linux-kernel - Re: Syslets, Threadlets, generic AIO support, v6

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Message-ID: <alpine.LFD.0.98.0705301422230.26602@woody.linux-foundation.org>
Date:	Wed, 30 May 2007 14:27:52 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ulrich Drepper <drepper@...hat.com>
cc:	Ingo Molnar <mingo@...e.hu>, Jeff Garzik <jeff@...zik.org>,
	Zach Brown <zach.brown@...cle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@....com.au>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Evgeniy Polyakov <johnpol@....mipt.ru>,
	"David S. Miller" <davem@...emloft.net>,
	Suparna Bhattacharya <suparna@...ibm.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: Syslets, Threadlets, generic AIO support, v6

On Wed, 30 May 2007, Ulrich Drepper wrote:
> 
> I don't like special cases.  For me things better come in quantities 0,
> 1, and unlimited (well, reasonable high limit).  Otherwise, who gets to
> use that special namespace?  The C library is not the only body of code
> which would want to use descriptors.

Well, don't think of it as a special case at all: think of bit 30 as a 
"the user asked for a non-linear fd".

In fact, to make it effective, I'd suggest literally scrambling the low 
bits (using, for example, some silly per-boot xor value to to actually 
generate the "true" index - the equivalent of a really stupid randomizer). 

That way you'd have the legacy "linear" space, and a separate "non-linear 
space" where people simply *cannot* make assumptions about contiguous fd 
allocations. There's no special case there - it's just an extension which 
explicitly allows us to say "if you do that, your fd's won't be allocated 
the traditional way any more, but you *can* mix the traditional and the 
non-linear allocation".

> And then the semantics: do these descriptors should show up in
> /proc/self/fd?  Are there separate directories for each namespace?  Do
> they count against the rlimit?

Oh, absolutely. The'd be real fd's in every way. People could use them 
100% equivalently (and concurrently) with the traditional ones. The whole, 
and the _only_ point, would be that it breaks the legacy guarantees of a 
dense fd space.

Most apps don't actually *need* that dense fd space in any case. But by 
defaulting to it, we wouldn't break those (few) apps that actually depend 
on it.

		Linus
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