Is it illegal to have kernel virtual address == 0 ?

[ranshalit@gmail.com (Ran Shalit)]

On Sun, Jan 15, 2017 at 12:54 PM, Greg KH <greg@kroah.com> wrote:
> On Sun, Jan 15, 2017 at 12:17:40PM +0200, Ran Shalit wrote:
>> Hello,
>>
>> I've seen some modules where it is tested if virtual address is 0, and
>> if yes, it is acted as if it is an error.
>> But can't virtual address be zero too ?
>>
>> For example, in the following module:
>> https://github.com/khilman/omap-test-dmatest/blob/master/main.c
>> it is checked the returned value of dma_alloc_coherent as following:
>>
>> dma_test[i].src_buf = (unsigned long) dma_alloc_coherent(NULL,
>> buf_size (dma_addr_t *)&dma_test[i].src_buf_phys, GFP_KERNEL |
>> GFP_DMA);
>> if (!dma_test[i].src_buf) {
>>    printk("dmatest_init(): get_zeroed_page() failed.\n");
>>    r = -ENOMEM;
>>    goto cleanup;
>> }
>>
>> As you can see , when  dma_alloc_coherent returns 0 , it is acted as
>> if it is an error.
>
> Yes, if dma_alloc_coherent returns NULL, an error happened and the
> memory could not be allocated.
>
> In-kernel memory pointers will never be NULL if they are valid.
>
> Hope this helps,

Thank you very much,
Ran

>
> greg k-h