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SubjectRe: [PATCH v3 5/5] mm/slub: do not create dma-kmalloc if no managed pages in DMA zone
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, 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.

Hello Christoph, Baoquan and I started this cleanup.
But we're a bit confused. I want to ask you something.

- Did you mean dma_map_* can handle arbitrary buffer, (and dma_map_* will
bounce buffer when necessary) Can we assume it on every architectures
and buses?

Reading at the DMA API documentation and code (dma_map_page_attrs(),
dma_direct_map_page()), I'm not sure about that.

In the documentation: (dma_map_single)
Further, the DMA address of the memory must be within the
dma_mask of the device (the dma_mask is a bit mask of the
addressable region for the device, i.e., if the DMA address of
the memory ANDed with the dma_mask is still equal to the DMA
address, then the device can perform DMA to the memory). To
ensure that the memory allocated by kmalloc is within the dma_mask,
the driver may specify various platform-dependent flags to restrict
the DMA address range of the allocation (e.g., on x86, GFP_DMA
guarantees to be within the first 16MB of available DMA addresses,
as required by ISA devices).

- In what function does the DMA API do bounce buffering?

Thanks a lot,
Hyeonggon

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