linux-kernel - Re: [PATCH 5/5] RISC-V: Implement sparsemem

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Date:   Thu, 11 Oct 2018 15:37:30 +0200
From:   Christoph Hellwig <hch@....de>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-riscv@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-sh@...r.kernel.org,
        Stephen Bates <sbates@...thlin.com>,
        Palmer Dabbelt <palmer@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Christoph Hellwig <hch@....de>,
        Andrew Waterman <andrew@...ive.com>,
        Olof Johansson <olof@...om.net>,
        Michael Clark <michaeljclark@....com>,
        Rob Herring <robh@...nel.org>, Zong Li <zong@...estech.com>
Subject: Re: [PATCH 5/5] RISC-V: Implement sparsemem

> +/*
> + * Log2 of the upper bound of the size of a struct page. Used for sizing
> + * the vmemmap region only, does not affect actual memory footprint.
> + * We don't use sizeof(struct page) directly since taking its size here
> + * requires its definition to be available at this point in the inclusion
> + * chain, and it may not be a power of 2 in the first place.
> + */
> +#define STRUCT_PAGE_MAX_SHIFT	6

I know this is copied from arm64, but wouldn't this be a good time
to move this next to the struct page defintion?

Also this:

arch/arm64/mm/init.c:   BUILD_BUG_ON(sizeof(struct page) > (1 << STRUCT_PAGE_MAX_SHIFT));

should move to comment code (or would have to be duplicated for riscv)

> +#define VMEMMAP_SIZE	(UL(1) << (CONFIG_VA_BITS - PAGE_SHIFT - 1 + \
> +				   STRUCT_PAGE_MAX_SHIFT))

Might be more readable with a another define, and without abuse of the
horrible UL macro:

#define VMEMMAP_SHIFT \
	(CONFIG_VA_BITS - PAGE_SHIFT - 1 + STRUCT_PAGE_MAX_SHIFT)
#define VMEMMAP_SIZE	(1UL << VMEMMAP_SHIFT)

> +#define VMEMMAP_END	(VMALLOC_START - 1)
> +#define VMEMMAP_START	(VMALLOC_START - VMEMMAP_SIZE)
> +
> +#define vmemmap		((struct page *)VMEMMAP_START)

This could also use some comments..

> @@ -0,0 +1,11 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef __ASM_SPARSEMEM_H
> +#define __ASM_SPARSEMEM_H
> +
> +#ifdef CONFIG_SPARSEMEM
> +#define MAX_PHYSMEM_BITS	CONFIG_PA_BITS
> +#define SECTION_SIZE_BITS	30
> +#endif
> +
> +#endif

For potentially wide-spanning ifdefs like inclusion headers it always
is nice to have a comment with the symbol on the endif line.