linux-kernel - Re: O_DIRECT question

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Date:	Fri, 12 Jan 2007 13:22:03 +0800
From:	Aubrey <aubreylee@...il.com>
To:	"Nick Piggin" <nickpiggin@...oo.com.au>
Cc:	"Linus Torvalds" <torvalds@...l.org>,
	"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 1/12/07, Nick Piggin <nickpiggin@...oo.com.au> wrote:
> Linus Torvalds wrote:
> >
> > On Fri, 12 Jan 2007, Nick Piggin wrote:
> >
> >>We are talking about about fragmentation. And limiting pagecache to try to
> >>avoid fragmentation is a bandaid, especially when the problem can be solved
> >>(not just papered over, but solved) in userspace.
> >
> >
> > It's not clear that the problem _can_ be solved in user space.
> >
> > It's easy enough to say "never allocate more than a page". But it's often
> > not REALISTIC.
>  >
> > Very basic issue: the perfect is the enemy of the good. Claiming that
> > there is a "proper solution" is usually a total red herring. Quite often
> > there isn't, and the "paper over" is actually not papering over, it's
> > quite possibly the best solution there is.
>
> 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.
>
Hmm..., aside from big order allocations from user space, if there is
a large application we need to run, it should be loaded into the
memory, so we have to allocate a big block to accommodate it. kernel
fun like load_elf_fdpic_binary() etc will request contiguous memory,
then if vfs eat up free memory, loading fails.

-Aubrey
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