[PATCH v3 RESEND 08/17] ARM: LPAE: use phys_addr_t in free_memmap()

[Russell King - ARM Linux]

On Mon, Sep 24, 2012 at 05:51:46PM +0100, Catalin Marinas wrote:
> I don't think that's needed. free_all_bootmem() in mm/nobootmem.c takes
> care of freeing lowmem but it has a different notion of max_low_pfn. So
> this hunk did the trick:
> 
> @@ -420,8 +366,8 @@ void __init bootmem_init(void)
>  	 * Note: max_low_pfn and max_pfn reflect the number of _pages_ in
>  	 * the system, not the maximum PFN.
>  	 */
> -	max_low_pfn = max_low - PHYS_PFN_OFFSET;
> -	max_pfn = max_high - PHYS_PFN_OFFSET;
> +	max_low_pfn = max_low;
> +	max_pfn = max_high;
>  }

Did you actually look to see where that's used before you made the change.
I don't think you did.

The reason we have that there is that much of the kernel assumes memory
always starts at physical zero, so the max*pfn variables can be used to
generate bitmasks to cover the range of system memory addresses - iow,
(1 << max_low_pfn) - 1.

Eg, in the block code:

        blk_max_low_pfn = max_low_pfn - 1;
        blk_max_pfn = max_pfn - 1;
...
        unsigned long b_pfn = dma_mask >> PAGE_SHIFT;

        if (b_pfn < blk_max_low_pfn)
                dma = 1;

Having a DMA mask for a peripheral which only has 24 bits wired (so
0x00ffffff) with a system memory offset of 0xc0000000 results in
apparantly _all_ system memory being DMA-able according to this test
unless max_low_pfn is defined as the _number_ of bits in the RAM
address.

In dma_get_required_mask():

        u32 low_totalram = ((max_pfn - 1) << PAGE_SHIFT);

                low_totalram = (1 << (fls(low_totalram) - 1));
                low_totalram += low_totalram - 1;

which results in (for a phys offset of 0xc0000000) low_totalram being
0xffffffff unconditionally no matter how much RAM you actually have.

fs/proc/kcore.c can be ignored because that's not supported on ARM.

So, for DMA masks to work correctly, max_pfn and max_low_pfn must be
defined to cover the number of signiciant address bits in the RAM
region, and not covering the number of significant bits of the last
RAM address.