On Sat, 2020-10-10 at 12:36 +0200, Ard Biesheuvel wrote: > On Fri, 9 Oct 2020 at 19:10, Catalin Marinas <catalin.marinas@xxxxxxx> wrote: > > On Fri, Oct 09, 2020 at 06:23:06PM +0200, Ard Biesheuvel wrote: > > > On Fri, 9 Oct 2020 at 17:24, Lorenzo Pieralisi > > > <lorenzo.pieralisi@xxxxxxx> wrote: > > > > We can move this check to IORT code and call it from arm64 if it > > > > can be made to work. > > > > > > Finding the smallest value in the IORT, and assigning it to > > > zone_dma_bits if it is < 32 should be easy. But as I understand it, > > > having these separate DMA and DMA32 zones is what breaks kdump, no? So > > > how is this going to fix the underlying issue? > > > > If zone_dma_bits is 32, ZONE_DMA32 disappears into ZONE_DMA (GFP_DMA32 > > allocations fall back to ZONE_DMA). > > > > kdump wants DMA-able memory and, without a 30-bit ZONE_DMA, that would > > be the bottom 32-bit. With the introduction of ZONE_DMA, this suddenly > > became 1GB. We could change kdump to allocate ZONE_DMA32 but this one > > may also be small as it lost 1GB to ZONE_DMA. However, the kdump kernel > > would need to be rebuilt without ZONE_DMA since it won't have any. IIRC > > (it's been a while since I looked), the kdump allocation couldn't span > > multiple zones. > > > > In a separate thread, we try to fix kdump to use allocations above 4G as > > a fallback but this only fixes platforms with enough RAM (and maybe it's > > only those platforms that care about kdump). > > > > One thing that strikes me as odd is that we are applying the same > shifting logic to ZONE_DMA as we are applying to ZONE_DMA32, i.e., if > DRAM starts outside of the zone, it is shifted upwards. > > On a typical ARM box, this gives me > > [ 0.000000] Zone ranges: > [ 0.000000] DMA [mem 0x0000000080000000-0x00000000bfffffff] > [ 0.000000] DMA32 [mem 0x00000000c0000000-0x00000000ffffffff] > [ 0.000000] Normal [mem 0x0000000100000000-0x0000000fffffffff] > > i.e., the 30-bit addressable range has bit 31 set, which is weird. Yes I agree it's weird, and IMO plain useless. I sent a series this summer to address this[1], which ultimately triggered the discussion we're having right now. Although with with your latest patch and the DT counterpart, we should be OK. It would be weird for a HW description to define DMA constraints that are impossible to reach on that system. > I wonder if it wouldn't be better (and less problematic in the general > case) to drop this logic for ZONE_DMA, and simply let it remain empty > unless there is really some memory there. From my experience, you can't have empty ZONE_DMA when enabled. Empty ZONE_DMA32 is OK though. Although I'm sure it's something that can be changed. Regards, Nicolas [1] https://lkml.org/lkml/2020/8/19/1022
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