Re: [PATCH 4/6] mm/page_alloc: Explicitly define what alloc flags deplete min reserves

[Vlastimil Babka]

On 11/29/22 16:16, Mel Gorman wrote:
> As there are more ALLOC_ flags that affect reserves, define what flags
> affect reserves and clarify the effect of each flag.

Seems to me this does more than a clarification, but also some functional
tweaks, so it could be helpful if those were spelled out in the changelog.

> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> ---
>  mm/internal.h   |  3 +++
>  mm/page_alloc.c | 36 +++++++++++++++++++++++-------------
>  2 files changed, 26 insertions(+), 13 deletions(-)
> 
> diff --git a/mm/internal.h b/mm/internal.h
> index 9a9d9b5ee87f..370500718732 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -757,6 +757,9 @@ 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 e2b65767dda0..85a87d0ac57a 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3944,15 +3944,14 @@ 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
> @@ -3976,25 +3975,36 @@ 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 (alloc_flags & ALLOC_RESERVES) {

Do we want to keep this unlikely() as alloc_harder did before?

> +		/*
> +		 * __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|ALLOC_HIGHATOMIC))
> +			min -= min / 4;

For example this seems to change the allowed dip to reserves for
ALLOC_HIGHATOMIC.

> +
> +		/*
> +		 * 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;
>  	}

(noted that this patch doesn't seem to change the concern I raised in
previous patch)

>  	/*
> @@ -5293,7 +5303,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>  		 * could deplete whole memory reserves which would just make
>  		 * 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|ALLOC_HARDER, ac);

And this AFAICS seems to give __GFP_NOFAIL 3/4 of min reserves instead of
1/4, which seems like a significant change (but hopefully ok) so worth
noting at least.

>  		if (page)
>  			goto got_pg;
>