;
>  
>  	ASSERT(!XFS_IS_REALTIME_INODE(ip));
>  	ASSERT(!xfs_get_extsz_hint(ip));
> @@ -560,7 +561,7 @@ xfs_file_iomap_begin_delay(
>  
>  	XFS_STATS_INC(mp, xs_blk_mapw);
>  
> -	if (!(ifp->if_flags & XFS_IFEXTENTS)) {
> +	if (!(ip->i_df.if_flags & XFS_IFEXTENTS)) {
>  		error = xfs_iread_extents(NULL, ip, XFS_DATA_FORK);
>  		if (error)
>  			goto out_unlock;
> @@ -568,51 +569,92 @@ xfs_file_iomap_begin_delay(
>  
>  	end_fsb = min(XFS_B_TO_FSB(mp, offset + count), maxbytes_fsb);
>  
> -	eof = !xfs_iext_lookup_extent(ip, ifp, offset_fsb, &icur, &got);
> +	/*
> +	 * Search the data fork fork first to look up our source mapping.  We
> +	 * always need the data fork map, as we have to return it to the
> +	 * iomap code so that the higher level write code can read data in to
> +	 * perform read-modify-write cycles for unaligned writes.
> +	 */
> +	eof = !xfs_iext_lookup_extent(ip, &ip->i_df, offset_fsb, &icur, &imap);
>  	if (eof)
> -		got.br_startoff = end_fsb; /* fake hole until the end */
> +		imap.br_startoff = end_fsb; /* fake hole until the end */
>  
> -	if (got.br_startoff <= offset_fsb) {
> +	/* We never need to allocate blocks for zeroing a hole. */
> +	if ((flags & IOMAP_ZERO) && imap.br_startoff > offset_fsb) {
> +		xfs_hole_to_iomap(ip, iomap, offset_fsb, imap.br_startoff);
> +		goto out_unlock;
> +	}
> +
> +	/*
> +	 * Search the COW fork extent list even if we did not find a data fork
> +	 * extent.  This serves two purposes: first this implements the
> +	 * speculative preallocation using cowextisze, so that we also unshare

"cowextsize"

