[PATCH v7 3/5] btrfs: create read policy sysfs attribute, pid

[Anand Jain <anand.jain@oracle.com>]

Add

 /sys/fs/btrfs/UUID/read_policy

attribute so that the read policy for the raid1 and raid10 chunks can be
tuned.

When this attribute is read, it shall show all available policies, with
active policy being with in [ ]. The read_policy attribute can be written
using one of the items listed in there.

For example:
  $cat /sys/fs/btrfs/UUID/read_policy
  [pid]
  $echo pid > /sys/fs/btrfs/UUID/read_policy

Signed-off-by: Anand Jain <anand.jain@oracle.com>
---
v5: 
  Title rename: old: btrfs: sysfs, add read_policy attribute
  Uses the btrfs_strmatch() helper (BTRFS_READ_POLICY_NAME_MAX dropped).
  Use the table for the policy names.
  Rename len to ret.
  Use a simple logic to prefix space in btrfs_read_policy_show()
  Reviewed-by: Josef Bacik <josef@toxicpanda.com> dropped.

v4:-
v3: rename [by_pid] to [pid]
v2: v2: check input len before strip and kstrdup

 fs/btrfs/sysfs.c | 49 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 49 insertions(+)

diff --git a/fs/btrfs/sysfs.c b/fs/btrfs/sysfs.c
index 7bb68cef98ab..c9a8850b186a 100644
--- a/fs/btrfs/sysfs.c
+++ b/fs/btrfs/sysfs.c
@@ -832,6 +832,54 @@ static int btrfs_strmatch(const char *given, const char *golden)
 	return -EINVAL;
 }
 
+static const char* const btrfs_read_policy_name[] = { "pid" };
+
+static ssize_t btrfs_read_policy_show(struct kobject *kobj,
+				      struct kobj_attribute *a, char *buf)
+{
+	int i;
+	ssize_t ret = 0;
+	struct btrfs_fs_devices *fs_devices = to_fs_devs(kobj);
+
+	for (i = 0; i < BTRFS_NR_READ_POLICY; i++) {
+		if (fs_devices->read_policy == i)
+			ret += snprintf(buf + ret, PAGE_SIZE - ret, "%s[%s]",
+					(ret == 0 ? "" : " "),
+					btrfs_read_policy_name[i]);
+		else
+			ret += snprintf(buf + ret, PAGE_SIZE - ret, "%s%s",
+					(ret == 0 ? "" : " "),
+					btrfs_read_policy_name[i]);
+	}
+
+	ret += snprintf(buf + ret, PAGE_SIZE - ret, "\n");
+
+	return ret;
+}
+
+static ssize_t btrfs_read_policy_store(struct kobject *kobj,
+				       struct kobj_attribute *a,
+				       const char *buf, size_t len)
+{
+	int i;
+	struct btrfs_fs_devices *fs_devices = to_fs_devs(kobj);
+
+	for (i = 0; i < BTRFS_NR_READ_POLICY; i++) {
+		if (btrfs_strmatch(buf, btrfs_read_policy_name[i]) == 0) {
+			if (i != fs_devices->read_policy) {
+				fs_devices->read_policy = i;
+				btrfs_info(fs_devices->fs_info,
+					   "read policy set to '%s'",
+					   btrfs_read_policy_name[i]);
+			}
+			return len;
+		}
+	}
+
+	return -EINVAL;
+}
+BTRFS_ATTR_RW(, read_policy, btrfs_read_policy_show, btrfs_read_policy_store);
+
 static const struct attribute *btrfs_attrs[] = {
 	BTRFS_ATTR_PTR(, label),
 	BTRFS_ATTR_PTR(, nodesize),
@@ -840,6 +888,7 @@ static const struct attribute *btrfs_attrs[] = {
 	BTRFS_ATTR_PTR(, quota_override),
 	BTRFS_ATTR_PTR(, metadata_uuid),
 	BTRFS_ATTR_PTR(, checksum),
+	BTRFS_ATTR_PTR(, read_policy),
 	NULL,
 };
 
-- 
2.23.0