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). -- Catalin
