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

Hyeonggon Yoo
2021-12-15T07:03:47Z

On Wed, Dec 15, 2021 at 04:48:26AM +0000, Hyeonggon Yoo wrote:
>
> Hello Baoquan and Vlastimil.
>
> I'm not sure allowing ZONE_DMA32 for kdump kernel is nice way to solve
> this problem. Devices that requires ZONE_DMA is rare but we still
> support them.
>
> If we allow ZONE_DMA32 for ZONE_DMA in kdump kernels,
> the problem will be hard to find.
>

Sorry, I sometimes forget validating my english writing :(

What I meant:

I'm not sure that allocating from ZONE_DMA32 instead of ZONE_DMA
for kdump kernel is nice way to solve this problem.

Devices that requires ZONE_DMA memory is rare but we still support them.

If we use ZONE_DMA32 memory instead of ZONE_DMA in kdump kernels,
It will be hard to the problem when we use devices that can use only
ZONE_DMA memory.

> What about one of those?:
>
> 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
>
> Thanks,
> Hyeonggon.
>
> > >>
> > >> Maybe the function get_capabilities() want to allocate memory
> > >> even if it's not from DMA zone, but other callers will not expect that.
> > >
> > > 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.
> > >
> > >
> > >>
> > >> > kmalloc_caches[KMALLOC_DMA][i] = create_kmalloc_cache(
> > >> > kmalloc_info[i].name[KMALLOC_DMA],
> > >> > kmalloc_info[i].size,
> > >> > --
> > >> > 2.17.2
> > >> >
> > >> >
> > >>
> > >
> >