question on "xen-blkback: enlarge the array size of blkback name"

question on "xen-blkback: enlarge the array size of blkback name"

[Jan Beulich]

Hello,

looking at this once again now that it's upstream, I still can't see what
this is good for: The only use the produced string has is to be passed
to kthread_run(), i.e. ultimately to be stored in a task's ->comm. That,
however, continues to be limited to 16 characters, so
blkback.domid.xvd[a-z] being up to 18 characters I don't understand
how the change results in the full name being shown. What am I
missing? Why aren't you instead shortening the string to e.g.
blkbk.domid.xvd[a-z] (which still is 16 characters, i.e. the last
character would still be chopped off for 5 digit domain IDs; perhaps
the first dot should then also be dropped).

Thanks, Jan

Re: question on "xen-blkback: enlarge the array size of blkback name"

[Roger Pau Monné]

El 27/05/15 a les 9.47, Jan Beulich ha escrit:
> Hello,
> 
> looking at this once again now that it's upstream, I still can't see what
> this is good for: The only use the produced string has is to be passed
> to kthread_run(), i.e. ultimately to be stored in a task's ->comm. That,
> however, continues to be limited to 16 characters, so
> blkback.domid.xvd[a-z] being up to 18 characters I don't understand
> how the change results in the full name being shown. What am I
> missing?

Right, I didn't realize there was a length limitation in task_struct.
How did you manage to get it working Chentao?

> Why aren't you instead shortening the string to e.g.
> blkbk.domid.xvd[a-z] (which still is 16 characters, i.e. the last
> character would still be chopped off for 5 digit domain IDs; perhaps
> the first dot should then also be dropped).

That sounds right.

Roger.

Re: question on "xen-blkback: enlarge the array size of blkback name"

[Ian Campbell]

On Wed, 2015-05-27 at 10:28 +0100, Roger Pau Monné wrote:
> El 27/05/15 a les 9.47, Jan Beulich ha escrit:
> > Hello,
> > 
> > looking at this once again now that it's upstream, I still can't see what
> > this is good for: The only use the produced string has is to be passed
> > to kthread_run(), i.e. ultimately to be stored in a task's ->comm. That,
> > however, continues to be limited to 16 characters, so
> > blkback.domid.xvd[a-z] being up to 18 characters I don't understand
> > how the change results in the full name being shown. What am I
> > missing?
> 
> Right, I didn't realize there was a length limitation in task_struct.
> How did you manage to get it working Chentao?
> 
> > Why aren't you instead shortening the string to e.g.
> > blkbk.domid.xvd[a-z] (which still is 16 characters, i.e. the last
> > character would still be chopped off for 5 digit domain IDs; perhaps
> > the first dot should then also be dropped).
> 
> That sounds right.

If you are pressed for space then the "xvd" is probably a bit redundant
in a string which starts blkbk.

The guest may not even call the device xvdN (iirc BSD has another
prefix) any how, so having blkback say so seems of limited use anyway.

Since this seems to not include a partition number how does this work in
the split partition scheme? (i.e. one where the guest is given xvda1 and
xvda2 rather than xvda with a partition table)

Perhaps something derived from one of the schemes in
http://xenbits.xen.org/docs/unstable/misc/vbd-interface.txt might be a
better fit?

Ian.

> 
> Roger.
> 
> 
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> Xen-devel mailing list
> Xen-devel@lists.xen.org
> http://lists.xen.org/xen-devel



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