linux-kernel - Re: O_DIRECT question

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Date:	Thu, 11 Jan 2007 21:18:52 -0800 (PST)
From:	Linus Torvalds <torvalds@...l.org>
To:	Nick Piggin <nickpiggin@...oo.com.au>
cc:	Bill Davidsen <davidsen@....com>, Andrew Morton <akpm@...l.org>,
	Hua Zhong <hzhong@...il.com>, Hugh Dickins <hugh@...itas.com>,
	linux-kernel@...r.kernel.org, hch@...radead.org,
	kenneth.w.chen@...el.com, mjt@....msk.ru
Subject: Re: O_DIRECT question

On Fri, 12 Jan 2007, Nick Piggin wrote:
> 
> Yeah *smallish* higher order allocations are fine, and we use them all the
> time for things like stacks or networking.
> 
> But Aubrey (who somehow got removed from the cc list) wants to do order 9
> allocations from userspace in his nommu environment. I'm just trying to be
> realistic when I say that this isn't going to be robust and a userspace
> solution is needed.

I do agree that order-9 allocations simply is unlikely to work without 
some pre-allocation notion or some serious work at active de-fragmentation 
(and the page cache is likely to be the _least_ of the problems people 
will hit - slab and other kernel allocations are likely to be much much 
harder to handle, since you can't free them in quite as directed a 
manner).

But for smallish-order (eg perhaps 3-4 possibly even more if you are 
careful in other places), the page cache limiter may well be a "good 
enough" solution in practice, especially if other allocations can be 
controlled by strict usage patterns (which is not realistic in a general- 
purpose kind of situation, but might be realistic in embedded).

		Linus
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