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

Michal Hocko
2023-01-11T15:38:06Z

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@techsingularity.net>

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

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