linux-kernel - Re: [PATCH v2 0/3] staging: zcache: xcfmalloc support

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Message-ID: <1317666154.16137.727.camel@nimitz>
Date:	Mon, 03 Oct 2011 11:22:34 -0700
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Nitin Gupta <ngupta@...are.org>
Cc:	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	Greg KH <greg@...ah.com>, gregkh@...e.de,
	devel@...verdev.osuosl.org, cascardo@...oscopio.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	brking@...ux.vnet.ibm.com, rcj@...ux.vnet.ibm.com
Subject: Re: [PATCH v2 0/3] staging: zcache: xcfmalloc support

On Mon, 2011-10-03 at 13:54 -0400, Nitin Gupta wrote:
> I think disabling preemption on the local CPU is the cheapest we can get
> to protect PCPU buffers. We may experiment with, say, multiple buffers
> per CPU, so we end up disabling preemption only in highly improbable
> case of getting preempted just too many times exactly within critical
> section.

I guess the problem is two-fold: preempt_disable() and
local_irq_save().  

> static int zcache_put_page(int cli_id, int pool_id, struct tmem_oid *oidp,
>                                 uint32_t index, struct page *page)
> {
>         struct tmem_pool *pool;
>         int ret = -1;
> 
>         BUG_ON(!irqs_disabled());

That tells me "zcache" doesn't work with interrupts on.  It seems like
awfully high-level code to have interrupts disabled.  The core page
allocator has some irq-disabling spinlock calls, but that's only really
because it has to be able to service page allocations from interrupts.
What's the high-level reason for zcache?

I'll save the discussion about preempt for when Seth posts his patch.

-- Dave

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