Device names
I just found out that all the device handling in btrfs is based on pathnames, but shorter pathnames (1024) that PATH_MAX (4096).
This is confusing, and concerning for multiple reasons:
1. pathnames are namespace-specific; what is a pathname in one namespace might not be in another.
2. different truncation rules in the rest of the kernel.
There seem to be no way one can do the equivalent of BTRFS_IOC_DEV_INFO but actually get a device number or other stable information for that device (that could be compared to a file descriptor for verification); is that an accurate observation?
-hpa
Signature · 2 lines
H. Peter Anvin, Intel Open Source Technology Center I work for Intel. I don't speak on their behalf.
Quoted 5 lines · H. Peter Anvin
I just found out that all the device handling in btrfs is based on pathnames, but shorter pathnames (1024) that
PATH_MAX(4096).This is confusing, and concerning for multiple reasons:
Quoted 8 lines
1. pathnames are namespace-specific; what is a pathname in one namespace might not be in another.
2. different truncation rules in the rest of the kernel.There seem to be no way one can do the equivalent of
BTRFS_IOC_DEV_INFObut actually get a device number or other stable information for that device (that could be compared to a file descriptor for verification); is that an accurate observation?
At mount time, we go through and verify the path names still belong to the filesystem you thought they belonged to. The bdev is locked during the verification, so it won't be able to go away or change.
This is a long way of saying right we don't spit out device numbers. Even device numbers can change. We can easily add a uuid based listing, which I think is what you want.
-chris
Quoted 9 lines · Chris Mason
At mount time, we go through and verify the path names still belong to the filesystem you thought they belonged to. The bdev is locked during the verification, so it won't be able to go away or change.
This is a long way of saying right we don't spit out device numbers. Even device numbers can change. We can easily add a uuid based listing, which I think is what you want.
No, I want to find the actual devices. I know I can get the UUID, but scanning all the block devices in the system looking for that UUID is a nonstarter.
Device path names can change while the system is operating (and, worse, are dependent on namespace changes and chroot); device *numbers* cannot as long as the device is in use (e.g. mounted.) They can indeed change while not in use, of course.
-hpa
Quoted 11 lines · H. Peter Anvin
Quoted 9 lines · Chris Mason
At mount time, we go through and verify the path names still belong to the filesystem you thought they belonged to. The bdev is locked during the verification, so it won't be able to go away or change.
This is a long way of saying right we don't spit out device numbers. Even device numbers can change. We can easily add a uuid based listing, which I think is what you want.
Quoted 8 lines
No, I want to find the actual devices. I know I can get the UUID, but scanning all the block devices in the system looking for that UUID is a nonstarter.
Device path names can change while the system is operating (and, worse, are dependent on namespace changes and chroot); device *numbers* cannot as long as the device is in use (e.g. mounted.) They can indeed change while not in use, of course.
Ok, my two choices for exporting this to you are a /sys/btrfs kind of directory (representing the mounted filesystems) or an ioctl. Which one is most usable for you?
You want to map from /some_dir to a definitive list of devices you need to find in syslinux to later boot off that FS, right?
-chris
Quoted 16 lines · Chris Mason
Quoted 15 lines · H. Peter Anvin
Quoted 9 lines · Chris Mason
At mount time, we go through and verify the path names still belong to the filesystem you thought they belonged to. The bdev is locked during the verification, so it won't be able to go away or change.
This is a long way of saying right we don't spit out device numbers. Even device numbers can change. We can easily add a uuid based listing, which I think is what you want.
No, I want to find the actual devices. I know I can get the UUID, but scanning all the block devices in the system looking for that UUID is a nonstarter.
Quoted 8 lines
Quoted 4 lines
Device path names can change while the system is operating (and, worse, are dependent on namespace changes and chroot); device *numbers* cannot as long as the device is in use (e.g. mounted.) They can indeed change while not in use, of course.
Ok, my two choices for exporting this to you are a
/sys/btrfskind of directory (representing the mounted filesystems) or an ioctl. Which one is most usable for you?
As short term solution, I suggest to update the BTRFS_IOC_DEV_INFO to export also the major:minor pair. This should be a minor change and also should be backward compatible (there is a lot of space in the struct btrfs_ioctl_dev_info_args )
fs/btrfs/ioctl.c +0 -0
@@ -2261,6 +2261,8 @@ static long btrfs_ioctl_dev_info(struct btrfs_root
*root,
di_args->devid = dev->devid;
di_args->bytes_used = dev->bytes_used;
di_args->total_bytes = dev->total_bytes;
+ di_args->major = imajor(dev->bdev->bd_inode);
+ di_args->minor = iminor(dev->bdev->bd_inode);
memcpy(di_args->uuid, dev->uuid, sizeof(di_args->uuid));
if (dev->name)
strncpy(di_args->path, dev->name, sizeof(di_args->path));fs/btrfs/ioctl.h +3 -1
@@ -98,7 +98,9 @@ struct btrfs_ioctl_dev_info_args {__u8 uuid[BTRFS_UUID_SIZE ];/* in/out */ __u64 bytes_used;/* out */ __u64 total_bytes;/* out */ -__u64 unused[379 ];/* pad to 4k */ +__u64 major;/* out */ +__u64 minor;/* out */ +__u64 unused[377 ];/* pad to 4k */ __u8 path[BTRFS_DEVICE_PATH_NAME_MAX ];/* out */ };
As long term solution, exporting all this kind of information in /sys/btrfs/... could be a more robust solution, which could simplify the "backward compatible" problem.
Only my 2¢...
GB
Quoted 11 lines
You want to map from /some_dir to a definitive list of devices you need to find in syslinux to later boot off that FS, right?
-chris
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