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From: Seth Jennings <sjenning@linux.vnet.ibm.com>
To: Minchan Kim <minchan@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Dan Magenheimer <dan.magenheimer@oracle.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	Nitin Gupta <ngupta@vflare.org>,
	Robert Jennings <rcj@linux.vnet.ibm.com>,
	linux-mm@kvack.org, devel@driverdev.osuosl.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/4] zsmalloc: add details to zs_map_object boiler plate
Date: Wed, 11 Jul 2012 09:15:43 -0500	[thread overview]
Message-ID: <4FFD8A8F.6030603@linux.vnet.ibm.com> (raw)
In-Reply-To: <4FFD2E65.5080307@kernel.org>

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


WARNING: multiple messages have this Message-ID
From: Seth Jennings <sjenning@linux.vnet.ibm.com>
To: Minchan Kim <minchan@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Dan Magenheimer <dan.magenheimer@oracle.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	Nitin Gupta <ngupta@vflare.org>,
	Robert Jennings <rcj@linux.vnet.ibm.com>,
	linux-mm@kvack.org, devel@driverdev.osuosl.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/4] zsmalloc: add details to zs_map_object boiler plate
Date: Wed, 11 Jul 2012 09:15:43 -0500	[thread overview]
Message-ID: <4FFD8A8F.6030603@linux.vnet.ibm.com> (raw)
In-Reply-To: <4FFD2E65.5080307@kernel.org>

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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  reply	other threads:[~2012-07-11 14:20 UTC|newest]

Thread overview: 62+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-07-02 21:15 [PATCH 0/4] zsmalloc improvements Seth Jennings
2012-07-02 21:15 ` Seth Jennings
2012-07-02 21:15 ` [PATCH 1/4] zsmalloc: remove x86 dependency Seth Jennings
2012-07-02 21:15   ` Seth Jennings
2012-07-10  2:21   ` Minchan Kim
2012-07-10  2:21     ` Minchan Kim
2012-07-10 15:29     ` Seth Jennings
2012-07-10 15:29       ` Seth Jennings
2012-07-11  7:27       ` Minchan Kim
2012-07-11  7:27         ` Minchan Kim
2012-07-11 18:26   ` Nitin Gupta
2012-07-11 18:26     ` Nitin Gupta
2012-07-11 20:32     ` Seth Jennings
2012-07-11 20:32       ` Seth Jennings
2012-07-11 22:42       ` Nitin Gupta
2012-07-11 22:42         ` Nitin Gupta
2012-07-12  0:23         ` Seth Jennings
2012-07-12  0:23           ` Seth Jennings
2012-07-02 21:15 ` [PATCH 2/4] zsmalloc: add single-page object fastpath in unmap Seth Jennings
2012-07-02 21:15   ` Seth Jennings
2012-07-10  2:25   ` Minchan Kim
2012-07-10  2:25     ` Minchan Kim
2012-07-02 21:15 ` [PATCH 3/4] zsmalloc: add details to zs_map_object boiler plate Seth Jennings
2012-07-02 21:15   ` Seth Jennings
2012-07-10  2:35   ` Minchan Kim
2012-07-10  2:35     ` Minchan Kim
2012-07-10 15:17     ` Seth Jennings
2012-07-10 15:17       ` Seth Jennings
2012-07-11  7:42       ` Minchan Kim
2012-07-11  7:42         ` Minchan Kim
2012-07-11 14:15         ` Seth Jennings [this message]
2012-07-11 14:15           ` Seth Jennings
2012-07-12  1:15           ` Minchan Kim
2012-07-12  1:15             ` Minchan Kim
2012-07-12 19:54             ` Dan Magenheimer
2012-07-12 19:54               ` Dan Magenheimer
2012-07-12 22:46               ` Dan Magenheimer
2012-07-12 22:46                 ` Dan Magenheimer
2012-07-02 21:15 ` [PATCH 4/4] zsmalloc: add mapping modes Seth Jennings
2012-07-02 21:15   ` Seth Jennings
2012-07-04  5:33 ` [PATCH 0/4] zsmalloc improvements Minchan Kim
2012-07-04  5:33   ` Minchan Kim
2012-07-04 20:43 ` Konrad Rzeszutek Wilk
2012-07-04 20:43   ` Konrad Rzeszutek Wilk
2012-07-06 15:07   ` Seth Jennings
2012-07-06 15:07     ` Seth Jennings
2012-07-09 13:58     ` Seth Jennings
2012-07-09 13:58       ` Seth Jennings
2012-07-11 19:42       ` Konrad Rzeszutek Wilk
2012-07-11 19:42         ` Konrad Rzeszutek Wilk
2012-07-11 20:48         ` Seth Jennings
2012-07-11 20:48           ` Seth Jennings
2012-07-12 10:40           ` Konrad Rzeszutek Wilk
2012-07-12 10:40             ` Konrad Rzeszutek Wilk
2012-07-11  7:03 ` Minchan Kim
2012-07-11  7:03   ` Minchan Kim
2012-07-11 14:00   ` Seth Jennings
2012-07-11 14:00     ` Seth Jennings
2012-07-12  1:01     ` Minchan Kim
2012-07-12  1:01       ` Minchan Kim
2012-07-11 19:16   ` Seth Jennings
2012-07-11 19:16     ` Seth Jennings

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