linux-kernel - Re: [PATCH] scatterlist: add a warning if sg

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Message-ID: <50C05BB8.6090908@linutronix.de>
Date:	Thu, 06 Dec 2012 09:47:52 +0100
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] scatterlist: add a warning if sg_virt() is used on highmem
 pages

On 12/05/2012 10:46 PM, Andrew Morton wrote:
>> --- a/include/linux/scatterlist.h
>> +++ b/include/linux/scatterlist.h
>> @@ -198,6 +198,9 @@ static inline dma_addr_t sg_phys(struct scatterlist *sg)
>>    **/
>>   static inline void *sg_virt(struct scatterlist *sg)
>>   {
>> +#ifdef CONFIG_DEBUG_SG
>> +	WARN_ON(PageHighMem(sg_page(sg)));
>> +#endif
>>   	return page_address(sg_page(sg)) + sg->offset;
>>   }
>
> Why won't it work?  page_address() will search the kmap table and will
> search for the page.  If the caller had previously kmapped that page,
> all is well.  If the caller has failed to kmap the page, sg_virt() will
> return a nearly-null pointer and presumably someone will later go oops.

I missed that it will work for previously mapped page. If you think
that an oops later is enough then okay, I drop this.

Sebastian
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