linux-ext4 - Re: booked-page-flag.patch

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Date:	Thu, 15 Feb 2007 15:23:52 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Alex Tomas <alex@...sterfs.com>
Cc:	Eric Sandeen <sandeen@...hat.com>, "Theodore Ts'o" <tytso@....edu>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: booked-page-flag.patch

On Fri, 16 Feb 2007 00:07:55 +0300
Alex Tomas <alex@...sterfs.com> wrote:

> >>>>> Andrew Morton (AM) writes:
> 
>  AM> If the page doesn't have buffer-heads, set PG_private and clear page->private
> 
>  AM> If the page does have buffer_heads, use BH_Delay.
> 
> I did exactly this way in the first version, but later
> got feeling that the community'd discard "ugly hack".

These things can be wrapped in nice helper functions in header files.  that
allows us to easily reimplement if a better idea comes along.

And we already do all sorts of horrid things to save space in the page
struct.

> one more question: how much of it you want in VFS?

As much as possible, really.  It's probably too late for XFS to reuse any
new infrastructure, but if the infrastructure is well-designed it _should_
be possible to us it in new filesytems, and to convert ext2.

> ->get_block(with BH_Delay) can be used to signal
> filesystem that no actual allocation is required.
> so, aware filesystem can just reserve space. then
> ->writepages() should walk through the pages like
> it does currently, collect contiugous sequences
> and again call ->get_block(w/o BH_Delay) with b_size
> covering all contiguous pages ...
> 

That sounds like it'd work, yes.

Except for an address_space which is using delayed allocation, its
->prepare_write wouldn't call get_block at all, so perhaps that isn't
needed.

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