xfs: convert growfs AG header init to use buffer lists

We currently write all new AG headers synchronously, which can be
slow for large grow operations. All we really need to do is ensure
all the headers are on disk before we run the growfs transaction, so
convert this to a buffer list and a delayed write operation. We
block waiting for the delayed write buffer submission to complete,
so this will fulfill the requirement to have all the buffers written
correctly before proceeding.
Signed-Off-By: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

fs/xfs/xfs_fsops.c

@@ -81,7 +81,8 @@ xfs_grow_ag_headers(
 	struct xfs_mount	*mp,
 	xfs_agnumber_t		agno,
 	xfs_extlen_t		agsize,
-	xfs_rfsblock_t		*nfree)
+	xfs_rfsblock_t		*nfree,
+	struct list_head	*buffer_list)
 {
 	struct xfs_agf		*agf;
 	struct xfs_agi		*agi;
@@ -135,11 +136,8 @@ xfs_grow_ag_headers(
 		agf->agf_refcount_level = cpu_to_be32(1);
 		agf->agf_refcount_blocks = cpu_to_be32(1);
 	}
-
-	error = xfs_bwrite(bp);
+	xfs_buf_delwri_queue(bp, buffer_list);
 	xfs_buf_relse(bp);
-	if (error)
-		goto out_error;
 
 	/*
 	 * AG freelist header block
@@ -164,10 +162,8 @@ xfs_grow_ag_headers(
 	for (bucket = 0; bucket < xfs_agfl_size(mp); bucket++)
 		agfl_bno[bucket] = cpu_to_be32(NULLAGBLOCK);
 
-	error = xfs_bwrite(bp);
+	xfs_buf_delwri_queue(bp, buffer_list);
 	xfs_buf_relse(bp);
-	if (error)
-		goto out_error;
 
 	/*
 	 * AG inode header block
@@ -201,10 +197,8 @@ xfs_grow_ag_headers(
 	for (bucket = 0; bucket < XFS_AGI_UNLINKED_BUCKETS; bucket++)
 		agi->agi_unlinked[bucket] = cpu_to_be32(NULLAGINO);
 
-	error = xfs_bwrite(bp);
+	xfs_buf_delwri_queue(bp, buffer_list);
 	xfs_buf_relse(bp);
-	if (error)
-		goto out_error;
 
 	/*
 	 * BNO btree root block
@@ -226,10 +220,8 @@ xfs_grow_ag_headers(
 	arec->ar_blockcount = cpu_to_be32(
 		agsize - be32_to_cpu(arec->ar_startblock));
 
-	error = xfs_bwrite(bp);
+	xfs_buf_delwri_queue(bp, buffer_list);
 	xfs_buf_relse(bp);
-	if (error)
-		goto out_error;
 
 	/*
 	 * CNT btree root block
@@ -251,10 +243,8 @@ xfs_grow_ag_headers(
 		agsize - be32_to_cpu(arec->ar_startblock));
 	*nfree += be32_to_cpu(arec->ar_blockcount);
 
-	error = xfs_bwrite(bp);
+	xfs_buf_delwri_queue(bp, buffer_list);
 	xfs_buf_relse(bp);
-	if (error)
-		goto out_error;
 
 	/* RMAP btree root block */
 	if (xfs_sb_version_hasrmapbt(&mp->m_sb)) {
@@ -326,10 +316,8 @@ xfs_grow_ag_headers(
 			be16_add_cpu(&block->bb_numrecs, 1);
 		}
 
-		error = xfs_bwrite(bp);
+		xfs_buf_delwri_queue(bp, buffer_list);
 		xfs_buf_relse(bp);
-		if (error)
-			goto out_error;
 	}
 
 	/*
@@ -345,11 +333,8 @@ xfs_grow_ag_headers(
 	}
 
 	xfs_btree_init_block(mp, bp, XFS_BTNUM_INO , 0, 0, agno, 0);
-
-	error = xfs_bwrite(bp);
+	xfs_buf_delwri_queue(bp, buffer_list);
 	xfs_buf_relse(bp);
-	if (error)
-		goto out_error;
 
 	/*
 	 * FINO btree root block
@@ -364,13 +349,9 @@ xfs_grow_ag_headers(
 			goto out_error;
 		}
 
-		xfs_btree_init_block(mp, bp, XFS_BTNUM_FINO,
-					     0, 0, agno, 0);
-
-		error = xfs_bwrite(bp);
+		xfs_btree_init_block(mp, bp, XFS_BTNUM_FINO, 0, 0, agno, 0);
+		xfs_buf_delwri_queue(bp, buffer_list);
 		xfs_buf_relse(bp);
-		if (error)
-			goto out_error;
 	}
 
 	/*
@@ -386,13 +367,9 @@ xfs_grow_ag_headers(
 			goto out_error;
 		}
 
-		xfs_btree_init_block(mp, bp, XFS_BTNUM_REFC,
-				     0, 0, agno, 0);
-
-		error = xfs_bwrite(bp);
+		xfs_btree_init_block(mp, bp, XFS_BTNUM_REFC, 0, 0, agno, 0);
+		xfs_buf_delwri_queue(bp, buffer_list);
 		xfs_buf_relse(bp);
-		if (error)
-			goto out_error;
 	}
 
 out_error:
@@ -419,6 +396,7 @@ xfs_growfs_data_private(
 	xfs_agnumber_t		oagcount;
 	int			pct;
 	xfs_trans_t		*tp;
+	LIST_HEAD		(buffer_list);
 
 	nb = in->newblocks;
 	pct = in->imaxpct;
@@ -459,9 +437,16 @@ xfs_growfs_data_private(
 		return error;
 
 	/*
-	 * Write new AG headers to disk. Non-transactional, but written
-	 * synchronously so they are completed prior to the growfs transaction
-	 * being logged.
+	 * Write new AG headers to disk. Non-transactional, but need to be
+	 * written and completed prior to the growfs transaction being logged.
+	 * To do this, we use a delayed write buffer list and wait for
+	 * submission and IO completion of the list as a whole. This allows the
+	 * IO subsystem to merge all the AG headers in a single AG into a single
+	 * IO and hide most of the latency of the IO from us.
+	 *
+	 * This also means that if we get an error whilst building the buffer
+	 * list to write, we can cancel the entire list without having written
+	 * anything.
 	 */
 	nfree = 0;
 	for (agno = nagcount - 1; agno >= oagcount; agno--, new -= agsize) {
@@ -472,10 +457,17 @@ xfs_growfs_data_private(
 		else
 			agsize = mp->m_sb.sb_agblocks;
 
-		error = xfs_grow_ag_headers(mp, agno, agsize, &nfree);
-		if (error)
+		error = xfs_grow_ag_headers(mp, agno, agsize, &nfree,
+					    &buffer_list);
+		if (error) {
+			xfs_buf_delwri_cancel(&buffer_list);
 			goto error0;
+		}
 	}
+	error = xfs_buf_delwri_submit(&buffer_list);
+	if (error)
+		goto error0;
+
 	xfs_trans_agblocks_delta(tp, nfree);
 
 	/*