[PATCH] bcache: make stripe_size configurable and persistent for hardware raid5/6

[Eric Wheeler]

From: Eric Wheeler <git@xxxxxxxxxxxxxxxxxx>

While some drivers set queue_limits.io_opt (e.g., md raid5), there are
currently no SCSI/RAID controller drivers that do.  Previously stripe_size
and partial_stripes_expensive were read-only values and could not be
tuned by users (eg, for hardware RAID5/6).

This patch enables users to save the optimal IO size via sysfs through
the backing device attributes stripe_size and partial_stripes_expensive
into the bcache superblock.

Superblock changes are backwards-compatable:

*  partial_stripes_expensive: One bit was used in the superblock flags field

*  stripe_size: There are eight 64-bit "pad" fields for future use in
   the superblock which default to 0; from those, 32-bits are now used
   to save the stripe_size and load at device registration time.

Signed-off-by: Eric Wheeler <bcache@xxxxxxxxxxxxxxxxxx>
---
 Documentation/admin-guide/bcache.rst | 21 +++++++++++++++++++++
 drivers/md/bcache/super.c            | 15 ++++++++++++++-
 drivers/md/bcache/sysfs.c            | 33 +++++++++++++++++++++++++++++++--
 include/uapi/linux/bcache.h          |  6 ++++--
 4 files changed, 70 insertions(+), 5 deletions(-)

diff --git a/Documentation/admin-guide/bcache.rst b/Documentation/admin-guide/bcache.rst
index c0ce64d..ef82022 100644
--- a/Documentation/admin-guide/bcache.rst
+++ b/Documentation/admin-guide/bcache.rst
@@ -420,6 +420,12 @@ dirty_data
 label
   Name of underlying device.
 
+partial_stripes_expensive
+  Flag to bcache that partial or unaligned stripe_size'd
+  writes to the backing device are expensive (e.g., RAID5/6 incur
+  read-copy-write). Writing this sysfs attribute updates the superblock
+  and also takes effect immediately.  See also stripe_size, below.
+
 readahead
   Size of readahead that should be performed.  Defaults to 0.  If set to e.g.
   1M, it will round cache miss reads up to that size, but without overlapping
@@ -458,6 +464,21 @@ stop
   Write to this file to shut down the bcache device and close the backing
   device.
 
+stripe_size
+  The stripe size in bytes of the backing device for optimial
+  write performance (also known as the "stride width"). This is set
+  automatically when using a device driver sets blk_limits_io_opt
+  (e.g., md, rbd, skd, zram, virtio_blk).  No hardware RAID controller
+  sets blk_limits_io_opt as of 2019-06-15, so configure this to suit
+  your needs.  Note that you must unregister and re-register the backing
+  device after making a change to stripe_size.
+
+  Where N is the number of data disks,
+    RAID5: stripe_size = (N-1)*RAID_CHUNK_SIZE.
+    RAID6: stripe_size = (N-2)*RAID_CHUNK_SIZE.
+
+  See also partial_stripes_expensive, above.
+
 writeback_delay
   When dirty data is written to the cache and it previously did not contain
   any, waits some number of seconds before initiating writeback. Defaults to
diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index 1b63ac8..d0b9501 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -80,6 +80,7 @@ static const char *read_super(struct cache_sb *sb, struct block_device *bdev,
 
 	sb->flags		= le64_to_cpu(s->flags);
 	sb->seq			= le64_to_cpu(s->seq);
+	sb->stripe_size		= le32_to_cpu(s->stripe_size);
 	sb->last_mount		= le32_to_cpu(s->last_mount);
 	sb->first_bucket	= le16_to_cpu(s->first_bucket);
 	sb->keys		= le16_to_cpu(s->keys);
@@ -221,6 +222,7 @@ static void __write_super(struct cache_sb *sb, struct bio *bio)
 
 	out->flags		= cpu_to_le64(sb->flags);
 	out->seq		= cpu_to_le64(sb->seq);
+	out->stripe_size	= cpu_to_le32(sb->stripe_size);
 
 	out->last_mount		= cpu_to_le32(sb->last_mount);
 	out->first_bucket	= cpu_to_le16(sb->first_bucket);
@@ -1258,7 +1260,18 @@ static int cached_dev_init(struct cached_dev *dc, unsigned int block_size)
 
 	dc->disk.stripe_size = q->limits.io_opt >> 9;
 
