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

[Baoquan He <bhe@redhat.com>]

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.
>