> +	 * block adjacent to shared blocks instead of just the shared blocks
> +	 * themselves.  Second the lookup in the extent list is generally faster
> +	 * than going out to the shared extent tree.
> +	 */
> +	if (xfs_is_reflink_inode(ip)) {
> +		cow_eof = !xfs_iext_lookup_extent(ip, ip->i_cowfp, offset_fsb,
> +				&ccur, &cmap);
> +		if (!cow_eof && cmap.br_startoff <= offset_fsb) {
> +			trace_xfs_reflink_cow_found(ip, &cmap);
> +			whichfork = XFS_COW_FORK;
> +			goto done;
> +		}
> +	}
> +
> +	if (imap.br_startoff <= offset_fsb) {
>  		/*
>  		 * For reflink files we may need a delalloc reservation when
>  		 * overwriting shared extents.   This includes zeroing of
>  		 * existing extents that contain data.
>  		 */
> -		if (xfs_is_reflink_inode(ip) &&
> -		    ((flags & IOMAP_WRITE) ||
> -		     got.br_state != XFS_EXT_UNWRITTEN)) {
> -			xfs_trim_extent(&got, offset_fsb, end_fsb - offset_fsb);
> -			error = xfs_reflink_reserve_cow(ip, &got);
> -			if (error)
> -				goto out_unlock;
> +		if (!xfs_is_reflink_inode(ip) ||
> +		    ((flags & IOMAP_ZERO) && imap.br_state != XFS_EXT_NORM)) {
> +			trace_xfs_iomap_found(ip, offset, count, XFS_DATA_FORK,
> +					&imap);
> +			goto done;
>  		}
>  
> -		trace_xfs_iomap_found(ip, offset, count, XFS_DATA_FORK, &got);
> -		goto done;
> -	}
> +		xfs_trim_extent(&imap, offset_fsb, end_fsb - offset_fsb);
>  
> -	if (flags & IOMAP_ZERO) {
> -		xfs_hole_to_iomap(ip, iomap, offset_fsb, got.br_startoff);
> -		goto out_unlock;
> +		/* Trim the mapping to the nearest shared extent boundary. */
> +		error = xfs_reflink_trim_around_shared(ip, &imap, &shared);
> +		if (error)
> +			goto out_unlock;
> +
> +		/* Not shared?  Just report the (potentially capped) extent. */
> +		if (!shared) {
> +			trace_xfs_iomap_found(ip, offset, count, XFS_DATA_FORK,
> +					&imap);
> +			goto done;
> +		}
> +
> +		/*
> +		 * Fork all the shared blocks from our write offset until the
> +		 * end of the extent.
> +		 */
> +		whichfork = XFS_COW_FORK;
> +		end_fsb = imap.br_startoff + imap.br_blockcount;
> +	} else {
> +		/*
> +		 * We cap the maximum length we map here to MAX_WRITEBACK_PAGES
> +		 * pages to keep the chunks of work done where somewhat
> +		 * symmetric with the work writeback does.  This is a completely
> +		 * arbitrary number pulled out of thin air.
> +		 *
> +		 * Note that the values needs to be less than 32-bits wide until
> +		 * the lower level functions are updated.
> +		 */
> +		count = min_t(loff_t, count, 1024 * PAGE_SIZE);
> +		end_fsb = min(XFS_B_TO_FSB(mp, offset + count), maxbytes_fsb);
>  	}
>  
>  	error = xfs_qm_dqattach_locked(ip, false);
>  	if (error)
>  		goto out_unlock;
>  
> -	/*
> -	 * We cap the maximum length we map here to MAX_WRITEBACK_PAGES pages
> -	 * to keep the chunks of work done where somewhat symmetric with the
> -	 * work writeback does. This is a completely arbitrary number pulled
> -	 * out of thin air as a best guess for initial testing.
> -	 *
> -	 * Note that the values needs to be less than 32-bits wide until
> -	 * the lower level functions are updated.
> -	 */
> -	count = min_t(loff_t, count, 1024 * PAGE_SIZE);
> -	end_fsb = min(XFS_B_TO_FSB(mp, offset + count), maxbytes_fsb);
> -
> -	if (eof) {
> +	if (eof && whichfork == XFS_DATA_FORK) {
>  		prealloc_blocks = xfs_iomap_prealloc_size(ip, offset, count,
>  				&icur);
>  		if (prealloc_blocks) {
> @@ -635,9 +677,11 @@ xfs_file_iomap_begin_delay(
>  	}
>  
>  retry:
> -	error = xfs_bmapi_reserve_delalloc(ip, XFS_DATA_FORK, offset_fsb,
> -			end_fsb - offset_fsb, prealloc_blocks, &got, &icur,
> -			eof);
> +	error = xfs_bmapi_reserve_delalloc(ip, whichfork, offset_fsb,
> +			end_fsb - offset_fsb, prealloc_blocks,
> +			whichfork == XFS_DATA_FORK ? &imap : &cmap,
> +			whichfork == XFS_DATA_FORK ? &icur : &ccur,
> +			whichfork == XFS_DATA_FORK ? eof : cow_eof);
>  	switch (error) {
>  	case 0:
>  		break;
> @@ -659,9 +703,19 @@ xfs_file_iomap_begin_delay(
>  	 * them out if the write happens to fail.
>  	 */
>  	iomap->flags |= IOMAP_F_NEW;
> -	trace_xfs_iomap_alloc(ip, offset, count, XFS_DATA_FORK, &got);
> +	trace_xfs_iomap_alloc(ip, offset, count, whichfork, &imap);

I'm confused by this, if whichfork == COW then won't ftrace report
results from the wrong fork?

