xfs: simplify xfs_file_iomap_begin() logic

The current logic that determines whether allocation should be done
has grown somewhat spaghetti like with the addition of IOMAP_NOWAIT
functionality. Separate out each of the different cases into single,
obvious checks to get rid most of the nested IOMAP_NOWAIT checks
in the allocation logic.
Signed-Off-By: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

fs/xfs/xfs_iomap.c

@@ -1040,6 +1040,10 @@ xfs_file_iomap_begin(
 			goto out_unlock;
 	}
 
+	/* Non-modifying mapping requested, so we are done */
+	if (!(flags & (IOMAP_WRITE | IOMAP_ZERO)))
+		goto out_found;
+
 	if (xfs_is_reflink_inode(ip) &&
 	    ((flags & IOMAP_WRITE) ||
 	     ((flags & IOMAP_ZERO) && needs_cow_for_zeroing(&imap, nimaps)))) {
@@ -1068,29 +1072,33 @@ xfs_file_iomap_begin(
 		length = XFS_FSB_TO_B(mp, end_fsb) - offset;
 	}
 
-	if ((flags & IOMAP_WRITE) && imap_needs_alloc(inode, &imap, nimaps)) {
-		/*
-		 * If nowait is set bail since we are going to make
-		 * allocations.
-		 */
+	/* Don't need to allocate over holes when doing zeroing operations. */
+	if (flags & IOMAP_ZERO)
+		goto out_found;
+
+	if (!imap_needs_alloc(inode, &imap, nimaps))
+		goto out_found;
+
+	/* If nowait is set bail since we are going to make allocations. */
 	if (flags & IOMAP_NOWAIT) {
 		error = -EAGAIN;
 		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.
+	 * We cap the maximum length we map to a sane size  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.
+	 * Note that the values needs to be less than 32-bits wide until the
+	 * lower level functions are updated.
 	 */
 	length = min_t(loff_t, length, 1024 * PAGE_SIZE);
+
 	/*
-		 * xfs_iomap_write_direct() expects the shared lock. It
-		 * is unlocked on return.
+	 * xfs_iomap_write_direct() expects the shared lock. It is unlocked on
+	 * return.
 	 */
 	if (lockmode == XFS_ILOCK_EXCL)
 		xfs_ilock_demote(ip, lockmode);
@@ -1101,13 +1109,8 @@ xfs_file_iomap_begin(
 
 	iomap->flags = IOMAP_F_NEW;
 	trace_xfs_iomap_alloc(ip, offset, length, 0, &imap);
-	} else {
-		ASSERT(nimaps);
-
-		xfs_iunlock(ip, lockmode);
-		trace_xfs_iomap_found(ip, offset, length, 0, &imap);
-	}
 
+out_finish:
 	if (xfs_ipincount(ip) && (ip->i_itemp->ili_fsync_fields
 				& ~XFS_ILOG_TIMESTAMP))
 		iomap->flags |= IOMAP_F_DIRTY;
@@ -1117,6 +1120,13 @@ xfs_file_iomap_begin(
 	if (shared)
 		iomap->flags |= IOMAP_F_SHARED;
 	return 0;
+
+out_found:
+	ASSERT(nimaps);
+	xfs_iunlock(ip, lockmode);
+	trace_xfs_iomap_found(ip, offset, length, 0, &imap);
+	goto out_finish;
+
 out_unlock:
 	xfs_iunlock(ip, lockmode);
 	return error;