On Wed, Dec 15, 2021 at 07:03:35AM +0000, Hyeonggon Yoo wrote: > I'm not sure that allocating from ZONE_DMA32 instead of ZONE_DMA > for kdump kernel is nice way to solve this problem. What is the problem with zones in kdump kernels? > Devices that requires ZONE_DMA memory is rare but we still support them. Indeed. > > 1) Do not call warn_alloc in page allocator if will always fail > > to allocate ZONE_DMA pages. > > > > > > 2) let's check all callers of kmalloc with GFP_DMA > > if they really need GFP_DMA flag and replace those by DMA API or > > just remove GFP_DMA from kmalloc() > > > > 3) Drop support for allocating DMA memory from slab allocator > > (as Christoph Hellwig said) and convert them to use DMA32 > > (as Christoph Hellwig said) and convert them to use *DMA API* > > > and see what happens This is the right thing to do, but it will take a while. In fact I dont think we really need the warning in step 1, a simple grep already allows to go over them. I just looked at the uses of GFP_DMA in drivers/scsi for example, and all but one look bogus. > > > > Yeah, I have the same guess too for get_capabilities(), not sure about other > > > > callers. Or, as ChristophL and ChristophH said(Sorry, not sure if this is > > > > the right way to call people when the first name is the same. Correct me if > > > > it's wrong), any buffer requested from kmalloc can be used by device driver. > > > > Means device enforces getting memory inside addressing limit for those > > > > DMA transferring buffer which is usually large, Megabytes level with > > > > vmalloc() or alloc_pages(), but doesn't care about this kind of small > > > > piece buffer memory allocated with kmalloc()? Just a guess, please tell > > > > a counter example if anyone happens to know, it could be easy. The way this works is that the dma_map* calls will bounce buffer memory that does to fall into the addressing limitations. This is a performance overhead, but allows drivers to address all memory in a system. If the driver controls memory allocation it should use one of the dma_alloc_* APIs that allocate addressable memory from the start. The allocator will dip into ZONE_DMA and ZONE_DMA32 when needed.
