linux-kernel - Re: [PATCH RFC 1/3] x86: Make page cache mode a real type

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Date:	Thu, 21 Aug 2014 16:09:21 -0600
From:	Toshi Kani <toshi.kani@...com>
To:	jgross@...e.com
Cc:	stefan.bader@...onical.com, linux-kernel@...r.kernel.org,
	xen-devel@...ts.xensource.com, konrad.wilk@...cle.com,
	ville.syrjala@...ux.intel.com, hpa@...or.com, x86@...nel.org
Subject: Re: [PATCH RFC 1/3] x86: Make page cache mode a real type

On Tue, 2014-08-19 at 15:25 +0200, jgross@...e.com wrote:
> From: Juergen Gross <jgross@...e.com>
> 
> At the moment there are a lot of places that handle setting or getting
> the page cache mode by treating the pgprot bits equal to the cache mode.
> This is only true because there are a lot of assumptions about the setup
> of the PAT MSR. Otherwise the cache type needs to get translated into
> pgprot bits and vice versa.
> 
> This patch tries to prepare for that by introducing a seperate type
> for the cache mode and adding functions to translate between those and pgprot
> values.
> 
> To avoid too much performance penalty the translation between cache mode
> and pgprot values is done via tables which contain the relevant information.
> Write-back cache mode is hard-wired to be 0, all other modes are configurable
> via those tables. For large pages there are translation functions as the
> PAT bit is located at different positions in the ptes of 4k and large pages.

One more comment below..

> diff --git a/arch/x86/include/asm/cacheflush.h b/arch/x86/include/asm/cacheflush.h
 :
> -static inline void set_page_memtype(struct page *pg, unsigned long memtype)
> +static inline void set_page_memtype(struct page *pg,
> +				    enum page_cache_mode memtype)
>  {
>  	unsigned long memtype_flags = _PGMT_DEFAULT;
>  	unsigned long old_flags;
>  	unsigned long new_flags;
>  
>  	switch (memtype) {
> -	case _PAGE_CACHE_WC:
> +	case _PAGE_CACHE_MODE_WC:
>  		memtype_flags = _PGMT_WC;
>  		break;
> -	case _PAGE_CACHE_UC_MINUS:
> +	case _PAGE_CACHE_MODE_UC_MINUS:
>  		memtype_flags = _PGMT_UC_MINUS;
>  		break;
> -	case _PAGE_CACHE_WB:
> +	case _PAGE_CACHE_MODE_WB:
> +	default:
>  		memtype_flags = _PGMT_WB;
>  		break;
>  	}

Adding the "default" case handled as _PGMT_WB is not correct here.
free_ram_pages_type() calls set_page_memtype() with -1, which needs to
be set to _PGMT_DEFAULT.

Thanks,
-Toshi




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