Re: [PATCH 2/2 RESEND Rebased] btrfs-progs: add readmirror policy

[Anand Jain]

On 23/7/19 9:53 PM, David Sterba wrote:

On Wed, Jun 26, 2019 at 04:37:23PM +0800, Anand Jain wrote:

This sets the readmirror=<parm> as a btrfs.<attr> extentded attribute.

Signed-off-by: Anand Jain <anand.j...@oracle.com>
---
  props.c | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
  1 file changed, 49 insertions(+)

diff --git a/props.c b/props.c
index 3a498bd9e904..1d1a2c7f9d14 100644
--- a/props.c
+++ b/props.c
@@ -178,6 +178,53 @@ out:
        return ret;
  }

+static int prop_readmirror(enum prop_object_type type, const char *object,

+                          const char *name, const char *value)
+{
+       int fd;
+       int ret;
+       char buf[256] = {0};
+       char *xattr_name;
+       DIR *dirstream = NULL;
+
+       fd = open_file_or_dir3(object, &dirstream, value ? O_RDWR : O_RDONLY);
+       if (fd < 0) {
+               ret = -errno;
+               error("failed to open %s: %m", object);
+               return ret;
+       }
+
+       xattr_name = alloc_xattr_name(name);
+       if (IS_ERR(xattr_name)) {
+               error("failed to alloc xattr_name %s: %m", object);
+               return PTR_ERR(xattr_name);
+       }
+
+       ret = 0;
+       if (value) {
+               if (fsetxattr(fd, xattr_name, value, strlen(value), 0) < 0) {
+                       ret = -errno;
+                       error("failed to set readmirror for %s: %m", object);
+               }
+       } else {
+               if (fgetxattr(fd, xattr_name, buf, 256) < 0) {
+                       if (errno != ENOATTR) {
+                               ret = -errno;
+                               error("failed to get readmirror for %s: %m",
+                                     object);
+                       }
+               } else {
+                       fprintf(stdout, "readmirror=%.*s\n", (int) strlen(buf),
+                               buf);
+               }
+       }
+
+       free(xattr_name);
+       close_file_or_dir(fd, dirstream);
+
+       return ret;
+}
+
  const struct prop_handler prop_handlers[] = {
        {"ro", "Set/get read-only flag of subvolume.", 0, prop_object_subvol,
         prop_read_only},
@@ -185,5 +232,7 @@ const struct prop_handler prop_handlers[] = {
         prop_object_dev | prop_object_root, prop_label},
        {"compression", "Set/get compression for a file or directory", 0,
         prop_object_inode, prop_compression},
+       {"readmirror", "set/get readmirror policy for filesystem", 0,
+        prop_object_root, prop_readmirror},

For some unknown reason the object type for filesystem-wide props is
called prop_object_root, which is correct, but it got me confused first.

So the most reliable way to set it is

   $ btrfs prop set -t filesystem /path readmirror <VALUE>

and

   $ btrfs prop set /path readmirror <VALUE>

will auto-detect the object type by /path, but I'm not sure what exactly
does it do in case it's a mount point but not the toplevel subvolume.

If its not toplevel subvolume it fails in kernel with -EINVAL.

----
+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;
-----