--D

>  done:
> -	error = xfs_bmbt_to_iomap(ip, iomap, &got, false);
> +	if (whichfork == XFS_COW_FORK) {
> +		if (imap.br_startoff > offset_fsb) {
> +			xfs_trim_extent(&cmap, offset_fsb,
> +					imap.br_startoff - offset_fsb);
> +			error = xfs_bmbt_to_iomap(ip, iomap, &cmap, false);
> +			goto out_unlock;
> +		}
> +		/* ensure we only report blocks we have a reservation for */
> +		xfs_trim_extent(&imap, cmap.br_startoff, cmap.br_blockcount);
> +	}
> +	error = xfs_bmbt_to_iomap(ip, iomap, &imap, false);
>  out_unlock:
>  	xfs_iunlock(ip, XFS_ILOCK_EXCL);
>  	return error;
> diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
> index bdbaff1b3fb7..d59b556d42cb 100644
> --- a/fs/xfs/xfs_reflink.c
> +++ b/fs/xfs/xfs_reflink.c
> @@ -234,73 +234,6 @@ xfs_reflink_trim_around_shared(
>  	}
>  }
>  
> -/*
> - * Trim the passed in imap to the next shared/unshared extent boundary, and
> - * if imap->br_startoff points to a shared extent reserve space for it in the
> - * COW fork.
> - *
> - * Note that imap will always contain the block numbers for the existing blocks
> - * in the data fork, as the upper layers need them for read-modify-write
> - * operations.
> - */
> -int
> -xfs_reflink_reserve_cow(
> -	struct xfs_inode	*ip,
> -	struct xfs_bmbt_irec	*imap)
> -{
> -	struct xfs_ifork	*ifp = XFS_IFORK_PTR(ip, XFS_COW_FORK);
> -	struct xfs_bmbt_irec	got;
> -	int			error = 0;
> -	bool			eof = false;
> -	struct xfs_iext_cursor	icur;
> -	bool			shared;
> -
> -	/*
> -	 * Search the COW fork extent list first.  This serves two purposes:
> -	 * first this implement the speculative preallocation using cowextisze,
> -	 * so that we also unshared block adjacent to shared blocks instead
> -	 * of just the shared blocks themselves.  Second the lookup in the
> -	 * extent list is generally faster than going out to the shared extent
> -	 * tree.
> -	 */
> -
> -	if (!xfs_iext_lookup_extent(ip, ifp, imap->br_startoff, &icur, &got))
> -		eof = true;
> -	if (!eof && got.br_startoff <= imap->br_startoff) {
> -		trace_xfs_reflink_cow_found(ip, imap);
> -		xfs_trim_extent(imap, got.br_startoff, got.br_blockcount);
> -		return 0;
> -	}
> -
> -	/* Trim the mapping to the nearest shared extent boundary. */
> -	error = xfs_reflink_trim_around_shared(ip, imap, &shared);
> -	if (error)
> -		return error;
> -
> -	/* Not shared?  Just report the (potentially capped) extent. */
> -	if (!shared)
> -		return 0;
> -
> -	/*
> -	 * Fork all the shared blocks from our write offset until the end of
> -	 * the extent.
> -	 */
> -	error = xfs_qm_dqattach_locked(ip, false);
> -	if (error)
> -		return error;
> -
> -	error = xfs_bmapi_reserve_delalloc(ip, XFS_COW_FORK, imap->br_startoff,
> -			imap->br_blockcount, 0, &got, &icur, eof);
> -	if (error == -ENOSPC || error == -EDQUOT)
> -		trace_xfs_reflink_cow_enospc(ip, imap);
> -	if (error)
> -		return error;
> -
> -	xfs_trim_extent(imap, got.br_startoff, got.br_blockcount);
> -	trace_xfs_reflink_cow_alloc(ip, &got);
> -	return 0;
> -}
> -
>  /* Convert part of an unwritten CoW extent to a real one. */
>  STATIC int
>  xfs_reflink_convert_cow_extent(
> diff --git a/fs/xfs/xfs_trace.h b/fs/xfs/xfs_trace.h
> index 870865913bd8..36e74fc90700 100644
> --- a/fs/xfs/xfs_trace.h
> +++ b/fs/xfs/xfs_trace.h
> @@ -3179,7 +3179,6 @@ DEFINE_INODE_ERROR_EVENT(xfs_reflink_unshare_error);
>  
>  /* copy on write */
>  DEFINE_INODE_IREC_EVENT(xfs_reflink_trim_around_shared);
> -DEFINE_INODE_IREC_EVENT(xfs_reflink_cow_alloc);
>  DEFINE_INODE_IREC_EVENT(xfs_reflink_cow_found);
>  DEFINE_INODE_IREC_EVENT(xfs_reflink_cow_enospc);
>  DEFINE_INODE_IREC_EVENT(xfs_reflink_convert_cow);
> -- 
> 2.19.1
>