linux-kernel - Re: [PATCH] Use extents for recording what swap is allocated.

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Message-Id: <1161727596.22729.11.camel@nigel.suspend2.net>
Date:	Wed, 25 Oct 2006 08:06:36 +1000
From:	Nigel Cunningham <ncunningham@...uxmail.org>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Andrew Morton <akpm@...l.org>, "Rafael J. Wysocki" <rjw@...k.pl>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Use extents for recording what swap is allocated.

Hi.

On Tue, 2006-10-24 at 21:42 +0100, Christoph Hellwig wrote:
> On Mon, Oct 23, 2006 at 02:14:17PM +1000, Nigel Cunningham wrote:
> > Switch from bitmaps to using extents to record what swap is allocated;
> > they make more efficient use of memory, particularly where the allocated
> > storage is small and the swap space is large.
> >     
> > This is also part of the ground work for implementing support for
> > supporting multiple swap devices.
> 
> In addition to the very useful comments from Rafael there's some observations
> of my own:
> 
>  - there's an awful lot of opencoded list manipulation, any chance you
>    could use list.h instead?

IIRC, I avoided list.h because I only wanted a singly linked list (it
never gets traversed backwards). List.h looks to me like all doubly
linked lists. Do you know if there are any other singly linked list
implementations I could piggy-back?

That said, since there's normally not that many extents, I could switch
quite easily and it wouldn't normally waste much memory.

>  - what unit are the extent values in?  The usage of unsigned long rings
>    warning bells for me, shouldn't this be something like pgoff_t or
>    sector_t depending on what you describe with it?

For this use, they're swap extents. For another use I have in suspend2,
they're sector_t >> block_device_size. That's why I picked ul; something
generic in what's supposed to be a generic implementation.

Thanks for the comments.

Nigel

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