given a pointer to xfs_inode_t, how to determine path?

We're running 2.6.27 (upgrading not currently possible, embedded product).

We had a situation arise where we could see in stack traces that a number of tasks were stuck in vn_iowait(). That function takes a pointer to xfs_inode_t. Given that, would it be possible to work backwards to determine a filesystem path corresponding to that inode? I realize it would likely only go back to the head of the filesystem, but that would be fine.

Hi Chris,

Quoted 8 lines · Chris Friesen

We're running 2.6.27 (upgrading not currently possible, embedded product).

We had a situation arise where we could see in stack traces that a number of tasks were stuck in vn_iowait(). That function takes a pointer to xfs_inode_t. Given that, would it be possible to work backwards to determine a filesystem path corresponding to that inode? I realize it would likely only go back to the head of the filesystem, but that would be fine.

Yep, it's possible.

echo t > /proc/sysrq-trigger won't give you the full stack, but you may be able to get an idea of what is going on, well enough to make some guesses as to the culprit. That's probably your best first step.

If you have crash installed:

1) disassemble vn_iowait and find out which register the xfs_inode_t pointer is in.
2) print out the full stacks of all the processes in the system. 'bt -f', I think.

3) grep for the xfs_inode_t pointer on those stacks.

Quoted 12 lines · Ben Myers

Hi Chris,

Quoted 8 lines · Chris Friesen

We're running 2.6.27 (upgrading not currently possible, embedded product).

We had a situation arise where we could see in stack traces that a number of tasks were stuck in vn_iowait(). That function takes a pointer to xfs_inode_t. Given that, would it be possible to work backwards to determine a filesystem path corresponding to that inode? I realize it would likely only go back to the head of the filesystem, but that would be fine.

Quoted 5 lines

Yep, it's possible.

echo t > /proc/sysrq-trigger won't give you the full stack, but you may be able to get an idea of what is going on, well enough to make some guesses as to the culprit. That's probably your best first step.

Doesn't give you the path of the inode, or anything that can tell you what the inode number is so you could run "find <dir> -inum <num>" after the system has recovered.

Quoted 4 lines

If you have crash installed:

1) disassemble vn_iowait and find out which register the xfs_inode_t pointer is in.
2) print out the full stacks of all the processes in the system. 'bt -f', I think.

'bt -F <pid>' tells you what entries on the stack are of a particular symbol. i.e. they'll show up as [xfs_inode] instead of a number for all addresses from inside the xfs inode slab cache. comapring the output of 'bt -F' and 'bt -f' will get you the inode address without needing to be able to read x86-64 assembly language...

Quoted 1 line

3) grep for the xfs_inode_t pointer on those stacks.

Which doesn't get you the path, either. From there, follow to the struct inode, and from there to the struct dentry and grab the file name, then walk the d_parent dentry links back up to the root and construct the path that way.

Alternatively, I think you can walk from the top down by looking that the open files that a given process has and working down to the xfs_inode, but it's been a long time since I've had to come from that direction. Indeed, you might even be able to use lsof to do this and get an idea from userspace what files are currently open on the hung processes.