linux-kernel - Re: [PATCH v9 7/10] x86, mm, asm: Add WT support to set_page

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Date:	Fri, 22 May 2015 09:35:59 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Toshi Kani <toshi.kani@...com>
cc:	hpa@...or.com, mingo@...hat.com, akpm@...ux-foundation.org,
	arnd@...db.de, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	x86@...nel.org, linux-nvdimm@...1.01.org, jgross@...e.com,
	stefan.bader@...onical.com, luto@...capital.net, hmh@....eng.br,
	yigal@...xistor.com, konrad.wilk@...cle.com, Elliott@...com,
	mcgrof@...e.com, hch@....de
Subject: Re: [PATCH v9 7/10] x86, mm, asm: Add WT support to
 set_page_memtype()

On Wed, 13 May 2015, Toshi Kani wrote:
> + * X86 PAT uses page flags arch_1 and uncached together to keep track of
> + * memory type of pages that have backing page struct. X86 PAT supports 4
> + * different memory types, _PAGE_CACHE_MODE_WT, _PAGE_CACHE_MODE_WC,
> + * _PAGE_CACHE_MODE_UC_MINUS and _PAGE_CACHE_MODE_WB where page's memory
> + * type has not been changed from its default.

This is a horrible sentence.

 * X86 PAT supports 4 different memory types:
 *  - _PAGE_CACHE_MODE_WB
 *  - _PAGE_CACHE_MODE_WC
 *  - _PAGE_CACHE_MODE_UC_MINUS
 *  - _PAGE_CACHE_MODE_WT
 *
 * _PAGE_CACHE_MODE_WB is the default type.
 */
Hmm?

>   * Note we do not support _PAGE_CACHE_MODE_UC here.

This can be removed as it is completely redundant.

>   */
>  
> -#define _PGMT_DEFAULT		0
> +#define _PGMT_WB		0	/* default */

We just established two lines above that this is the default

>  #define _PGMT_WC		(1UL << PG_arch_1)
>  #define _PGMT_UC_MINUS		(1UL << PG_uncached)
> -#define _PGMT_WB		(1UL << PG_uncached | 1UL << PG_arch_1)
> +#define _PGMT_WT		(1UL << PG_uncached | 1UL << PG_arch_1)
>  #define _PGMT_MASK		(1UL << PG_uncached | 1UL << PG_arch_1)
>  #define _PGMT_CLEAR_MASK	(~_PGMT_MASK)
>  
> @@ -88,14 +88,14 @@ static inline enum page_cache_mode get_page_memtype(struct page *pg)
>  {
>  	unsigned long pg_flags = pg->flags & _PGMT_MASK;
>  
> -	if (pg_flags == _PGMT_DEFAULT)
> -		return -1;
> +	if (pg_flags == _PGMT_WB)
> +		return _PAGE_CACHE_MODE_WB;
>  	else if (pg_flags == _PGMT_WC)
>  		return _PAGE_CACHE_MODE_WC;
>  	else if (pg_flags == _PGMT_UC_MINUS)
>  		return _PAGE_CACHE_MODE_UC_MINUS;
>  	else
> -		return _PAGE_CACHE_MODE_WB;
> +		return _PAGE_CACHE_MODE_WT;
>  }
>  
>  static inline void set_page_memtype(struct page *pg,
> @@ -112,11 +112,12 @@ static inline void set_page_memtype(struct page *pg,
>  	case _PAGE_CACHE_MODE_UC_MINUS:
>  		memtype_flags = _PGMT_UC_MINUS;
>  		break;
> -	case _PAGE_CACHE_MODE_WB:
> -		memtype_flags = _PGMT_WB;
> +	case _PAGE_CACHE_MODE_WT:
> +		memtype_flags = _PGMT_WT;
>  		break;
> +	case _PAGE_CACHE_MODE_WB:
>  	default:
> -		memtype_flags = _PGMT_DEFAULT;
> +		memtype_flags = _PGMT_WB;	/* default */

What's the value of that  comment?

       default:
		 /* default */

Aside of the, please do not use tail comments. They make code harder
to parse.

>  /*
>   * For RAM pages, we use page flags to mark the pages with appropriate type.
> - * The page flags are limited to three types, WB, WC and UC-.
> - * WT and WP requests fail with -EINVAL, and UC gets redirected to UC-.
> + * The page flags are limited to four types, WB (default), WC, WT and UC-.
> + * WP request fails with -EINVAL, and UC gets redirected to UC-.

> + * A new memtype can only be set to the default memtype WB.

I have no idea what that line means.

> @@ -582,13 +583,6 @@ static enum page_cache_mode lookup_memtype(u64 paddr)
>  		struct page *page;
>  		page = pfn_to_page(paddr >> PAGE_SHIFT);
>  		rettype = get_page_memtype(page);

		return get_page_memtype(page);

And while you are at it please add the missing newline between the
variable declaration and code.

Thanks,

	tglx
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