Messages in this thread | | | Subject | Re: [PATCH] dma-pool: Fix too large DMA pools on medium systems | From | Robin Murphy <> | Date | Mon, 8 Jun 2020 13:53:17 +0100 |
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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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