linux-kernel - [PATCH, take2] VFS : Delay the dentry name generation on sockets and pipes.

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Message-Id: <200703091118.58990.dada1@cosmosbay.com>
Date:	Fri, 9 Mar 2007 11:18:58 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Christoph Hellwig <hch@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: [PATCH, take2] VFS : Delay the dentry name generation on sockets and pipes.

Hi Andrew

Please find a new version of this patch : I realized d_path() has very 
uncommon semantic (it seems nobody caught the point in previous patches), and 
had to change the documentation and pipefs_dname() / sockfs_dname() 
accordingly.

Now, readlink("/proc/pid/fd/xx", buffer, 4096) returns the exact number of 
bytes, not the whole 4095 bytes :)

Extract of new Documentation:

	CAUTION : d_path() logic is quite  tricky. 
	The correct way to return for example "Hello" is to put it
	at the end of the buffer, and returns a pointer to the first char.

       Example :

static char *somefs_dname(struct dentry *dent, char *buffer, int buflen)
{
       char *string = "Hello";
       int sz = strlen(string) + 1;
       if (sz > buflen)
               return ERR_PTR(-ENAMETOOLONG);
       buffer += (buflen - sz);
       return memcpy(buffer, string, sz);
}

Thank you

[PATCH] VFS : Delay the dentry name generation on sockets and pipes.

1) Introduces a new method in 'struct dentry_operations'. This method called 
d_dname() might be called from d_path() to build a pathname 
for special filesystems. It is called without locks.

Future patches (if we succeed in having one common dentry for all 
pipes/sockets) may need to change prototype of this method, but we now use :
char *d_dname(struct dentry *dentry, char *buffer, int buflen);

2) Use this new method for sockets : No more sprintf() at socket creation. 
This is delayed up to the moment someone does an access to /proc/pid/fd/...

3) Use this new method for pipes : No more sprintf() at pipe creation. This is 
delayed up to the moment someone does an access to /proc/pid/fd/...

A benchmark consisting of 1.000.000 calls to pipe()/close()/close() gives a 
*nice* speedup on my Pentium(M) 1.6 Ghz :

3.090 s instead of 3.450 s

Signed-off-by: Eric Dumazet <dada1@...mosbay.com>
Acked-by: Christoph Hellwig <hch@...radead.org>
Acked-by: Linus Torvalds <torvalds@...ux-foundation.org>

 Documentation/filesystems/Locking |    2 ++
 Documentation/filesystems/vfs.txt |   26 +++++++++++++++++++++++++-
 fs/dcache.c                       |   10 ++++++++++
 fs/pipe.c                         |   23 +++++++++++++++++------
 include/linux/dcache.h            |    1 +
 net/socket.c                      |   25 ++++++++++++++++++-------
 6 files changed, 73 insertions(+), 14 deletions(-)

View attachment "introduce_d_dname_2.patch" of type "text/plain" (6948 bytes)