[PATCH 8/8] vm: Add an tuning knob for vm.max_writeback_mb

[Jens Axboe <jens.axboe@oracle.com>]

From: Theodore Ts'o <tytso@mit.edu>

Originally, MAX_WRITEBACK_PAGES was hard-coded to 1024 because of a
concern of not holding I_SYNC for too long.  (At least, that was the
comment previously.)  This doesn't make sense now because the only
time we wait for I_SYNC is if we are calling sync or fsync, and in
that case we need to write out all of the data anyway.  Previously
there may have been other code paths that waited on I_SYNC, but not
any more.

According to Christoph, the current writeback size is way too small,
and XFS had a hack that bumped out nr_to_write to four times the value
sent by the VM to be able to saturate medium-sized RAID arrays.  This
value was also problematic for ext4 as well, as it caused large files
to be come interleaved on disk by in 8 megabyte chunks (we bumped up
the nr_to_write by a factor of two).

So, in this patch, we make the MAX_WRITEBACK_PAGES a tunable,
max_writeback_mb, and set it to a default value of 128 megabytes.

http://bugzilla.kernel.org/show_bug.cgi?id=13930

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
---
 fs/fs-writeback.c         |    9 +--------
 include/linux/writeback.h |    1 +
 kernel/sysctl.c           |    8 ++++++++
 mm/page-writeback.c       |    6 ++++++
 4 files changed, 16 insertions(+), 8 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index ce68f60..790d379 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -641,14 +641,7 @@ void writeback_inodes_wbc(struct writeback_control *wbc)
 	writeback_inodes_wb(&bdi->wb, wbc);
 }
 
-/*
- * The maximum number of pages to writeout in a single bdi flush/kupdate
- * operation.  We do this so we don't hold I_SYNC against an inode for
- * enormous amounts of time, which would block a userspace task which has
- * been forced to throttle against that inode.  Also, the code reevaluates
- * the dirty each time it has written this many pages.
- */
-#define MAX_WRITEBACK_PAGES     1024
+#define MAX_WRITEBACK_PAGES	(max_writeback_mb << (20 - PAGE_SHIFT))
 
 static inline bool over_bground_thresh(void)
 {
diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index 78b1e46..fbed759 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -104,6 +104,7 @@ extern int vm_dirty_ratio;
 extern unsigned long vm_dirty_bytes;
 extern unsigned int dirty_writeback_interval;
 extern unsigned int dirty_expire_interval;
+extern unsigned int max_writeback_mb;
 extern int vm_highmem_is_dirtyable;
 extern int block_dump;
 extern int laptop_mode;
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 58be760..315fc30 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1104,6 +1104,14 @@ static struct ctl_table vm_table[] = {
 		.proc_handler	= &proc_dointvec,
 	},
 	{
+		.ctl_name	= CTL_UNNUMBERED,
+		.procname	= "max_writeback_mb",
+		.data		= &max_writeback_mb,
+		.maxlen		= sizeof(max_writeback_mb),
+		.mode		= 0644,
+		.proc_handler	= &proc_dointvec,
+	},
+	{
 		.ctl_name	= VM_NR_PDFLUSH_THREADS,
 		.procname	= "nr_pdflush_threads",
 		.data		= &nr_pdflush_threads,
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 2c287d9..38fe4e8 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -55,6 +55,12 @@ static inline long sync_writeback_pages(void)
 /* The following parameters are exported via /proc/sys/vm */
 
 /*
+ * The maximum amount of memory (in megabytes) to write out in a
+ * single bdflush/kupdate operation.
+ */
+unsigned int max_writeback_mb = 128;
+
+/*
  * Start background writeback (via pdflush) at this percentage
  */
 int dirty_background_ratio = 10;
-- 
1.6.4.1.207.g68ea