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Thu, 07 Apr 2022 05:27:00 -0700 (PDT) Date: Thu, 7 Apr 2022 21:26:53 +0900 From: Hyeonggon Yoo <42.hyeyoo@gmail.com> To: Catalin Marinas Cc: Will Deacon , Marc Zyngier , Arnd Bergmann , Greg Kroah-Hartman , Andrew Morton , Linus Torvalds , linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 08/10] mm/slab: Allow dynamic kmalloc() minimum alignment Message-ID: References: <20220405135758.774016-1-catalin.marinas@arm.com> <20220405135758.774016-9-catalin.marinas@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Stat-Signature: tfyq51cxq6syhbdx4h5oegk4r9tf6cp5 X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 1F3FB180003 Authentication-Results: imf24.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b=Bqewqm4s; dmarc=pass (policy=none) header.from=gmail.com; spf=pass (imf24.hostedemail.com: domain of 42.hyeyoo@gmail.com designates 209.85.215.171 as permitted sender) smtp.mailfrom=42.hyeyoo@gmail.com X-Rspam-User: X-HE-Tag: 1649334422-740428 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, Apr 07, 2022 at 10:35:04AM +0100, Catalin Marinas wrote: > On Thu, Apr 07, 2022 at 06:18:16PM +0900, Hyeonggon Yoo wrote: > > On Thu, Apr 07, 2022 at 09:50:23AM +0100, Catalin Marinas wrote: > > > On Thu, Apr 07, 2022 at 03:46:37AM +0000, Hyeonggon Yoo wrote: > > > > On Tue, Apr 05, 2022 at 02:57:56PM +0100, Catalin Marinas wrote: > > > > > --- a/mm/slab_common.c > > > > > +++ b/mm/slab_common.c > > > > > @@ -838,9 +838,18 @@ void __init setup_kmalloc_cache_index_table(void) > > > > > } > > > > > } > > > > > > > > > > -static void __init > > > > > +unsigned int __weak arch_kmalloc_minalign(void) > > > > > +{ > > > > > + return ARCH_KMALLOC_MINALIGN; > > > > > +} > > > > > + > > > > > > > > As ARCH_KMALLOC_ALIGN and arch_kmalloc_minalign() may not be same after > > > > patch 10, I think s/ARCH_KMALLOC_ALIGN/arch_kmalloc_minalign/g > > > > for every user of it would be more correct? > > > > > > Not if the code currently using ARCH_KMALLOC_MINALIGN needs a constant. > > > Yes, there probably are a few places where the code can cope with a > > > dynamic arch_kmalloc_minalign() but there are two other cases where a > > > constant is needed: > > > > > > 1. As a BUILD_BUG check because the code is storing some flags in the > > > bottom bits of a pointer. A smaller ARCH_KMALLOC_MINALIGN works just > > > fine here. > > > > > > 2. As a static alignment for DMA requirements. That's where the newly > > > exposed ARCH_DMA_MINALIGN should be used. > > > > > > Note that this series doesn't make the situation any worse than before > > > since ARCH_DMA_MINALIGN stays at 128 bytes for arm64. Current users can > > > evolve to use a dynamic alignment in future patches. My main aim with > > > this series is to be able to create kmalloc-64 caches on arm64. > > > > AFAIK there are bunch of drivers that directly calls kmalloc(). > > Well, lots of drivers call kmalloc() ;). > > > It becomes tricky when e.g.) a driver allocates just 32 bytes, > > but architecture requires it to be 128-byte aligned. > > That's the current behaviour, a 32 byte allocation would return an > object from kmalloc-128. I want to reduce this to at least kmalloc-64 > (or smaller) if the CPU/SoC allows it. Yeah I agree the change is worth :) Thanks for the work. > > That's why everything allocated from kmalloc() need to be aligned in > > ARCH_DMA_MINALIGN. > > I don't get your conclusion here. Would you mind explaining? What I wanted to say was that, why ARCH_DMA_MINALIGN should be different from ARCH_KMALLOC_MINALIGN. I thought the two were basically same thing. Instead of decoupling them, I thought just decreasing them in runtime makes more sense. > > So I'm yet skeptical on decoupling ARCH_DMA/KMALLOC_MINALIGN. Instead > > of decoupling it, I'm more into dynamically decreasing it. > > The reason for decoupling is mostly that there are some static uses of > ARCH_KMALLOC_MINALIGN as per point 1 above. The other is the > __assume_kmalloc_alignment attribute. We shouldn't have such assumed > alignment larger than what a dynamic kmalloc() would return. To me it > makes a lot more sense for ARCH_KMALLOC_MINALIGN to be the minimum > guaranteed in a kernel build but kmalloc() returning a larger alignment > at run-time than the other way around. But yeah, considering the problems you mentioned, it seems unavoidable to decouple them. Thank you for explanation and I will review slab part soon. > Thanks. > > -- > Catalin -- Thanks, Hyeonggon