linux-kernel - Re: [PATCH] scsi: fix sense

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Date:	Mon, 7 Apr 2008 14:15:56 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Pekka Enberg <penberg@...helsinki.fi>
cc:	Hugh Dickins <hugh@...itas.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Lameter <clameter@....com>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	Jens Axboe <jens.axboe@...cle.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] scsi: fix sense_slab/bio swapping livelock

On Tue, 8 Apr 2008, Pekka Enberg wrote:
> 
> I wonder if we can get away with a SLAB_IO flag that you can use to annotate
> caches that participate in writeout and the allocator could keep some spare
> pages around that can be handed out for them in case of OOM...

Actually, I don't think this is SLAB-specific, since there are potentially 
the same issues for pages.

I suspect the right thing to do is not to mark them for "IO", but mark 
them for "short-lived", and allow short-lived allocations that don't have 
extended lifetimes to succeed even when a "real" allocation wouldn't.

When we are under memory pressure, a normal allocation generally needs to 
clear up enough memory from elsewhere to succeed. But a short-lived 
allocation without any long-term lifetimes would be known to release its 
memory back to the pool, so it can be allowed to go ahead when a normal 
memory allocation would not.

Examples of short-lived allocations:
 - IO requests
 - temporary network packets that don't get queued up (eg "ACK" packet) as 
   opposed to those that do get queued (incoming _or_ outgoing queues)
 - things like the temporary buffers for "getname()/putname()" etc.

That said, even a lot of temporary allocations can at times have issues. 
If your IO layer is dead, your IO request queues that _should_ have been 
very temporary may end up staying around for a long time. But I do think 
that it makes sense to prioritize allocations that are known to be short- 
lived.

(It might also allows us to allocate them from different pools and just 
generally have other heuristics for cache behaviour - short-lived 
allocations simply have different behaviour)

			Linus
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