linux-kernel - Re: [PATCH 3/4] zsmalloc: add details to zs_map

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Message-ID: <4FFD8A8F.6030603@linux.vnet.ibm.com>
Date:	Wed, 11 Jul 2012 09:15:43 -0500
From:	Seth Jennings <sjenning@...ux.vnet.ibm.com>
To:	Minchan Kim <minchan@...nel.org>
CC:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Nitin Gupta <ngupta@...are.org>,
	Robert Jennings <rcj@...ux.vnet.ibm.com>, linux-mm@...ck.org,
	devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] zsmalloc: add details to zs_map_object boiler plate

On 07/11/2012 02:42 AM, Minchan Kim wrote:
> On 07/11/2012 12:17 AM, Seth Jennings wrote:
>> On 07/09/2012 09:35 PM, Minchan Kim wrote:
>>> Maybe we need local_irq_save/restore in zs_[un]map_object path.
>>
>> I'd rather not disable interrupts since that will create
>> unnecessary interrupt latency for all users, even if they
> 
> Agreed.
> Although we guide k[un]map atomic is so fast, it isn't necessary
> to force irq_[enable|disable]. Okay.
> 
>> don't need interrupt protection.  If a particular user uses
>> zs_map_object() in an interrupt path, it will be up to that
>> user to disable interrupts to ensure safety.
> 
> Nope. It shouldn't do that.
> Any user in interrupt context can't assume that there isn't any other user using per-cpu buffer
> right before interrupt happens.
> 
> The concern is that if such bug happens, it's very hard to find a bug.
> So, how about adding this?
> 
> void zs_map_object(...)
> {
> 	BUG_ON(in_interrupt());
> }

I not completely following you, but I think I'm following
enough.  Your point is that the per-cpu buffers are shared
by all zsmalloc users and one user doesn't know if another
user is doing a zs_map_object() in an interrupt path.

However, I think what you are suggesting is to disallow
mapping in interrupt context.  This is a problem for zcache
as it already does mapping in interrupt context, namely for
page decompression in the page fault handler.

What do you think about making the per-cpu buffers local to
each zsmalloc pool? That way each user has their own per-cpu
buffers and don't step on each other's toes.

Thanks,
Seth

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