Re: [RFC] Slimming down struct inode

Stefan Richter
2006-06-10T06:27:59Z

Theodore Tso wrote at lkml:
[saving space in the numerously instantiated struct inode]
> On Sat, Jun 10, 2006 at 02:27:57AM +0100, Al Viro wrote:
>>>5) Nuke i_cindex. This is only used by ieee1394's
>>> ieee_file_to_instance. There must be another place where we can
>>> store this --- say, in a ieee1394-specific field in struct file? Or
>>> maybe it can be derived some other way, but to chew up 4 bytes in
>>> i_cindex for all inodes just for ieee1394's benefit seems like the
>>> Wrong Thing(tm).
>>
>>No, it's actually the right thing for _all_ char devices. And it's used
>>before we get a struct file. If anything, ->i_rdev should go long-term...
>
> i_cindex is set by fs/char_dev.h, and the only user of that field (I
> grepped the entire sources) is ./drivers/ieee1394/ieee1394_core.h:
>
> /* return the index (within a minor number block) of a file */
> static inline unsigned char ieee1394_file_to_instance(struct file *file)
> {
> return file->f_dentry->d_inode->i_cindex;
> }
...

Also, the places where ieee1394_file_to_instance() is used appear not to
be fast paths. (It's the struct file_operations.open() hook of two
protocol drivers.) I.e. there is no harm in replacing it by whatever
more expensive lookup method.

Dv1394 calls ieee1394_file_to_instance() from within spin_lock_irqsave()
/unlock_irqrestore(), but it can easily moved out of it if need be.
--
Stefan Richter
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