linux-kernel - Re: [patch] epoll use a single inode ...

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Date:	Tue, 6 Mar 2007 16:51:50 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Davide Libenzi <davidel@...ilserver.org>
cc:	Avi Kivity <avi@...o.co.il>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...IV.linux.org.uk>
Subject: Re: [patch] epoll use a single inode ...

[ Al Viro added to Cc: as the arbiter of good taste in the VFS layer. He 
  has veto powers even over my proposals ;^]

On Tue, 6 Mar 2007, Davide Libenzi wrote:
> 
> I currently have the dentry to carry the name of the file* class it is 
> linked to. I'd prefer to keep it that way, unless there are huge factors 
> against.

I assume that the *only* reason for having multiple dentries is really 
just the output in /proc/<pid>/fd/, right? Or is there any other reason to 
have separate dentries for these pseudo-files?

It's a bit sad to waste that much memory (and time) on something like 
that. I bet that the dentry setup is a noticeable part of the whole 
sigfd()/timerfd() setup. It's likely also a big part of any memory 
footprint if you have lots of them.

So how about just doing:
 - do a single dentry
 - make a "struct file_operations" member function that prints out the 
   name of the thing in /proc/<pid>/fd/, and which *defaults* to just 
   doing the d_path() on the dentry, but special filesystems like this 
   could do something else (like print out a fake inode number from the 
   "file->f_private_data" information)

There seems to really be no downsides to that approach. No existing 
filesystem will even notice (they'll all have NULL in the new f_op 
member), and it would allow pipes etc to be sped up and use less memory.

			Linus
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