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

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

[Geert Uytterhoeven]

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));
 	}
-- 
2.17.1

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

[Robin Murphy]

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

[Geert Uytterhoeven]

Hi Robin,

On Mon, Jun 8, 2020 at 2:04 PM Robin Murphy <robin.murphy@arm.com> wrote:
> 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>

> > --- 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

">> (x - PAGE_SHIFT)" is a commonly used construct in the kernel.

> 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;

That multiplication will overflow on 32-bit systems (perhaps even on
large 64-bit systems; any 47-bit addressing?).

        unsigned long pages = totalram_pages() / (SZ_1GB / SZ_128K);

>         atomic_pool_size = min(pages, MAX_ORDER_NR_PAGES) << PAGE_SHIFT;
>         atomic_pool_size = max_t(size_t, atomic_pool_size, SZ_128K);

I agree this part is an improvement.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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Re: [PATCH] dma-pool: Fix too large DMA pools on medium systems

[Robin Murphy]

On 2020-06-08 13:25, Geert Uytterhoeven wrote:
> Hi Robin,
> 
> On Mon, Jun 8, 2020 at 2:04 PM Robin Murphy <robin.murphy@arm.com> wrote:
>> 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>
> 
>>> --- 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
> 
> ">> (x - PAGE_SHIFT)" is a commonly used construct in the kernel.

Sure, but when "x" is a magic number there's still extra cognitive load 
in determining whether it's the *right* magic number ;)

Mostly, though, it was just the fact that an expression involving 5 
different units (bytes, pages, "gigs", bits, and whatever MAX_ORDER is) 
is inherently more challenging to follow than the equivalent thing 
framed in fewer, especially when it can be reasonably done in just two 
(bytes and pages).

Robin.

>> 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;
> 
> That multiplication will overflow on 32-bit systems (perhaps even on
> large 64-bit systems; any 47-bit addressing?).
> 
>          unsigned long pages = totalram_pages() / (SZ_1GB / SZ_128K);
> 
>>          atomic_pool_size = min(pages, MAX_ORDER_NR_PAGES) << PAGE_SHIFT;
>>          atomic_pool_size = max_t(size_t, atomic_pool_size, SZ_128K);
> 
> I agree this part is an improvement.
> 
> Gr{oetje,eeting}s,
> 
>                          Geert
> 
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