Re: [PATCH] dma-pool: Fix too large DMA pools on medium systems

[Robin Murphy <robin.murphy@arm.com>]

On 2020-06-08 09:52, Geert Uytterhoeven wrote:
> On systems with at least 32 MiB, but less than 32 GiB of RAM, the DMA
> memory pools are much larger than intended (e.g. 2 MiB instead of 128
> KiB on a 256 MiB system).
> 
> Fix this by correcting the calculation of the number of GiBs of RAM in
> the system.
> 
> Fixes: 1d659236fb43c4d2 ("dma-pool: scale the default DMA coherent pool size with memory capacity")
> Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
> ---
>   kernel/dma/pool.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/dma/pool.c b/kernel/dma/pool.c
> index 35bb51c31fff370f..1c7eab2cc0498003 100644
> --- a/kernel/dma/pool.c
> +++ b/kernel/dma/pool.c
> @@ -175,8 +175,8 @@ static int __init dma_atomic_pool_init(void)
>   	 * sizes to 128KB per 1GB of memory, min 128KB, max MAX_ORDER-1.
>   	 */
>   	if (!atomic_pool_size) {
> -		atomic_pool_size = max(totalram_pages() >> PAGE_SHIFT, 1UL) *
> -					SZ_128K;
> +		unsigned long gigs = totalram_pages() >> (30 - PAGE_SHIFT);
> +		atomic_pool_size = max(gigs, 1UL) * SZ_128K;
>   		atomic_pool_size = min_t(size_t, atomic_pool_size,
>   					 1 << (PAGE_SHIFT + MAX_ORDER-1));
>   	}

Nit: although this probably is right, it seems even less readable than 
the broken version (where at least some at-a-glance 'dimensional 
analysis' flags up "(number of pages) >> PAGE_SHIFT" as rather 
suspicious). How about a something a little more self-explanatory, e.g.:

	unsigned long pages = totalram_pages() * SZ_128K / SZ_1GB;
	atomic_pool_size = min(pages, MAX_ORDER_NR_PAGES) << PAGE_SHIFT;
	atomic_pool_size = max_t(size_t, atomic_pool_size, SZ_128K);

?

Robin.
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