[PATCH 2/3 RESEND Rebased] btrfs: add readmirror property framework

[Anand Jain]

Function call chain  __btrfs_map_block()->find_live_mirror() uses
thread %pid to determine the %mirror_num for the read when mirror_num=0
in the argument.

This patch introduces a framework so that readmirror is a configurable
parameter, with default set to pid.

Signed-off-by: Anand Jain <anand.j...@oracle.com>
---
 fs/btrfs/props.c   | 41 +++++++++++++++++++++++++++++++++++++++++
 fs/btrfs/volumes.c |  9 ++++++++-
 fs/btrfs/volumes.h |  6 ++++++
 3 files changed, 55 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/props.c b/fs/btrfs/props.c
index f9143f7c006d..0dc26a154a98 100644
--- a/fs/btrfs/props.c
+++ b/fs/btrfs/props.c
@@ -10,6 +10,7 @@
 #include "ctree.h"
 #include "xattr.h"
 #include "compression.h"
+#include "volumes.h"
 
 #define BTRFS_PROP_HANDLERS_HT_BITS 8
 static DEFINE_HASHTABLE(prop_handlers_ht, BTRFS_PROP_HANDLERS_HT_BITS);
@@ -312,6 +313,39 @@ static const char *prop_compression_extract(struct inode 
*inode)
        return NULL;
 }
 
+static int prop_readmirror_validate(struct inode *inode, const char *value,
+                                   size_t len)
+{
+       struct btrfs_root *root = BTRFS_I(inode)->root;
+
+       if (root->root_key.objectid != BTRFS_FS_TREE_OBJECTID)
+               return -EINVAL;
+
+       if (!len)
+               return 0;
+
+       return -EINVAL;
+}
+
+static int prop_readmirror_apply(struct inode *inode, const char *value,
+                                size_t len)
+{
+       struct btrfs_fs_devices *fs_devices = btrfs_sb(inode->i_sb)->fs_devices;
+
+       fs_devices->readmirror_policy = BTRFS_READMIRROR_DEFAULT;
+
+       return 0;
+}
+
+static const char *prop_readmirror_extract(struct inode *inode)
+{
+       /*
+        * readmirror policy is applied for the whole FS, inheritance is not
+        * applicable.
+        */
+       return NULL;
+}
+
 static struct prop_handler prop_handlers[] = {
        {
                .xattr_name = XATTR_BTRFS_PREFIX "compression",
@@ -320,6 +354,13 @@ static struct prop_handler prop_handlers[] = {
                .extract = prop_compression_extract,
                .inheritable = 1
        },
+       {
+               .xattr_name = XATTR_BTRFS_PREFIX "readmirror",
+               .validate = prop_readmirror_validate,
+               .apply = prop_readmirror_apply,
+               .extract = prop_readmirror_extract,
+               .inheritable = 0
+       },
 };
 
 static int inherit_props(struct btrfs_trans_handle *trans,
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index a13ddba1ebc3..d72850ed4f88 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -5490,7 +5490,14 @@ static int find_live_mirror(struct btrfs_fs_info 
*fs_info,
        else
                num_stripes = map->num_stripes;
 
-       preferred_mirror = first + current->pid % num_stripes;
+       switch(fs_info->fs_devices->readmirror_policy) {
+       case BTRFS_READMIRROR_DEFAULT:
+               /* fall through */
+       default:
+               /* readmirror as per thread pid */
+               preferred_mirror = first + current->pid % num_stripes;
+               break;
+       }
 
        if (dev_replace_is_ongoing &&
            fs_info->dev_replace.cont_reading_from_srcdev_mode ==
diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index 7f6aa1816409..e985d2133c0a 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -219,6 +219,10 @@ BTRFS_DEVICE_GETSET_FUNCS(total_bytes);
 BTRFS_DEVICE_GETSET_FUNCS(disk_total_bytes);
 BTRFS_DEVICE_GETSET_FUNCS(bytes_used);
 
+enum btrfs_readmirror_policy {
+       BTRFS_READMIRROR_DEFAULT,
+};
+
 struct btrfs_fs_devices {
        u8 fsid[BTRFS_FSID_SIZE]; /* FS specific uuid */
        u8 metadata_uuid[BTRFS_FSID_SIZE];
@@ -269,6 +273,8 @@ struct btrfs_fs_devices {
        struct kobject fsid_kobj;
        struct kobject *device_dir_kobj;
        struct completion kobj_unregister;
+
+       int readmirror_policy;
 };
 
 #define BTRFS_BIO_INLINE_CSUM_SIZE     64
-- 
2.20.1 (Apple Git-117)