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

[Christoph Hellwig]

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.