Re: [PATCH v6 1/4] mm: hugetlb_vmemmap: introduce STRUCT_PAGE_SIZE_IS_POWER_OF_2

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 30 Mar 2022 23:37:42 +0800 Muchun Song <songmuchun@bytedance.com> wrote:

> If the size of "struct page" is not the power of two and this
> feature is enabled,

What is "this feature"?   Let's spell it out?

> then the vmemmap pages of HugeTLB will be
> corrupted after remapping (panic is about to happen in theory).
> But this only exists when !CONFIG_MEMCG && !CONFIG_SLUB on
> x86_64.  However, it is not a conventional configuration nowadays.
> So it is not a real word issue, just the result of a code review.
> But we have to prevent anyone from configuring that combined
> configuration.  In order to avoid many checks like "is_power_of_2
> (sizeof(struct page))" through mm/hugetlb_vmemmap.c.  Introduce
> STRUCT_PAGE_SIZE_IS_POWER_OF_2 to detect if the size of struct
> page is power of 2 and make this feature depends on this new
> macro.  Then we could prevent anyone do any unexpected
> configuration.
> 
> ...
>
> --- /dev/null
> +++ b/mm/struct_page_size.c
> @@ -0,0 +1,20 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Generate definitions needed by the preprocessor.
> + * This code generates raw asm output which is post-processed
> + * to extract and format the required data.
> + */
> +
> +#define __GENERATING_STRUCT_PAGE_SIZE_IS_POWER_OF_2_H
> +/* Include headers that define the enum constants of interest */
> +#include <linux/mm_types.h>
> +#include <linux/kbuild.h>
> +#include <linux/log2.h>
> +
> +int main(void)
> +{
> +	if (is_power_of_2(sizeof(struct page)))
> +		DEFINE(STRUCT_PAGE_SIZE_IS_POWER_OF_2, is_power_of_2(sizeof(struct page)));

Why not

	DEFINE(STRUCT_PAGE_SIZE_IS_POWER_OF_2, 1);

?

> +	return 0;
> +}
> -- 
> 2.11.0