RE: [PATCH v4 1/2] dma-direct: provide the ability to reserve per-numa CMA

["Song Bao Hua (Barry Song)" <song.bao.hua@hisilicon.com>]

> -----Original Message-----
> From: Christoph Hellwig [mailto:hch@lst.de]
> Sent: Wednesday, July 29, 2020 12:23 AM
> To: Song Bao Hua (Barry Song) <song.bao.hua@hisilicon.com>
> Cc: Christoph Hellwig <hch@lst.de>; m.szyprowski@samsung.com;
> robin.murphy@arm.com; will@kernel.org; ganapatrao.kulkarni@cavium.com;
> catalin.marinas@arm.com; iommu@lists.linux-foundation.org; Linuxarm
> <linuxarm@huawei.com>; linux-arm-kernel@lists.infradead.org;
> linux-kernel@vger.kernel.org; Zengtao (B) <prime.zeng@hisilicon.com>;
> huangdaode <huangdaode@huawei.com>; Jonathan Cameron
> <jonathan.cameron@huawei.com>; Nicolas Saenz Julienne
> <nsaenzjulienne@suse.de>; Steve Capper <steve.capper@arm.com>; Andrew
> Morton <akpm@linux-foundation.org>; Mike Rapoport <rppt@linux.ibm.com>
> Subject: Re: [PATCH v4 1/2] dma-direct: provide the ability to reserve
> per-numa CMA
> 
> On Tue, Jul 28, 2020 at 12:19:03PM +0000, Song Bao Hua (Barry Song) wrote:
> > I am sorry I haven't got your point yet. Do you mean something like the
> below?
> >
> > arch/arm64/Kconfig:
> > config CMDLINE
> > 	string "Default kernel command string"
> > -	default ""
> > +	default "pernuma_cma=16M"
> > 	help
> > 	  Provide a set of default command-line options at build time by
> > 	  entering them here. As a minimum, you should specify the the
> > 	  root device (e.g. root=/dev/nfs).
> 
> Yes.
> 
> > A background of the current code is that Linux distributions can usually use
> arch/arm64/configs/defconfig
> > directly to build kernel. cmdline can be easily ignored during the generation
> of Linux distributions.
> 
> I've not actually heard of a distro shipping defconfig yet..
> 
> >
> > > if a way to expose this in the device tree might be useful, but people
> > > more familiar with the device tree and the arm code will have to chime
> > > in on that.
> >
> > Not sure if it is an useful user case as we are using ACPI but not device tree
> since it is an ARM64
> > server with NUMA.
> 
> Well, than maybe ACPI experts need to chime in on this.
> 
> > > This seems to have lost the dma_contiguous_default_area NULL check.
> >
> > cma_alloc() is doing the check by returning NULL if cma is NULL.
> >
> > struct page *cma_alloc(struct cma *cma, size_t count, unsigned int align,
> > 		       bool no_warn)
> > {
> > 	...
> > 	if (!cma || !cma->count)
> > 		return NULL;
> > }
> >
> > But I agree here the code can check before calling cma_alloc_aligned.
> 
> Oh, indeed.  Please split the removal of the NULL check in to a prep
> patch then.

Do you mean removing the NULL check in cma_alloc()? If so, it seems lot of places
need to be changed:

struct page *dma_alloc_from_contiguous(struct device *dev, size_t count,
				       unsigned int align, bool no_warn)
{
	if (align > CONFIG_CMA_ALIGNMENT)
		align = CONFIG_CMA_ALIGNMENT;
+ code to check dev_get_cma_area(dev) is not NULL
	return cma_alloc(dev_get_cma_area(dev), count, align, no_warn);
}

bool dma_release_from_contiguous(struct device *dev, struct page *pages,
				 int count)
{
+ code to check dev_get_cma_area(dev) is not NULL
	return cma_release(dev_get_cma_area(dev), pages, count);
}

bool cma_release(struct cma *cma, const struct page *pages, unsigned int count)
{
	unsigned long pfn;
+ do we need to remove this !cma too if we remove it in cma_alloc()?
	if (!cma || !pages)
		return false;
	...
}

And some other places where cma_alloc() and cma_release() are called:

arch/powerpc/kvm/book3s_hv_builtin.c
drivers/dma-buf/heaps/cma_heap.c
drivers/s390/char/vmcp.c
drivers/staging/android/ion/ion_cma_heap.c
mm/hugetlb.c

it seems many code were written with the assumption that cma_alloc/release will
check if cma is null so they don't check it before calling cma_alloc().

And I am not sure if kernel robot will report error like pointer reference before checking
it if !cma is removed in cma_alloc().

Thanks
Barry

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