linux-ext4 - RE: Cleancache [PATCH 2/7] (was Transcendent Memory): core files

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Date:	Mon, 24 May 2010 13:02:34 -0700 (PDT)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	chris.mason@...cle.com, akpm@...ux-foundation.org, adilger@....com,
	tytso@....edu, mfasheh@...e.com, joel.becker@...cle.com,
	matthew@....cx, linux-btrfs@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-ext4@...r.kernel.org, ocfs2-devel@....oracle.com,
	linux-mm@...ck.org, ngupta@...are.org, jeremy@...p.org,
	JBeulich@...ell.com, kurt.hackel@...cle.com, npiggin@...e.de,
	dave.mccracken@...cle.com, riel@...hat.com
Subject: RE: Cleancache [PATCH 2/7] (was Transcendent Memory): core files

> From: Al Viro [mailto:viro@...IV.linux.org.uk]
> Subject: Re: Cleancache [PATCH 2/7] (was Transcendent Memory): core files

Hi Al!

Thanks for the feedback!  Sorry for the delayed response.

> ...again, use sane types...

Good point.  Will fix types for next rev (using size_t, ino_t,
and pgoff_t).

> > +	int (*get_page)(int, unsigned long, unsigned long, struct page *);
> 
> Ugh.  First of all, presumably you have some structure behind that
> index, don't you?  Might be a better way to do it.

Not quite sure what you mean here.  The index is really
just part of a unique handle for cleancache to identify
the (page of) data.

> What's more, use of ->i_ino is simply wrong.  How stable do you want that
> to be and how much do you want it to outlive struct address_space in question?
> From my reading of your code, it doesn't outlive that anyway, so...

Unless I misunderstand your point, no, the inode never outlives
the address space because the specification requires the kernel
to ensure coherency; if the inode were about to outlive the
address space, the cleancache_flush operations must be invoked
(and I think the patch covers all the necessary cases).

> The third one is pgoff_t; again, use sane types, _if_ you actually want
> the argument #3 at all - it can be derived from struct page you are
> passing there as well.

I thought it best to declare the _ops so that the struct page
is opaque to the "backend" (driver).  The kernel-side ("frontend")
defines the handle and ensures coherency, so the backend shouldn't
be allowed to derive or muck with the three-tuple passed by the
kernel. In the existing (Xen tmem) driver, the only operation
performed on the struct page parameter is page_to_pfn().  OTOH,
I could go one step further and pass a pfn_t instead of a
struct page, since it is really only the physical page frame that
the backend needs to know about and (synchronously) read/write from/to.

Thoughts?

Thanks again!
Dan

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