linux-kernel - Re: [PATCH 6/8] mm/highmem: make kmap cache coloring aware

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Date:	Tue, 22 Jul 2014 12:35:15 -0700
From:	Leonid Yegoshin <Leonid.Yegoshin@...tec.com>
To:	Max Filippov <jcmvbkbc@...il.com>
CC:	<linux-xtensa@...ux-xtensa.org>, Chris Zankel <chris@...kel.net>,
	"Marc Gauthier" <marc@...ence.com>, <linux-kernel@...r.kernel.org>,
	<linux-mm@...ck.org>, <linux-arch@...r.kernel.org>,
	<linux-mips@...ux-mips.org>, David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH 6/8] mm/highmem: make kmap cache coloring aware

On 07/22/2014 12:01 PM, Max Filippov wrote:
> From: Leonid Yegoshin <Leonid.Yegoshin@...tec.com>
>
> Provide hooks that allow architectures with aliasing cache to align
> mapping address of high pages according to their color. Such architectures
> may enforce similar coloring of low- and high-memory page mappings and
> reuse existing cache management functions to support highmem.
>
> Cc: linux-mm@...ck.org
> Cc: linux-arch@...r.kernel.org
> Cc: linux-mips@...ux-mips.org
> Cc: David Rientjes <rientjes@...gle.com>
> Signed-off-by: Leonid Yegoshin <Leonid.Yegoshin@...tec.com>
> [ Max: extract architecture-independent part of the original patch, clean
>    up checkpatch and build warnings. ]
> Signed-off-by: Max Filippov <jcmvbkbc@...il.com>
> ---
> Changes since the initial version:
> - define set_pkmap_color(pg, cl) as do { } while (0) instead of /* */;
> - rename is_no_more_pkmaps to no_more_pkmaps;
> - change 'if (count > 0)' to 'if (count)' to better match the original
>    code behavior;
>
>   mm/highmem.c | 19 ++++++++++++++++---
>   1 file changed, 16 insertions(+), 3 deletions(-)
>
> diff --git a/mm/highmem.c b/mm/highmem.c
> index b32b70c..88fb62e 100644
> --- a/mm/highmem.c
> +++ b/mm/highmem.c
> @@ -44,6 +44,14 @@ DEFINE_PER_CPU(int, __kmap_atomic_idx);
>    */
>   #ifdef CONFIG_HIGHMEM
>   
> +#ifndef ARCH_PKMAP_COLORING
> +#define set_pkmap_color(pg, cl)		do { } while (0)
> +#define get_last_pkmap_nr(p, cl)	(p)
> +#define get_next_pkmap_nr(p, cl)	(((p) + 1) & LAST_PKMAP_MASK)
> +#define no_more_pkmaps(p, cl)		(!(p))
> +#define get_next_pkmap_counter(c, cl)	((c) - 1)
> +#endif
> +
>   unsigned long totalhigh_pages __read_mostly;
>   EXPORT_SYMBOL(totalhigh_pages);
>   
> @@ -161,19 +169,24 @@ static inline unsigned long map_new_virtual(struct page *page)
>   {
>   	unsigned long vaddr;
>   	int count;
> +	int color __maybe_unused;
> +
> +	set_pkmap_color(page, color);
> +	last_pkmap_nr = get_last_pkmap_nr(last_pkmap_nr, color);
>   
>   start:
>   	count = LAST_PKMAP;
>   	/* Find an empty entry */
>   	for (;;) {
> -		last_pkmap_nr = (last_pkmap_nr + 1) & LAST_PKMAP_MASK;
> -		if (!last_pkmap_nr) {
> +		last_pkmap_nr = get_next_pkmap_nr(last_pkmap_nr, color);
> +		if (no_more_pkmaps(last_pkmap_nr, color)) {
>   			flush_all_zero_pkmaps();
>   			count = LAST_PKMAP;
>   		}
>   		if (!pkmap_count[last_pkmap_nr])
>   			break;	/* Found a usable entry */
> -		if (--count)
> +		count = get_next_pkmap_counter(count, color);
> +		if (count)
>   			continue;
>   
>   		/*
I would like to return back to "if (count >0)".

The reason is in easy way to jump through the same coloured pages - next 
element is calculated via decrementing by non "1" value of colours and 
it can easy become negative on last page available:

#define     get_next_pkmap_counter(c,cl)    (c - FIX_N_COLOURS)

where FIX_N_COLOURS is a max number of page colours.

Besides that it is a good practice in stopping cycle.

- Leonid.

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