linux-kernel - Re: [PATCH 22/43] CacheFiles: Add a hook to write a single page of data to an inode [ver #46]

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Message-ID: <6488.1238688153@redhat.com>
Date:	Thu, 02 Apr 2009 17:02:33 +0100
From:	David Howells <dhowells@...hat.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	dhowells@...hat.com, hch@...radead.org, viro@...iv.linux.org.uk,
	nfsv4@...ux-nfs.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 22/43] CacheFiles: Add a hook to write a single page of data to an inode [ver #46]

Nick Piggin <nickpiggin@...oo.com.au> wrote:

> Hmm, I guess not all filesystems define write_begin/write_end. But if you
> only need to use ones that do define them?

That would be a reasonable restriction.  As would excluding NFS, AFS, CIFS,
etc..

> Yes, you framed the changelog as introducing this new callback because it
> allows a highly optimised code that takes advantage of page aligned write.
> So I went on a tangent thinking you were going to use it later to avoid
> the data copy or something.

I would love to avoid the data copies and do asynchronous direct I/O.  However
the DIO interface appears very highly userspace centric, and is very hard to
use from within the kernel.

> You are knowingly squashing together fscache and the backing filesystem if
> you do something like introduce a flag like PG_owner_priv_2 and disallow the
> backing filesystem from reusing it.

The backing filesystem isn't disallowed from reusing it; the *netfs* is.

Understand that there are two filesystems involved:  The netfs, which makes
requests of FS-Cache, and the backing filesystem which cachefiles uses to
store data.  

FS-Cache places restrictions such as having to yield up FS_private_2 and
FS_owner_priv_2 on the netfs.

Cachefiles places restrictions such as must have bmap(), setxattr(),
write_one_page() on the backing fs.

> So at which point you don't have to keep up illusions about being totally
> filesystem agnostic.

I'm trying to keep things as filesystem agnostic as I can, at both ends.

> Many filesystems don't need the file argument to write_begin/write_end.

That probably includes all those I care about for supporting the backing fs.
The problem is how does cachefiles tell?  Perhaps a file_system_type flag?

	#define FS_SUPPORTS_CACHEFILES 65536

or 

	#define FS_NULL_FILEPTR_OKAY 65536

perhaps.

David
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