Re: [PATCH 4/7] mm/page_alloc: Explicitly define what alloc flags deplete min reserves

[Michal Hocko]

On Mon 09-01-23 15:16:28, Mel Gorman wrote:
> As there are more ALLOC_ flags that affect reserves, define what flags
> affect reserves and clarify the effect of each flag.

I like this! It makes the code much easier to follow.

> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

Thanks!

> ---
>  mm/internal.h   |  3 +++
>  mm/page_alloc.c | 34 ++++++++++++++++++++++------------
>  2 files changed, 25 insertions(+), 12 deletions(-)
> 
> diff --git a/mm/internal.h b/mm/internal.h
> index 178484d9fd94..8706d46863df 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -749,6 +749,9 @@ unsigned int reclaim_clean_pages_from_list(struct zone *zone,
>  #define ALLOC_HIGHATOMIC	0x200 /* Allows access to MIGRATE_HIGHATOMIC */
>  #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)
> +
>  enum ttu_flags;
>  struct tlbflush_unmap_batch;
>  
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 0ef4f3236a5a..6f41b84a97ac 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3949,15 +3949,14 @@ ALLOW_ERROR_INJECTION(should_fail_alloc_page, TRUE);
>  static inline long __zone_watermark_unusable_free(struct zone *z,
>  				unsigned int order, unsigned int alloc_flags)
>  {
> -	const bool alloc_harder = (alloc_flags & (ALLOC_HARDER|ALLOC_OOM));
>  	long unusable_free = (1 << order) - 1;
>  
>  	/*
> -	 * If the caller does not have rights to ALLOC_HARDER then subtract
> -	 * the high-atomic reserves. This will over-estimate the size of the
> -	 * atomic reserve but it avoids a search.
> +	 * If the caller does not have rights to reserves below the min
> +	 * watermark then subtract the high-atomic reserves. This will
> +	 * over-estimate the size of the atomic reserve but it avoids a search.
>  	 */
> -	if (likely(!alloc_harder))
> +	if (likely(!(alloc_flags & ALLOC_RESERVES)))
>  		unusable_free += z->nr_reserved_highatomic;
>  
>  #ifdef CONFIG_CMA
> @@ -3981,25 +3980,36 @@ bool __zone_watermark_ok(struct zone *z, unsigned int order, unsigned long mark,
>  {
>  	long min = mark;
>  	int o;
> -	const bool alloc_harder = (alloc_flags & (ALLOC_HARDER|ALLOC_OOM));
>  
>  	/* free_pages may go negative - that's OK */
>  	free_pages -= __zone_watermark_unusable_free(z, order, alloc_flags);
>  
> -	if (alloc_flags & ALLOC_MIN_RESERVE)
> -		min -= min / 2;
> +	if (unlikely(alloc_flags & ALLOC_RESERVES)) {
> +		/*
> +		 * __GFP_HIGH allows access to 50% of the min reserve as well
> +		 * as OOM.
> +		 */
> +		if (alloc_flags & ALLOC_MIN_RESERVE)
> +			min -= min / 2;
>  
> -	if (unlikely(alloc_harder)) {
>  		/*
> -		 * OOM victims can try even harder than normal ALLOC_HARDER
> +		 * 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;
> +
> +		/*
> +		 * OOM victims can try even harder than the normal reserve
>  		 * users on the grounds that it's definitely going to be in
>  		 * the exit path shortly and free memory. Any allocation it
>  		 * makes during the free path will be small and short-lived.
>  		 */
>  		if (alloc_flags & ALLOC_OOM)
>  			min -= min / 2;
> -		else
> -			min -= min / 4;
>  	}
>  
>  	/*
> -- 
> 2.35.3

-- 
Michal Hocko
SUSE Labs