linux-kernel - Re: [RFC PATCH 2/6] pipes: use alloc-file instead of duplicating code

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Date:	Fri, 04 Dec 2009 13:54:43 -0500
From:	Eric Paris <eparis@...hat.com>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, viro@...iv.linux.org.uk, jmorris@...ei.org,
	npiggin@...e.de, zohar@...ibm.com, jack@...e.cz,
	jmalicki@...acarta.com, dsmith@...hat.com, serue@...ibm.com,
	hch@....de, john@...nmccutchan.com, rlove@...ve.org,
	ebiederm@...ssion.com, heiko.carstens@...ibm.com,
	penguin-kernel@...ove.SAKURA.ne.jp, mszeredi@...e.cz,
	jens.axboe@...cle.com, akpm@...ux-foundation.org, matthew@....cx,
	hugh.dickins@...cali.co.uk, kamezawa.hiroyu@...fujitsu.com,
	nishimura@....nes.nec.co.jp, davem@...emloft.net, arnd@...db.de,
	eric.dumazet@...il.com
Subject: Re: [RFC PATCH 2/6] pipes: use alloc-file instead of duplicating
 code

On Fri, 2009-12-04 at 07:08 +0100, Miklos Szeredi wrote:
> On Thu, 03 Dec 2009, Eric Paris wrote:
> > The pipe code duplicates the functionality of alloc-file and init-file.  Use
> > the generic vfs functions instead of duplicating code.
> > 
> > Signed-off-by: Eric Paris <eparis@...hat.com>
> 
> Acked-by: Miklos Szeredi <miklos@...redi.hu>
> 
> As a side note: I wonder why we aren't passing a "struct path" to
> alloc_file() and why are the refcount rules wrt. dentries/vfsmounts so
> weird?

It's probably because of the slightly weird refcnt rules that it asks
for the dentry and vfsmount separately rather than as a struct path.
The rules make perfect sense if you consider

d_alloc()  <-- reference on dentry
d_instantiate()
alloc_file() <-- reference on vfsmount
  so here file->f_path() is all good.

Which a number of callers user.  They make less sense when you consider
something that is not allocating the dentry right there (like this path)

dget(dentry);  <-- reference here
alloc_file() <-- reference on vfsmount;
  so here file->f_path is all good.

It would be a reasonable interface if it took a struct path and then
took a reference on the struct path.  The second case would look more
clean, but the first case would turn into

d_alloc()
d_instantiate()
alloc_file()
d_put() /* matches d_alloc() */

and

alloc_file()

Is this better?  I'll gladly do it if other think so it makes more
sense....

-Eric

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