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

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Message-ID: <4FFC478C.4050505@linux.vnet.ibm.com>
Date:	Tue, 10 Jul 2012 10:17:32 -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 07/09/2012 09:35 PM, Minchan Kim wrote:
> On 07/03/2012 06:15 AM, Seth Jennings wrote:
>> Add information on the usage limits of zs_map_object()
>>
>> Signed-off-by: Seth Jennings <sjenning@...ux.vnet.ibm.com>
>> ---
>>  drivers/staging/zsmalloc/zsmalloc-main.c |    7 ++++++-
>>  1 file changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/staging/zsmalloc/zsmalloc-main.c b/drivers/staging/zsmalloc/zsmalloc-main.c
>> index 4942d41..abf7c13 100644
>> --- a/drivers/staging/zsmalloc/zsmalloc-main.c
>> +++ b/drivers/staging/zsmalloc/zsmalloc-main.c
>> @@ -747,7 +747,12 @@ EXPORT_SYMBOL_GPL(zs_free);
>>   *
>>   * Before using an object allocated from zs_malloc, it must be mapped using
>>   * this function. When done with the object, it must be unmapped using
>> - * zs_unmap_object
>> + * zs_unmap_object.
>> + *
>> + * Only one object can be mapped per cpu at a time. There is no protection
>> + * against nested mappings.
>> + *
>> + * This function returns with preemption and page faults disabled.
>>  */
>>  void *zs_map_object(struct zs_pool *pool, unsigned long handle)
>>  {
>>
> 
> The comment is good but I hope we can detect it automatically with DEBUG
> option. It wouldn't be hard but it's a debug patch so not critical
> until we receive some report about the bug.

Yes, we could implement some detection scheme later.

> 
> The possibility for nesting is that it is used by irq context.
> 
> A uses the mapping
> .
> .
> .
> IRQ happen
> 	B uses the mapping in IRQ context
> 	.
> 	.
> 	.
> 
> 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
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.

Thanks,
Seth


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