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

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Date:	Tue, 10 Jul 2012 11:35:43 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Seth Jennings <sjenning@...ux.vnet.ibm.com>
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 07/03/2012 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.

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.

-- 
Kind regards,
Minchan Kim


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