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Date:   Fri, 13 Jan 2023 14:06:11 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Mel Gorman <mgorman@techsingularity.net>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        NeilBrown <neilb@suse.de>,
        Thierry Reding <thierry.reding@gmail.com>,
        Matthew Wilcox <willy@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Linux-MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 5/6] mm/page_alloc: Explicitly define how __GFP_HIGH
 non-blocking allocations accesses reserves
Message-ID: <Y8FXQ4XrSuoPux/7@dhcp22.suse.cz>
References: <20230113111217.14134-1-mgorman@techsingularity.net>
 <20230113111217.14134-6-mgorman@techsingularity.net>
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On Fri 13-01-23 11:12:16, Mel Gorman wrote:
> GFP_ATOMIC allocations get flagged ALLOC_HARDER which is a vague
> description. In preparation for the removal of GFP_ATOMIC redefine
> __GFP_ATOMIC to simply mean non-blocking and renaming ALLOC_HARDER to
> ALLOC_NON_BLOCK accordingly. __GFP_HIGH is required for access to reserves
> but non-blocking is granted more access. For example, GFP_NOWAIT is
> non-blocking but has no special access to reserves. A __GFP_NOFAIL
> blocking allocation is granted access similar to __GFP_HIGH if the
> only alternative is an OOM kill.
> 
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>

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

Thanks!

> ---
>  mm/internal.h   |  7 +++++--
>  mm/page_alloc.c | 44 ++++++++++++++++++++++++--------------------
>  2 files changed, 29 insertions(+), 22 deletions(-)
> 
> diff --git a/mm/internal.h b/mm/internal.h
> index 8706d46863df..23a37588073a 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -735,7 +735,10 @@ unsigned int reclaim_clean_pages_from_list(struct zone *zone,
>  #define ALLOC_OOM		ALLOC_NO_WATERMARKS
>  #endif
>  
> -#define ALLOC_HARDER		 0x10 /* try to alloc harder */
> +#define ALLOC_NON_BLOCK		 0x10 /* Caller cannot block. Allow access
> +				       * to 25% of the min watermark or
> +				       * 62.5% if __GFP_HIGH is set.
> +				       */
>  #define ALLOC_MIN_RESERVE	 0x20 /* __GFP_HIGH set. Allow access to 50%
>  				       * of the min watermark.
>  				       */
> @@ -750,7 +753,7 @@ unsigned int reclaim_clean_pages_from_list(struct zone *zone,
>  #define ALLOC_KSWAPD		0x800 /* allow waking of kswapd, __GFP_KSWAPD_RECLAIM set */
>  
>  /* Flags that allow allocations below the min watermark. */
> -#define ALLOC_RESERVES (ALLOC_HARDER|ALLOC_MIN_RESERVE|ALLOC_HIGHATOMIC|ALLOC_OOM)
> +#define ALLOC_RESERVES (ALLOC_NON_BLOCK|ALLOC_MIN_RESERVE|ALLOC_HIGHATOMIC|ALLOC_OOM)
>  
>  enum ttu_flags;
>  struct tlbflush_unmap_batch;
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 6f41b84a97ac..b9ae0ba0a2ab 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3989,18 +3989,19 @@ bool __zone_watermark_ok(struct zone *z, unsigned int order, unsigned long mark,
>  		 * __GFP_HIGH allows access to 50% of the min reserve as well
>  		 * as OOM.
>  		 */
> -		if (alloc_flags & ALLOC_MIN_RESERVE)
> +		if (alloc_flags & ALLOC_MIN_RESERVE) {
>  			min -= min / 2;
>  
> -		/*
> -		 * Non-blocking allocations can access some of the reserve
> -		 * with more access if also __GFP_HIGH. The reasoning is that
> -		 * a non-blocking caller may incur a more severe penalty
> -		 * if it cannot get memory quickly, particularly if it's
> -		 * also __GFP_HIGH.
> -		 */
> -		if (alloc_flags & ALLOC_HARDER)
> -			min -= min / 4;
> +			/*
> +			 * Non-blocking allocations (e.g. GFP_ATOMIC) can
> +			 * access more reserves than just __GFP_HIGH. Other
> +			 * non-blocking allocations requests such as GFP_NOWAIT
> +			 * or (GFP_KERNEL & ~__GFP_DIRECT_RECLAIM) do not get
> +			 * access to the min reserve.
> +			 */
> +			if (alloc_flags & ALLOC_NON_BLOCK)
> +				min -= min / 4;
> +		}
>  
>  		/*
>  		 * OOM victims can try even harder than the normal reserve
> @@ -4851,28 +4852,30 @@ gfp_to_alloc_flags(gfp_t gfp_mask, unsigned int order)
>  	 * 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_MIN_RESERVE(__GFP_HIGH).
> +	 * set both ALLOC_NON_BLOCK and ALLOC_MIN_RESERVE(__GFP_HIGH).
>  	 */
>  	alloc_flags |= (__force int)
>  		(gfp_mask & (__GFP_HIGH | __GFP_KSWAPD_RECLAIM));
>  
> -	if (gfp_mask & __GFP_ATOMIC) {
> +	if (!(gfp_mask & __GFP_DIRECT_RECLAIM)) {
>  		/*
>  		 * Not worth trying to allocate harder for __GFP_NOMEMALLOC even
>  		 * if it can't schedule.
>  		 */
>  		if (!(gfp_mask & __GFP_NOMEMALLOC)) {
> -			alloc_flags |= ALLOC_HARDER;
> +			alloc_flags |= ALLOC_NON_BLOCK;
>  
>  			if (order > 0)
>  				alloc_flags |= ALLOC_HIGHATOMIC;
>  		}
>  
>  		/*
> -		 * Ignore cpuset mems for GFP_ATOMIC rather than fail, see the
> -		 * comment for __cpuset_node_allowed().
> +		 * Ignore cpuset mems for non-blocking __GFP_HIGH (probably
> +		 * GFP_ATOMIC) rather than fail, see the comment for
> +		 * __cpuset_node_allowed().
>  		 */
> -		alloc_flags &= ~ALLOC_CPUSET;
> +		if (alloc_flags & ALLOC_MIN_RESERVE)
> +			alloc_flags &= ~ALLOC_CPUSET;
>  	} else if (unlikely(rt_task(current)) && in_task())
>  		alloc_flags |= ALLOC_MIN_RESERVE;
>  
> @@ -5303,12 +5306,13 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>  		WARN_ON_ONCE_GFP(costly_order, gfp_mask);
>  
>  		/*
> -		 * Help non-failing allocations by giving them access to memory
> -		 * reserves but do not use ALLOC_NO_WATERMARKS because this
> +		 * Help non-failing allocations by giving some access to memory
> +		 * reserves normally used for high priority non-blocking
> +		 * allocations but do not use ALLOC_NO_WATERMARKS because this
>  		 * could deplete whole memory reserves which would just make
> -		 * the situation worse
> +		 * the situation worse.
>  		 */
> -		page = __alloc_pages_cpuset_fallback(gfp_mask, order, ALLOC_HARDER, ac);
> +		page = __alloc_pages_cpuset_fallback(gfp_mask, order, ALLOC_MIN_RESERVE, ac);
>  		if (page)
>  			goto got_pg;
>  
> -- 
> 2.35.3

-- 
Michal Hocko
SUSE Labs