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

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Message-ID: <47FA8B5A.5090104@cs.helsinki.fi>
Date:	Tue, 08 Apr 2008 00:00:10 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Hugh Dickins <hugh@...itas.com>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Lameter <clameter@....com>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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

Hi Hugh,

On Mon, 2008-04-07 at 20:40 +0100, Hugh Dickins wrote:
> > > If we had a SLAB_NOMERGE flag, would we want to apply it to the
> > > bio cache or to the scsi_sense_cache or to both?  My difficulty
> > > in answering that makes me wonder whether such a flag is right.

On Mon, 7 Apr 2008, Peter Zijlstra wrote:
> > If this is critical to avoid memory deadlocks, I would suggest using
> > mempools (or my reserve framework).

Hugh Dickins wrote:
> No, the critical part of it has been dealt with (small fix to scsi
> free_list handling: which resembles a mempool, but done its own way).
> 
> What remains is about "unsightly" behaviour, the system having a
> tendency to collapse briefly into far-from-efficient operation
> when out of memory.

Although you weren't convinced by my arguments, I still have 
difficulties understanding why this kind of bad behavior would be 
acceptable in an embedded environment and why we don't need to fix it 
for the SLOB case as well.

But you do bring up a good point of SLUB changing the behavior on OOM 
situations for which SLAB_NOMERGE sounds like a good-enough stop-gap 
measure for the short term. I would prefer some other fix even if it 
means getting rid of slab merging competely (which would suck as it's 
very nice for making memory footprint smaller).

		Pekka
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