[PATCH v2 3/3] xfs: Let the max iomap length be consistent with the writeback code

[Tang Yizhou]

From: Tang Yizhou <yizhou.tang@xxxxxxxxxx>

Since commit 1a12d8bd7b29 ("writeback: scale IO chunk size up to half
device bandwidth"), macro MAX_WRITEBACK_PAGES has been removed from the
writeback path. Therefore, the MAX_WRITEBACK_PAGES comments in
xfs_direct_write_iomap_begin() and xfs_buffered_write_iomap_begin() appear
outdated.

In addition, Christoph mentioned that the xfs iomap process should be
similar to writeback, so xfs_max_map_length() was written following the
logic of writeback_chunk_size().

v2: Thanks for Christoph's advice. Resync with the writeback code.

Signed-off-by: Tang Yizhou <yizhou.tang@xxxxxxxxxx>
---
 fs/fs-writeback.c         |  5 ----
 fs/xfs/xfs_iomap.c        | 52 ++++++++++++++++++++++++---------------
 include/linux/writeback.h |  5 ++++
 3 files changed, 37 insertions(+), 25 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index d8bec3c1bb1f..31c72e207e1b 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -31,11 +31,6 @@
 #include <linux/memcontrol.h>
 #include "internal.h"
 
-/*
- * 4MB minimal write chunk size
- */
-#define MIN_WRITEBACK_PAGES	(4096UL >> (PAGE_SHIFT - 10))
-
 /*
  * Passed into wb_writeback(), essentially a subset of writeback_control
  */
diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index 1e11f48814c0..80f759fa9534 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -4,6 +4,8 @@
  * Copyright (c) 2016-2018 Christoph Hellwig.
  * All Rights Reserved.
  */
+#include <linux/writeback.h>
+
 #include "xfs.h"
 #include "xfs_fs.h"
 #include "xfs_shared.h"
@@ -744,6 +746,34 @@ xfs_ilock_for_iomap(
 	return 0;
 }
 
+/*
+ * 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.
+ *
+ * Following the logic of writeback_chunk_size(), the length will be
+ * rounded to the nearest 4MB boundary.
+ *
+ * Note that the values needs to be less than 32-bits wide until the
+ * lower level functions are updated.
+ */
+static loff_t
+xfs_max_map_length(struct inode *inode, loff_t length)
+{
+	struct bdi_writeback *wb;
+	long pages;
+
+	spin_lock(&inode->i_lock);
+	wb = inode_to_wb(wb);
+	pages = min(wb->avg_write_bandwidth / 2,
+		    global_wb_domain.dirty_limit / DIRTY_SCOPE);
+	spin_unlock(&inode->i_lock);
+	pages = round_down(pages + MIN_WRITEBACK_PAGES, MIN_WRITEBACK_PAGES);
+
+	return min_t(loff_t, length, pages * PAGE_SIZE);
+}
+
 /*
  * Check that the imap we are going to return to the caller spans the entire
  * range that the caller requested for the IO.
@@ -878,16 +908,7 @@ xfs_direct_write_iomap_begin(
 	if (flags & (IOMAP_NOWAIT | IOMAP_OVERWRITE_ONLY))
 		goto out_unlock;
 
-	/*
-	 * 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.
-	 */
-	length = min_t(loff_t, length, 1024 * PAGE_SIZE);
+	length = xfs_max_map_length(inode, length);
 	end_fsb = xfs_iomap_end_fsb(mp, offset, length);
 
 	if (offset + length > XFS_ISIZE(ip))
@@ -1096,16 +1117,7 @@ xfs_buffered_write_iomap_begin(
 		allocfork = 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);
+		count = xfs_max_map_length(inode, count);
 		end_fsb = xfs_iomap_end_fsb(mp, offset, count);
 
 		if (xfs_is_always_cow_inode(ip))
diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index d6db822e4bb3..657bc4dd22d0 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -17,6 +17,11 @@ struct bio;
 
 DECLARE_PER_CPU(int, dirty_throttle_leaks);
 
+/*
+ * 4MB minimal write chunk size
+ */
+#define MIN_WRITEBACK_PAGES	(4096UL >> (PAGE_SHIFT - 10))
+
 /*
  * The global dirty threshold is normally equal to the global dirty limit,
  * except when the system suddenly allocates a lot of anonymous memory and
-- 
2.25.1