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

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Date:	Wed, 07 Mar 2007 08:16:14 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Eric Dumazet <dada1@...mosbay.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Davide Libenzi <davidel@...ilserver.org>,
	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 ...

Eric Dumazet a écrit :
> 
> I would definitly *love* saving dentries for pipes (and sockets too), 
> but how are you going to get the inode ?
> 
> pipes()/sockets() can use read()/write()/rw_verify_area() and thus need 
> file->f_path.dentry->d_inode (so each pipe needs a separate dentry)
> 
> Are you suggesting adding a new "struct file_operations" member to get 
> the inode ?
> Or re-intoducing an inode pointer in struct file ?

Crazy ideas : (some readers are going to kill me)

1) Use the low order bit of f_path.dentry to say : this pointer is not a 
pointer to a dentry but the inode pointer (with the low order bit set to 1)

OR

2) file->f_path.dentry set to NULL for this special files (so that we dont 
need to dput() and cache line ping pong the common dentry each time we 
__fput() a pipe/socket.

Same trick could be used for file->f_path.mnt, because there is a big SMP 
cache line ping/pong to maintain a mnt_count on pipe/sockets mountpoint while 
these file systems cannot be un-mounted)

If dentry is NULL, we get the inode pointer from an overlay of struct 
file_ra_state    f_ra; (because for this special files readahead is unused)

This adds some conditional branches of course, but being able to save ram and 
better use cpu caches might be worth them.


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