On 12/17/21 at 11:38am, Hyeonggon Yoo wrote: > On Wed, Dec 15, 2021 at 08:27:10AM +0100, Christoph Hellwig wrote: > > 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, > > Hmm I think step 1) will be needed if someone is allocating pages from > DMA zone not using kmalloc or DMA API. (for example directly allocating > from buddy allocator) is there such cases? I think Christoph meant to take off the warning. I will post a patch to mute the warning if it's requesting page from DMA zone which has no managed pages. > > > 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. > > > > That's good. this cleanup will also remove unnecessary limitations. I searched and investigated several callsites where kmalloc(GFP_DMA) is called. E.g drivers/scsi/sr.c: sr_probe(). The scsi sr driver doesn't check DMA supporting capibility at all, e.g the dma limit, to set the dma mask or coherent_dma_mask. If we want to convert the kmalloc(GFP_DMA) to dma_alloc* API, scsi sr drvier developer/expert's suggestion and help is necessary. Either someone who knows this well help to change it, or give suggestion how to change so that I can do it. > > > > > > > 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. > _______________________________________________ kexec mailing list kexec@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/kexec
