linux-kernel - Re: [PATCH 1/7] mm/page_alloc: Rename ALLOC_HIGH to ALLOC_MIN

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Message-ID: <Y77TWhr9/dPcthiF@dhcp22.suse.cz>
Date:   Wed, 11 Jan 2023 16:18:50 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Linux-MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        NeilBrown <neilb@...e.de>,
        Thierry Reding <thierry.reding@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/7] mm/page_alloc: Rename ALLOC_HIGH to ALLOC_MIN_RESERVE

On Mon 09-01-23 15:16:25, Mel Gorman wrote:
> __GFP_HIGH aliases to ALLOC_HIGH but the name does not really hint
> what it means. As ALLOC_HIGH is internal to the allocator, rename
> it to ALLOC_MIN_RESERVE to document that the min reserves can
> be depleted.
> 
> Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
> Acked-by: Vlastimil Babka <vbabka@...e.cz>

Naming is hard but ALLOC_HIGH is definitely much more confusing as it
can collide with high watermark. ALLOC_MIN_RESERVE says that some
reserves are involved. ALl the reserves are below min watermark by
defition but I cannot really come up with a better name. I do not think
we want to encode the amount of reserves into the name.

Acked-by: Michal Hocko <mhocko@...e.com>

Thanks!
> ---
>  mm/internal.h   | 4 +++-
>  mm/page_alloc.c | 8 ++++----
>  2 files changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/internal.h b/mm/internal.h
> index bcf75a8b032d..403e4386626d 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -736,7 +736,9 @@ unsigned int reclaim_clean_pages_from_list(struct zone *zone,
>  #endif
>  
>  #define ALLOC_HARDER		 0x10 /* try to alloc harder */
> -#define ALLOC_HIGH		 0x20 /* __GFP_HIGH set */
> +#define ALLOC_MIN_RESERVE	 0x20 /* __GFP_HIGH set. Allow access to 50%
> +				       * of the min watermark.
> +				       */
>  #define ALLOC_CPUSET		 0x40 /* check for correct cpuset */
>  #define ALLOC_CMA		 0x80 /* allow allocations from CMA areas */
>  #ifdef CONFIG_ZONE_DMA32
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 0745aedebb37..244c1e675dc8 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3976,7 +3976,7 @@ bool __zone_watermark_ok(struct zone *z, unsigned int order, unsigned long mark,
>  	/* free_pages may go negative - that's OK */
>  	free_pages -= __zone_watermark_unusable_free(z, order, alloc_flags);
>  
> -	if (alloc_flags & ALLOC_HIGH)
> +	if (alloc_flags & ALLOC_MIN_RESERVE)
>  		min -= min / 2;
>  
>  	if (unlikely(alloc_harder)) {
> @@ -4818,18 +4818,18 @@ gfp_to_alloc_flags(gfp_t gfp_mask)
>  	unsigned int alloc_flags = ALLOC_WMARK_MIN | ALLOC_CPUSET;
>  
>  	/*
> -	 * __GFP_HIGH is assumed to be the same as ALLOC_HIGH
> +	 * __GFP_HIGH is assumed to be the same as ALLOC_MIN_RESERVE
>  	 * and __GFP_KSWAPD_RECLAIM is assumed to be the same as ALLOC_KSWAPD
>  	 * to save two branches.
>  	 */
> -	BUILD_BUG_ON(__GFP_HIGH != (__force gfp_t) ALLOC_HIGH);
> +	BUILD_BUG_ON(__GFP_HIGH != (__force gfp_t) ALLOC_MIN_RESERVE);
>  	BUILD_BUG_ON(__GFP_KSWAPD_RECLAIM != (__force gfp_t) ALLOC_KSWAPD);
>  
>  	/*
>  	 * The caller may dip into page reserves a bit more if the caller
>  	 * cannot run direct reclaim, or if the caller has realtime scheduling
>  	 * policy or is asking for __GFP_HIGH memory.  GFP_ATOMIC requests will
> -	 * set both ALLOC_HARDER (__GFP_ATOMIC) and ALLOC_HIGH (__GFP_HIGH).
> +	 * set both ALLOC_HARDER (__GFP_ATOMIC) and ALLOC_MIN_RESERVE(__GFP_HIGH).
>  	 */
>  	alloc_flags |= (__force int)
>  		(gfp_mask & (__GFP_HIGH | __GFP_KSWAPD_RECLAIM));
> -- 
> 2.35.3

-- 
Michal Hocko
SUSE Labs