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SubjectRE: [RFC PATCH] cma: make number of CMA areas dynamic, remove CONFIG_CMA_AREAS
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> -----Original Message-----
> From: Mike Kravetz [mailto:mike.kravetz@oracle.com]
> Sent: Wednesday, September 9, 2020 6:29 AM
> To: Song Bao Hua (Barry Song) <song.bao.hua@hisilicon.com>;
> linux-mm@kvack.org; linux-kernel@vger.kernel.org;
> linux-arm-kernel@lists.infradead.org; linux-mips@vger.kernel.org
> Cc: Roman Gushchin <guro@fb.com>; Joonsoo Kim <js1304@gmail.com>; Rik
> van Riel <riel@surriel.com>; Aslan Bakirov <aslan@fb.com>; Michal Hocko
> <mhocko@kernel.org>; Andrew Morton <akpm@linux-foundation.org>
> Subject: Re: [RFC PATCH] cma: make number of CMA areas dynamic, remove
> CONFIG_CMA_AREAS
>
> On 9/3/20 6:58 PM, Song Bao Hua (Barry Song) wrote:
> >
> >> -----Original Message-----
> >> From: Mike Kravetz [mailto:mike.kravetz@oracle.com]
> >> Sent: Thursday, September 3, 2020 3:02 PM
> >> To: linux-mm@kvack.org; linux-kernel@vger.kernel.org;
> >> linux-arm-kernel@lists.infradead.org; linux-mips@vger.kernel.org
> >> Cc: Roman Gushchin <guro@fb.com>; Song Bao Hua (Barry Song)
> >> <song.bao.hua@hisilicon.com>; Joonsoo Kim <js1304@gmail.com>; Rik van
> >> Riel <riel@surriel.com>; Aslan Bakirov <aslan@fb.com>; Michal Hocko
> >> <mhocko@kernel.org>; Andrew Morton <akpm@linux-foundation.org>;
> Mike
> >> Kravetz <mike.kravetz@oracle.com>
> >> Subject: [RFC PATCH] cma: make number of CMA areas dynamic, remove
> >> CONFIG_CMA_AREAS
> >>
> >> The number of distinct CMA areas is limited by the constant
> >> CONFIG_CMA_AREAS. In most environments, this was set to a default
> >> value of 7. Not too long ago, support was added to allocate hugetlb
> >> gigantic pages from CMA. More recent changes to make
> dma_alloc_coherent
> >> NUMA-aware on arm64 added more potential users of CMA areas. Along
> >> with the dma_alloc_coherent changes, the default value of CMA_AREAS
> >> was bumped up to 19 if NUMA is enabled.
> >>
> >> It seems that the number of CMA users is likely to grow. Instead of
> >> using a static array for cma areas, use a simple linked list. These
> >> areas are used before normal memory allocators, so use the memblock
> >> allocator.
> >
> > Hello Mike, It seems it is a good idea. Thanks for addressing this.
> >
> > I was focusing on per-numa cma feature in my patchset and I didn't take care
> of this
> > while I thought we should do something for the number of cma areas.
> >
>
> Thanks for taking a look.
>
> One area where I could use some help is testing/verifying on arm. See the
> changes to arch/arm/mm/dma-mapping.c. I have tested the generic changes
> on
> my x86 platform, but do not have an arm platform for easy testing.
>
> >> void __init dma_contiguous_early_fixup(phys_addr_t base, unsigned long
> >> size)
> >> {
> >> - dma_mmu_remap[dma_mmu_remap_num].base = base;
> >> - dma_mmu_remap[dma_mmu_remap_num].size = size;
> >> - dma_mmu_remap_num++;
> >> + struct dma_contig_early_reserve *d;
> >> +
> >> + d = memblock_alloc(sizeof(struct dma_contig_early_reserve),
> >
> > sizeof(*d)?
>
> Yes. thanks.
>
> >> @@ -172,15 +173,14 @@ int __init cma_init_reserved_mem(phys_addr_t
> >> base, phys_addr_t size,
> >> struct cma *cma;
> >> phys_addr_t alignment;
> >>
> >> - /* Sanity checks */
> >> - if (cma_area_count == ARRAY_SIZE(cma_areas)) {
> >> - pr_err("Not enough slots for CMA reserved regions!\n");
> >> - return -ENOSPC;
> >> - }
> >> + /* Do not attempt allocations after memblock allocator is torn down */
> >> + if (slab_is_available())
> >> + return -EINVAL;
> >>
> >> if (!size || !memblock_is_region_reserved(base, size))
> >> return -EINVAL;
> >>
> >> +
> >
> > Is this empty line relevant?
>
> No, added by mistake.
>
> >> @@ -192,12 +192,17 @@ int __init cma_init_reserved_mem(phys_addr_t
> >> base, phys_addr_t size,
> >> if (ALIGN(base, alignment) != base || ALIGN(size, alignment) != size)
> >> return -EINVAL;
> >>
> >> + cma = memblock_alloc(sizeof(struct cma), sizeof(long));
> >
> > sizeof(*cma)?
>
> Yes, thanks.
>
> > It seems we are going to write cma-> count, order_per_bit, debugfs fields.
> > To avoid false sharing of the cacheline of struct cma, it is better to align with
> > SMP_CACHE_BYTES.
> >
> > On the other hand, it seems we are unlikely to write the cma
>
> I thought about using SMP_CACHE_BYTES, but the structures are simply
> defined
> as an array today. This should not be any worse. I do not believe access
> to the structures is performance sensitive.

That depends on how often people will write and read the cma structure indirectly via
dma_alloc/free_coherent() APIs, especially through multiple CPUs.
Anyway, we don't have benchmark data to check if this will be really a problem.
So I am ok with the code we use either SMP_CACHE_BYTES or long as the alignment.

>
> Thanks,
> --
> Mike Kravetz

Thanks
Barry
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