Re: dynamic reservation and allocation of physically contiguous memory using CMA

[Marek Szyprowski]

Hi Amit,

On 2018-06-18 20:23, Amit Chandra (amichand) wrote:
>
> Hi experts,
>
> I had a question related to CMA. I have been trying to use the CMA 
> infra to reserve and allocate physically contiguous memory dynamically 
> at runtime.
>
> I built a custom kernel based on linux-4.14.47 to invoke the cma 
> initialization apis at runtime from kernel loadable module.
>
> I invoke cma_declare_contiguous() followed by cma_init_reserved_areas().
>
> cma_declare_contiguous throws no surprises and succeeds. The issue 
> happens when cma_init_reserved_areas() is invoked post that.
>
> Here is the kernel log snippet post that call:
>
> Jun 15 03:30:31 ubuntu-quickstart kernel: [Â 384.593218] cma: 
> cma_declare_contiguous(size 0x0000000200000000, base 
> 0x0000000000000000, limit 0x0000000000000000 alignment 0x0000000000000000)
>
> Jun 15 03:30:31 ubuntu-quickstart kernel: [Â 384.593228] cma: Reserved 
> 8192 MiB at 0x0000001d4d000000
>
> Jun 15 03:30:31 ubuntu-quickstart kernel: [Â 384.593345] BUG: Bad page 
> state in process insmod pfn:1d4d000
>
> Jun 15 03:30:31 ubuntu-quickstart kernel: [Â 384.595758] 
> page:ffffefc335340000 count:0 mapcount:-127 mapping:ÂÂÂÂÂÂÂÂÂ (null) 
> index:0x0
>
> Jun 15 03:30:31 ubuntu-quickstart kernel: [Â 384.599193] flags: 
> 0x57fffc000000000()
>
> Jun 15 03:30:31 ubuntu-quickstart kernel: [Â 384.600751] raw: 
> 057fffc000000000 0000000000000000 0000000000000000 00000000ffffff80
>
> Jun 15 03:30:31 ubuntu-quickstart kernel: [Â 384.603946] raw: 
> ffffefc335330020 ffffefc335350020 000000000000000a 0000000000000000
>
> Jun 15 03:30:31 ubuntu-quickstart kernel: [Â 384.607152] page dumped 
> because: nonzero mapcount
>
> I am having a hard time trying to understand why the mapcount is less 
> than 0 here. I figured this is happening in the call to __free_pages() 
> from init_cma_reserved_pageblock().
>
> Any pointers here would be really helpful. If I am missing any step 
> for cma reservation, please do let me know.
>
> Thanks in advance.
>

CMA initialization is possible only on very early boot stage. CMA will 
not work as dynamic module.

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland