Re: [rfc] new stat*fs-like syscall?

From: Brad Boyer <flar@allandria.com>
To: "J. R. Okajima" <hooanon05@yahoo.co.jp>
Cc: Christoph Hellwig <hch@infradead.org>,
	Nick Piggin <npiggin@suse.de>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	Al Viro <viro@ZenIV.linux.org.uk>,
	Ulrich Drepper <drepper@redhat.com>,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [rfc] new stat*fs-like syscall?
Date: Mon, 5 Jul 2010 13:58:06 -0700	[thread overview]
Message-ID: <20100705205806.GA12517@cynthia.pants.nu> (raw)
In-Reply-To: <13226.1277556884@jrobl>

On Sat, Jun 26, 2010 at 09:54:44PM +0900, J. R. Okajima wrote:
> Christoph Hellwig:
> > That's really job for a pathconf system call that allows quering random
> > paramters.
> 
> Do you mean it should be implemented such like this?
> vfs_pathconf(struct dentry, int parm)
> --> return d_sb->s_op->pathconf(parm)

I would suggest making it an inode operation if we do actually add it. Most
cases are going to be per super-block, but it might be easier to transparently
handle things like _PC_PIPE_BUF in glibc if it could call an fpathconf type
system call on the pipe fd. I haven't looked at the current glibc code for
that particular selector. The only one I looked at in any detail was
_PC_LINK_MAX, which is the one you already discussed and is obviously a
per-sb option. The only drawback I can see is that making it an inode
operation would make the vfs_pathconf fail on a negative dentry, but that
seems like a very strange thing to support in any case.

> I am afraid it is overdesign because the actual parameter(for FS) is
> _PC_LINK_MAX only. All other params are already handled by VFS, glibc or
> sb->statfs.

	Brad Boyer
	flar@allandria.com