Re: booked-page-flag.patch

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 16 Feb 2007 00:07:55 +0300
Alex Tomas <alex@clusterfs.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.