inline as below.
On 10/17/2014 09:58 AM, Gui Hecheng wrote:
On Thu, 2014-09-04 at 20:02 +0800, Anand Jain wrote:
On 09/04/2014 05:58 PM, David Sterba wrote:
On Mon, Aug 18, 2014 at 04:38:18PM +0800, Anand Jain wrote:
ioctl BTRFS_IOC_FS_INFO return num_devices which does _not_ include seed
device, But the following ioctl BTRFS_IOC_DEV_INFO counts and gets seed
disk when probed. So in the userland we hit a count-slot missmatch
bug..
get_fs_info()
::
BUG_ON(ndevs >= fi_args->num_devices);
which hits this bug when we have mounted a seed device.
So to fix this problem here in this patch ioctl BTRFS_IOC_FS_INFO
will provide total_devices instead of num_devices.
The ioctl is very unclear what the 'num_device' actually means.
Right. Thats also true in kernel. very messy. very confusing.
tool btrfs-devlist would help understand whats going on.
----
$ egrep num_device *.c | egrep "total_device"
ioctl.c: fi_args->num_devices = fs_devices->total_devices;
super.c: ret = !(fs_devices->num_devices ==
fs_devices->total_devices);
volumes.c: total_devices = btrfs_super_num_devices(disk_super);
----
By the way about BTRFS_IOC_DEVICES_READY ioctl above its long time
broken with seed/replace, just waiting to get these patches integrated
first so to fix it later.
This would fix the problem partly. Partly because ealier num_devices
included the replacing device but now total_device does not include
the replacing device. Getting a count which includes a transient device
is rather too in efficient/wrong indeed, because there can be a race
condition where in the time between ioctl BTRFS_IOC_FS_INFO to
BTRFS_IOC_DEV_INFO the replace device operation might have been
completed. So to fix this problem its better that user land btrfs-progs
probes replacing device (at devid 0) separately.
v2:
Agree with Wang's comment. Its better to show seed disks under the
sprout fs, so that user can establish mapping of seed to sprout devices.
So here I am making BTRFS_IOC_FS_INFO to return the total_devices
which would count the seed devices (but not the replacing device).
This is even more confusing. I think we need to add another member to
the ioctl struct to reflect the number of regular devices (num_devices)
and the true total number of devices including seeding and replaced
devices.
that will be a better way. thanks.
The difference should be accompanied by a flag that would say
if there's a seeding or replace in progress.
There are some backward compatibility concerns. Setting num_devices to
total_devices changes semantics of the ioctl, so I think it should stay
as is for now,
As I have tested there is not backward compatibility issue.
But from semantics perspective .. agreed.
but the BUG_ON can be removed and replaced by code that
reallocates the buffer or allocates a few more items in advance.
We don't know how may seed devices are there for a sprout FS.
So thats not possible.
Will review resubmit.
Thanks for commenting.
Hi all,
Firtly, thanks for the fix, Anand, how's the new version going?
I've been testing the btrfs fi show cmd these days and find that
this patch has not been merged into linus's tree yet.
Since the suggested way of adding member to the ioctl struct brings
compatibility issues, it may need more discussion.
Thanks Gui for commenting.
Yes discussion is needed. Mainly on our long term plan for a better
btrfs kernel device and parameter read interface. which also means
current bugs can wait for this new interface instead of patching the
old structures and bring in a new compatibility mess.
Thanks, Anand
But since this fix really incluence much to the user, I consider merging
this version first to be a good idea.
What do you think, Chris?
Thanks,
Gui
Anand
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