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

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Date:   Mon, 8 Jun 2020 14:25:03 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        David Rientjes <rientjes@...gle.com>,
        Linux IOMMU <iommu@...ts.linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] dma-pool: Fix too large DMA pools on medium systems

Hi Robin,

On Mon, Jun 8, 2020 at 2:04 PM Robin Murphy <robin.murphy@....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@...ux-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@...ux-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