On Mon, Apr 16, 2012 at 09:07:07PM +0800, Fengguang Wu wrote:

On Mon, Apr 16, 2012 at 08:54:32AM -0400, Vivek Goyal wrote:

quoted

On Sat, Apr 14, 2012 at 02:25:14PM +0200, Peter Zijlstra wrote:

quoted

On Wed, 2012-04-11 at 11:40 -0400, Vivek Goyal wrote:

quoted

Ok, that's good to know. How would we configure this special bdi? I am
assuming there is no backing device visible in /sys/block/<device>/queue/?
Same is true for network file systems. 

root@twins:/usr/src/linux-2.6# awk '/nfs/ {print $3}' /proc/self/mountinfo | while read bdi ; do ls -la /sys/class/bdi/${bdi}/ ; done
ls: cannot access /sys/class/bdi/0:20/: No such file or directory
total 0
drwxr-xr-x  3 root root    0 2012-03-27 23:18 .
drwxr-xr-x 35 root root    0 2012-03-27 23:02 ..
-rw-r--r--  1 root root 4096 2012-04-14 14:22 max_ratio
-rw-r--r--  1 root root 4096 2012-04-14 14:22 min_ratio
drwxr-xr-x  2 root root    0 2012-04-14 14:22 power
-rw-r--r--  1 root root 4096 2012-04-14 14:22 read_ahead_kb
lrwxrwxrwx  1 root root    0 2012-03-27 23:18 subsystem -> ../../../../class/bdi
-rw-r--r--  1 root root 4096 2012-03-27 23:18 uevent

Ok, got it. So /proc/self/mountinfo has the information about st_dev and
one can use that to reach to associated bdi. Thanks Peter.

Vivek, I noticed these lines in cfq code

                sscanf(dev_name(bdi->dev), "%u:%u", &major, &minor);

Why not use bdi->dev->devt?  The problem is that dev_name() will
return "btrfs-X" for btrfs rather than "major:minor".

Sorry it's not that simple. btrfs reports its faked btrfs_fs_info.bdi
to upper layer which is different from the bdi's for
btrfs_fs_info.fs_devices.devices saw by cfq.

It's the faked btrfs bdi that is named "btrfs-X" by this function:

setup_bdi():
        bdi_setup_and_register(bdi, "btrfs", BDI_CAP_MAP_COPY);

It does impose difficulties to interpret btrfs mountinfo, where you
cannot directly get the block device major/minor numbers:

35 16 0:26 / /fs/sda3 rw,relatime - btrfs /dev/sda3 rw,noacl

Thanks,
Fengguang

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