Re: [PATCH 3/7] mm/page_alloc: Explicitly record high-order atomic allocations in alloc_flags

Michal Hocko
2023-01-11T15:36:15Z

On Mon 09-01-23 15:16:27, Mel Gorman wrote:
> A high-order ALLOC_HARDER allocation is assumed to be atomic. While that
> is accurate, it changes later in the series. In preparation, explicitly
> record high-order atomic allocations in gfp_to_alloc_flags(). There is
> a slight functional change in that OOM handling avoids using high-order
> reserve until it has to.

I do not follow the oom handling part. IIRC we are dropping highatomic
reserves before triggering oom. Something might have changed down the
path but I can still see unreserve_highatomic_pageblock in
should_reclaim_retry.

I do not have any objection to ALLOC_HIGHATOMIC itself, though.

> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
> ---
> mm/internal.h | 1 +
> mm/page_alloc.c | 29 +++++++++++++++++++++++------
> 2 files changed, 24 insertions(+), 6 deletions(-)
>
> diff --git a/mm/internal.h b/mm/internal.h
> index 403e4386626d..178484d9fd94 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -746,6 +746,7 @@ unsigned int reclaim_clean_pages_from_list(struct zone *zone,
> #else
> #define ALLOC_NOFRAGMENT 0x0
> #endif
> +#define ALLOC_HIGHATOMIC 0x200 /* Allows access to MIGRATE_HIGHATOMIC */
> #define ALLOC_KSWAPD 0x800 /* allow waking of kswapd, __GFP_KSWAPD_RECLAIM set */
>
> enum ttu_flags;
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 0040b4e00913..0ef4f3236a5a 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3706,10 +3706,20 @@ struct page *rmqueue_buddy(struct zone *preferred_zone, struct zone *zone,
> * reserved for high-order atomic allocation, so order-0
> * request should skip it.
> */
> - if (order > 0 && alloc_flags & ALLOC_HARDER)
> + if (alloc_flags & ALLOC_HIGHATOMIC)
> page = __rmqueue_smallest(zone, order, MIGRATE_HIGHATOMIC);
> if (!page) {
> page = __rmqueue(zone, order, migratetype, alloc_flags);
> +
> + /*
> + * If the allocation fails, allow OOM handling access
> + * to HIGHATOMIC reserves as failing now is worse than
> + * failing a high-order atomic allocation in the
> + * future.
> + */
> + if (!page && (alloc_flags & ALLOC_OOM))
> + page = __rmqueue_smallest(zone, order, MIGRATE_HIGHATOMIC);
> +
> if (!page) {
> spin_unlock_irqrestore(&zone->lock, flags);
> return NULL;
> @@ -4023,8 +4033,10 @@ bool __zone_watermark_ok(struct zone *z, unsigned int order, unsigned long mark,
> return true;
> }
> #endif
> - if (alloc_harder && !free_area_empty(area, MIGRATE_HIGHATOMIC))
> + if ((alloc_flags & (ALLOC_HIGHATOMIC|ALLOC_OOM)) &&
> + !free_area_empty(area, MIGRATE_HIGHATOMIC)) {
> return true;
> + }
> }
> return false;
> }
> @@ -4286,7 +4298,7 @@ get_page_from_freelist(gfp_t gfp_mask, unsigned int order, int alloc_flags,
> * If this is a high-order atomic allocation then check
> * if the pageblock should be reserved for the future
> */
> - if (unlikely(order && (alloc_flags & ALLOC_HARDER)))
> + if (unlikely(alloc_flags & ALLOC_HIGHATOMIC))
> reserve_highatomic_pageblock(page, zone, order);
>
> return page;
> @@ -4813,7 +4825,7 @@ static void wake_all_kswapds(unsigned int order, gfp_t gfp_mask,
> }
>
> static inline unsigned int
> -gfp_to_alloc_flags(gfp_t gfp_mask)
> +gfp_to_alloc_flags(gfp_t gfp_mask, unsigned int order)
> {
> unsigned int alloc_flags = ALLOC_WMARK_MIN | ALLOC_CPUSET;
>
> @@ -4839,8 +4851,13 @@ gfp_to_alloc_flags(gfp_t gfp_mask)
> * Not worth trying to allocate harder for __GFP_NOMEMALLOC even
> * if it can't schedule.
> */
> - if (!(gfp_mask & __GFP_NOMEMALLOC))
> + if (!(gfp_mask & __GFP_NOMEMALLOC)) {
> alloc_flags |= ALLOC_HARDER;
> +
> + if (order > 0)
> + alloc_flags |= ALLOC_HIGHATOMIC;
> + }
> +
> /*
> * Ignore cpuset mems for GFP_ATOMIC rather than fail, see the
> * comment for __cpuset_node_allowed().
> @@ -5048,7 +5065,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> * kswapd needs to be woken up, and to avoid the cost of setting up
> * alloc_flags precisely. So we do that now.
> */
> - alloc_flags = gfp_to_alloc_flags(gfp_mask);
> + alloc_flags = gfp_to_alloc_flags(gfp_mask, order);
>
> /*
> * We need to recalculate the starting point for the zonelist iterator
> --
> 2.35.3

--
Michal Hocko
SUSE Labs