linux-kernel - Re: [PATCH 7/9] swap_info: swap count continuations

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Date:	Fri, 16 Oct 2009 01:21:14 +0100 (BST)
From:	Hugh Dickins <hugh.dickins@...cali.co.uk>
To:	David Rientjes <rientjes@...gle.com>
cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nitin Gupta <ngupta@...are.org>, hongshin@...il.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 7/9] swap_info: swap count continuations

On Thu, 15 Oct 2009, David Rientjes wrote:
> On Thu, 15 Oct 2009, KAMEZAWA Hiroyuki wrote:
> > Hmm...maybe I don't understand the benefit of this style of data structure.
> > 
> > Do we need fine grain chain ? 
> > Is  array of "unsigned long" counter is bad ?  (too big?)
> 
> I'm wondering if flex_array can be used for this purpose, which can store 
> up to 261632 elements of size unsigned long with 4K pages, or whether 
> finding the first available bit or weight would be too expensive.

When flex_arrays were first mooted, I did briefly wonder if we could
use them instead of vmalloc for the swap_map; but no, their interface
would slow down scan_swap_map() unacceptably.

Extensions of the swap_map are a different matter, they are seldom
referenced, and referenced just an item at a time: much better suited
to a flex_array.  And looking at Jon's Doc, I see they're good for
sparse arrays, that would suit swap_map extensions very well.

However... that limit of 261632 elements rules them out here (or can
we have a flex_array of flex_arrays?), and the lack of support for
__GFP_HIGHMEM is disappointing - the current implementation of swap
count continuations does use highmem (though perhaps these pages
are so rarely needed that it actually doesn't matter).

It seems that the flex_array is a solution in search of a problem,
and that the swap_map extension is not the right problem for it.

Hugh
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