linux-kernel - Re: [PATCH -V19 00/15] Generic name to handle and open by handle syscalls

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Message-Id: <E1Ov26Q-0000Hd-IT@pomaz-ex.szeredi.hu>
Date:	Mon, 13 Sep 2010 08:01:18 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	"Aneesh Kumar K. V" <aneesh.kumar@...ux.vnet.ibm.com>
CC:	miklos@...redi.hu, miklos@...redi.hu, hch@...radead.org,
	viro@...iv.linux.org.uk, adilger@....com, corbet@....net,
	neilb@...e.de, npiggin@...nel.dk, hooanon05@...oo.co.jp,
	bfields@...ldses.org, miklos@...redi.hu,
	linux-fsdevel@...r.kernel.org, sfrench@...ibm.com,
	philippe.deniel@....FR, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -V19 00/15] Generic name to handle and open by handle syscalls

On Mon, 13 Sep 2010, Aneesh Kumar K. V wrote:
> As per your suggestion i started looking at handlefs details and below
> is my take on the approach.
> 
> handlefs would be an internal kernel mount like pipefs and would have
> inode object mapping to the returned file descriptor of
> open_by_handle_at syscall for symlinks. For regular files we can do what
> we already does and for symlinks we will create inodes in handlefs and
> their inode operation will in turn result in call out of inode operations
> of the actual symlinks. Based on the above
> 
> a) We still need open_by_handle_at syscall
> b) We still need handle based link syscall, because we need to support
>    creating hardlinks based on handle, and the existing linkat syscall
>    takes the oldpath name.
> c) We still need handle based readlink syscall, because the existing
>    readlinkat syscall takes pathname.

You can implement ->read() on the symlink file instead.

> d) we can drop stat, chown and xattr syscall because they are introduced
>    specially for symlinks as we don't allow open on symlinks.
> e) It would be nice to have handle based stat syscall to avoid two
>    syscall overhead for fetching file attributes when implementing a
>    file server, where fetching file attribute is a common operation. 

Syscall overhead is generally insignificant compared to other effects.
The server can also cache open files for commonly used handles.

> 
> With the above from the current patch series we can drop chown and
> xattr syscalls. Would it be ok if we get the series with the those two
> syscall patches dropped upstream as i work on supporting symlinks with
> handlefs approach ?

Try it.

Al seems to only be active to the outside world around the merge
window, so that's the best time to ask him to pull.

Thanks,
Miklos
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