Re: [PATCH] scatterlist: add a warning if sg_virt() is used on highmem pages

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

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