[PATCH 0/3 RESEND Rebased] readmirror feature

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

These patches are tested to be working fine.

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

This patch introduces a framework so that we can add policies to determine
the %mirror_num. And adds the devid as the readmirror policy.

The property is stored as an extented attributes of root inode
(BTRFS_FS_TREE_OBJECTID).
User provided devid list is validated against the fs_devices::dev_list.

 For example:
   Usage:
     btrfs property set <mnt> readmirror devid<n>[,<m>...]
     btrfs property set <mnt> readmirror ""

   mkfs.btrfs -fq -draid1 -mraid1 /dev/sd[b-d] && mount /dev/sdb /btrfs
   btrfs prop set /btrfs readmirror devid1,2
   btrfs prop get /btrfs readmirror
    readmirror=devid1,2
   getfattr -n btrfs.readmirror --absolute-names /btrfs
    btrfs.readmirror="devid1,2"
   btrfs prop set /btrfs readmirror ""
   getfattr -n btrfs.readmirror --absolute-names /btrfs
    /btrfs: btrfs.readmirror: No such attribute
   btrfs prop get /btrfs readmirror

RFC->v1:
  Drops pid as one of the readmirror policy choices and as usual remains
  as default. And when the devid is reset the readmirror policy falls back
  to pid.
  Drops the mount -o readmirror idea, it can be added at a later point of
  time.
  Property now accepts more than 1 devid as readmirror device. As shown
  in the example above.

Anand Jain (3):
  btrfs: add inode pointer to prop_handler::validate()
  btrfs: add readmirror property framework
  btrfs: add readmirror devid property

 fs/btrfs/props.c   | 120 +++++++++++++++++++++++++++++++++++++++++++--
 fs/btrfs/props.h   |   4 +-
 fs/btrfs/volumes.c |  25 +++++++++-
 fs/btrfs/volumes.h |   8 +++
 fs/btrfs/xattr.c   |   2 +-
 5 files changed, 150 insertions(+), 9 deletions(-)

-- 
2.20.1 (Apple Git-117)