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SubjectRe: [PATCH 22/43] CacheFiles: Add a hook to write a single page of data to an inode [ver #46]
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Nick Piggin <nickpiggin@yahoo.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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