-	if (dc->disk.stripe_size)
+	if (dc->sb.stripe_size) {
+		if (dc->disk.stripe_size &&
+		    dc->disk.stripe_size != dc->sb.stripe_size) {
+			pr_warn("superblock stripe_size (%d) overrides bdev stripe_size (%d)\n",
+				(int)dc->sb.stripe_size,
+				(int)dc->disk.stripe_size);
+		}
+
+		dc->disk.stripe_size = dc->sb.stripe_size;
+		dc->partial_stripes_expensive =
+			(unsigned int)BDEV_PARTIAL_STRIPES_EXPENSIVE(&dc->sb);
+	} else if (dc->disk.stripe_size)
 		dc->partial_stripes_expensive =
 			q->limits.raid_partial_stripes_expensive;
 
diff --git a/drivers/md/bcache/sysfs.c b/drivers/md/bcache/sysfs.c
index bfb437f..4ebca52 100644
--- a/drivers/md/bcache/sysfs.c
+++ b/drivers/md/bcache/sysfs.c
@@ -111,8 +111,8 @@
 rw_attribute(writeback_rate_minimum);
 read_attribute(writeback_rate_debug);
 
-read_attribute(stripe_size);
-read_attribute(partial_stripes_expensive);
+rw_attribute(stripe_size);
+rw_attribute(partial_stripes_expensive);
 
 rw_attribute(synchronous);
 rw_attribute(journal_delay_ms);
@@ -343,6 +343,35 @@ static ssize_t bch_snprint_string_list(char *buf,
 		}
 	}
 
+	if (attr == &sysfs_stripe_size) {
+		int v = strtoul_or_return(buf);
+
+		if (v & 0x1FF) {
+			pr_err("stripe_size must be a muliple of 512-byte sectors");
+			return -EINVAL;
+		}
+
+		v >>= 9;
+
+		if (v != dc->sb.stripe_size) {
+			dc->sb.stripe_size = v;
+			pr_info("stripe_size=%d, re-register to take effect.",
+				v<<9);
+			bch_write_bdev_super(dc, NULL);
+		} else
+			pr_info("stripe_size is already set to %d.", v<<9);
+	}
+
+	if (attr == &sysfs_partial_stripes_expensive) {
+		int v = strtoul_or_return(buf);
+
+		if (v != BDEV_PARTIAL_STRIPES_EXPENSIVE(&dc->sb)) {
+			SET_BDEV_PARTIAL_STRIPES_EXPENSIVE(&dc->sb, v);
+			dc->partial_stripes_expensive = v;
+			bch_write_bdev_super(dc, NULL);
+		}
+	}
+
 	if (attr == &sysfs_stop_when_cache_set_failed) {
 		v = __sysfs_match_string(bch_stop_on_failure_modes, -1, buf);
 		if (v < 0)
diff --git a/include/uapi/linux/bcache.h b/include/uapi/linux/bcache.h
index 5d4f58e..ee60914 100644
--- a/include/uapi/linux/bcache.h
+++ b/include/uapi/linux/bcache.h
@@ -172,7 +172,9 @@ struct cache_sb {
 
 	__u64			flags;
 	__u64			seq;
-	__u64			pad[8];
+	__u32			stripe_size;
+	__u32			pad_u32;
+	__u64			pad_u64[7];
 
 	union {
 	struct {
@@ -230,7 +232,7 @@ static inline _Bool SB_IS_BDEV(const struct cache_sb *sb)
 #define BDEV_STATE_CLEAN		1U
 #define BDEV_STATE_DIRTY		2U
 #define BDEV_STATE_STALE		3U
-
+BITMASK(BDEV_PARTIAL_STRIPES_EXPENSIVE,	struct cache_sb, flags, 60, 1);
 /*
  * Magic numbers
  *
-- 
1.8.3.1