On Thu, Dec 09, 2021 at 01:59:58PM +0100, Christoph Lameter wrote: > On Thu, 9 Dec 2021, Baoquan He wrote: > > > > The slab allocators guarantee that all kmalloc allocations are DMA able > > > indepent of specifying ZONE_DMA/ZONE_DMA32 > > > > Here you mean we guarantee dma-kmalloc will be DMA able independent of > > specifying ZONE_DMA/DMA32, or the whole sla/ub allocator? > > All memory obtained via kmalloc --independent of "dma-alloc", ZONE_DMA > etc-- must be dmaable. > > > With my understanding, isn't the reasonable sequence zone DMA firstly if > > GFP_DMA, then zone DMA32, finaly zone NORMAL. At least, on x86_64, I > > believe device driver developer prefer to see this because most of time, > > zone DMA and zone DMA32 are both used for dma buffer allocation, if > > IOMMU is not enabled. However, memory got from zone NORMAL when required > > with GFP_DMA, and it succeeds, does it mean that the developer doesn't > > take the GFP_DMA flag seriously, just try to get buffer for allocation? > > ZONE_NORMAL is also used for DMA allocations. ZONE_DMA and ZONE_DMA32 are > only used if the physical range of memory supported by a device does not > include all of normal memory. > > > > The size of ZONE_DMA is traditionally depending on the platform. On some > > > it is 16MB, on some 1G and on some 4GB. ZONE32 is always 4GB and should > > > only be used if ZONE_DMA has already been used. > > > > As said at above, ia64 and riscv don't have ZONE_DMA at all, they just > > cover low 4G with ZONE_DMA32 alone. > > If you do not have devices that are crap and cannot address the full > memory then you dont need these special zones. > > Sorry this subject has caused confusion multiple times over the years and > there are still arches that are not implementing this in a consistent way. Hello Baoquan and Christoph. I'm the confused one here too. :) So the point is that ZONE_NORMAL is also dma-able if the device can access normal memory. (which is false for ISA devices, ancient PCI devices, ...etc.) Then if I understand right, I think the patch 5/5 (mm/slub: Avoid ...) should be removing GFP_DMA flag from the function sr_probe() -> get_capabilities, rather than copying copying normal kmalloc caches to dma kmalloc caches. (If the device does not have limitation in its address space.) Please let me know If I got it wrong :) Thanks, Hyeonggon. _______________________________________________ kexec mailing list kexec@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/kexec
