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@xxxxxxx> 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@xxxxxxxxxxxxxx>
>
> > > --- 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