linux-kernel - Re: [PATCH] Containment measures for slab objects on scatter gather lists

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Message-Id: <20070628.223734.21928089.davem@davemloft.net>
Date:	Thu, 28 Jun 2007 22:37:34 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	akpm@...ux-foundation.org
Cc:	clameter@....com, hugh@...itas.com, James.Bottomley@...eleye.com,
	rmk+lkml@....linux.org.uk, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Containment measures for slab objects on scatter
 gather lists

From: Andrew Morton <akpm@...ux-foundation.org>
Date: Thu, 28 Jun 2007 22:24:24 -0700

> So what happens when two quite different threads of control are doing
> IO against two hunks of kmalloced memory which happen to come from the same
> page?  Either some (kernel-wide) locking is needed, or that pageframe needs
> to be treated as readonly?

Or you put an atomic_t at the beginning or tail of every SLAB
object.  It's a space cost not a runtime cost for the common
case which is:

	smp_rmb();
	if (atomic_read(&slab_obj->count) == 1)
		really_free_it();
	else if (atomic_dec_and_test(...))

Note I don't like this variant either. :)

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