linux-kernel - Re: [patch 1/3] bdi patches

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Message-Id: <E1Iy0tl-0003qA-2U@dorka.pomaz.szeredi.hu>
Date:	Fri, 30 Nov 2007 09:06:57 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	neilb@...e.de
CC:	a.p.zijlstra@...llo.nl, fengguang.wu@...il.com,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 1/3] bdi patches

> > Provide a place in sysfs for the backing_dev_info object.
> > This allows us to see and set the various BDI specific variables.
> 
> You don't say what the place is, and I'm not quite familiar enough
> with sysfs internals to figure it out my self.  Help?

/sys/class/bdi  (which sometimes links into /sys/devices/virtual/bdi)

> And while I was looking I noticed that bdi_register (and bdi_init_fmt)
> takes a second argument 'parent', which is always NULL, and which is
> undocumented as to purpose.
> If no-one would ever add another call to bdi_register, why have the
> second arg, and if they might, how would they know what to put there?
> 
> Finally, the omission of NFS bothers me - and makes me wonder if the
> choice of name in sysfs is appropriate.
> 
> Would a program ever want to generate the name (in sysfs) for a
> particular bdi?  If so, how would it do it.

I'll let Peter answer these, when he gets back.

> It seems to me after a fairly quick look that a bdi is always
> associated with a device number.  For block devices the device number
> is obvious.  For NFS and FUSE, the device number is an anon device
> number allocated at mount time.
> Maybe the name of the bdi should be based on that number.  Then it
> would be possible to map directly from e.g. a file to the bdi that the
> file would be written to. 

Sounds like a good idea to me.

Miklos
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