Re: [PATCH v6 01/17] mm/slab: Decouple ARCH_KMALLOC_MINALIGN from ARCH_DMA_MINALIGN

From: Vlastimil Babka <vbabka@suse.cz>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Christoph Hellwig <hch@lst.de>,
	Robin Murphy <robin.murphy@arm.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Will Deacon <will@kernel.org>, Marc Zyngier <maz@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Herbert Xu <herbert@gondor.apana.org.au>,
	Ard Biesheuvel <ardb@kernel.org>,
	Isaac Manjarres <isaacmanjarres@google.com>,
	Saravana Kannan <saravanak@google.com>,
	Alasdair Kergon <agk@redhat.com>, Daniel Vetter <daniel@ffwll.ch>,
	Joerg Roedel <joro@8bytes.org>, Mark Brown <broonie@kernel.org>,
	Mike Snitzer <snitzer@kernel.org>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	Jonathan Cameron <jic23@kernel.org>,
	linux-mm@kvack.org, iommu@lists.linux.dev,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v6 01/17] mm/slab: Decouple ARCH_KMALLOC_MINALIGN from ARCH_DMA_MINALIGN
Date: Fri, 9 Jun 2023 16:13:34 +0200	[thread overview]
Message-ID: <e9e264d3-1d20-4c1d-74bf-41ffc050f4c2@suse.cz> (raw)
In-Reply-To: <ZIMv2SQnM+qSOI4M@arm.com>

On 6/9/23 15:57, Catalin Marinas wrote:
> On Fri, Jun 09, 2023 at 02:44:01PM +0100, Catalin Marinas wrote:
>> On Fri, Jun 09, 2023 at 02:32:57PM +0200, Vlastimil Babka wrote:
>> > On 5/31/23 17:48, Catalin Marinas wrote:
>> > > diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
>> > > index 0ee20b764000..3288a1339271 100644
>> > > --- a/include/linux/dma-mapping.h
>> > > +++ b/include/linux/dma-mapping.h
>> > > @@ -545,7 +545,7 @@ static inline int dma_set_min_align_mask(struct device *dev,
>> > >  
>> > >  static inline int dma_get_cache_alignment(void)
>> > >  {
>> > > -#ifdef ARCH_DMA_MINALIGN
>> > > +#ifdef ARCH_HAS_DMA_MINALIGN
>> > >  	return ARCH_DMA_MINALIGN;
>> > >  #endif
>> > >  	return 1;
>> > > diff --git a/include/linux/slab.h b/include/linux/slab.h
>> > > index 6b3e155b70bf..50dcf9cfbf62 100644
>> > > --- a/include/linux/slab.h
>> > > +++ b/include/linux/slab.h
>> > > @@ -235,12 +235,20 @@ void kmem_dump_obj(void *object);
>> > >   * alignment larger than the alignment of a 64-bit integer.
>> > >   * Setting ARCH_DMA_MINALIGN in arch headers allows that.
>> > >   */
>> > > -#if defined(ARCH_DMA_MINALIGN) && ARCH_DMA_MINALIGN > 8
>> > > +#ifdef ARCH_DMA_MINALIGN
>> > > +#define ARCH_HAS_DMA_MINALIGN
>> > > +#if ARCH_DMA_MINALIGN > 8 && !defined(ARCH_KMALLOC_MINALIGN)
>> > >  #define ARCH_KMALLOC_MINALIGN ARCH_DMA_MINALIGN
>> > > -#define KMALLOC_MIN_SIZE ARCH_DMA_MINALIGN
>> > > -#define KMALLOC_SHIFT_LOW ilog2(ARCH_DMA_MINALIGN)
>> > > +#endif
>> > >  #else
>> > > +#define ARCH_DMA_MINALIGN __alignof__(unsigned long long)
>> > > +#endif
>> > 
>> > It seems weird to make slab.h responsible for this part, especially for
>> > #define ARCH_HAS_DMA_MINALIGN, which dma-mapping.h consumes. Maybe it would
>> > be difficult to do differently due to some dependency hell, but minimally I
>> > don't see dma-mapping.h including slab.h so the result is
>> > include-order-dependent? Maybe it's included transitively, but then it's
>> > fragile and would be better to do explicitly?
>> 
>> True, there's a risk that it doesn't get included with some future
>> header refactoring.
>> 
>> What about moving ARCH_DMA_MINALIGN to linux/cache.h? Alternatively, I
>> could create a new linux/dma-minalign.h file but I feel since this is
>> about caches, having it in cache.h makes more sense. asm/cache.h is also
>> where most archs define the constant (apart from mips, sh, microblaze).
> 
> Something like this (still compiling):

Yeah that would be great!