Re: [PATCH v1 4/5] mm/page_alloc: place pages to tail in __free_pages_core()

[Michal Hocko]

On Mon 28-09-20 20:21:09, David Hildenbrand wrote:
> __free_pages_core() is used when exposing fresh memory to the buddy
> during system boot and when onlining memory in generic_online_page().
> 
> generic_online_page() is used in two cases:
> 
> 1. Direct memory onlining in online_pages().
> 2. Deferred memory onlining in memory-ballooning-like mechanisms (HyperV
>    balloon and virtio-mem), when parts of a section are kept
>    fake-offline to be fake-onlined later on.
> 
> In 1, we already place pages to the tail of the freelist. Pages will be
> freed to MIGRATE_ISOLATE lists first and moved to the tail of the freelists
> via undo_isolate_page_range().
> 
> In 2, we currently don't implement a proper rule. In case of virtio-mem,
> where we currently always online MAX_ORDER - 1 pages, the pages will be
> placed to the HEAD of the freelist - undesireable. While the hyper-v
> balloon calls generic_online_page() with single pages, usually it will
> call it on successive single pages in a larger block.
> 
> The pages are fresh, so place them to the tail of the freelists and avoid
> the PCP. In __free_pages_core(), remove the now superflouos call to
> set_page_refcounted() and add a comment regarding page initialization and
> the refcount.
> 
> Note: In 2. we currently don't shuffle. If ever relevant (page shuffling
> is usually of limited use in virtualized environments), we might want to
> shuffle after a sequence of generic_online_page() calls in the
> relevant callers.

It took some time to get through all the freeing paths with subtle
differences but this looks reasonable. You are mentioning that this
influences a boot time free memory ordering as well but only very
briefly. I do not expect this to make a huge difference but who knows.
It makes some sense to add pages in the order they show up in the
physical address ordering.

> Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
> Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Wei Yang <richard.weiyang@xxxxxxxxxxxxxxxxx>
> Cc: Oscar Salvador <osalvador@xxxxxxx>
> Cc: Mike Rapoport <rppt@xxxxxxxxxx>
> Cc: "K. Y. Srinivasan" <kys@xxxxxxxxxxxxx>
> Cc: Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>
> Cc: Stephen Hemminger <sthemmin@xxxxxxxxxxxxx>
> Cc: Wei Liu <wei.liu@xxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

That being said I do not see any fundamental problems.
Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
>  mm/page_alloc.c | 37 ++++++++++++++++++++++++-------------
>  1 file changed, 24 insertions(+), 13 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index d5a5f528b8ca..8a2134fe9947 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -270,7 +270,8 @@ bool pm_suspended_storage(void)
>  unsigned int pageblock_order __read_mostly;
>  #endif
>  
> -static void __free_pages_ok(struct page *page, unsigned int order);
> +static void __free_pages_ok(struct page *page, unsigned int order,
> +			    fop_t fop_flags);
>  
>  /*
>   * results with 256, 32 in the lowmem_reserve sysctl:
> @@ -682,7 +683,7 @@ static void bad_page(struct page *page, const char *reason)
>  void free_compound_page(struct page *page)
>  {
>  	mem_cgroup_uncharge(page);
> -	__free_pages_ok(page, compound_order(page));
> +	__free_pages_ok(page, compound_order(page), FOP_NONE);
>  }
>  
>  void prep_compound_page(struct page *page, unsigned int order)
> @@ -1419,17 +1420,15 @@ static void free_pcppages_bulk(struct zone *zone, int count,
>  	spin_unlock(&zone->lock);
>  }
>  
> -static void free_one_page(struct zone *zone,
> -				struct page *page, unsigned long pfn,
> -				unsigned int order,
> -				int migratetype)
> +static void free_one_page(struct zone *zone, struct page *page, unsigned long pfn,
> +			  unsigned int order, int migratetype, fop_t fop_flags)
>  {
>  	spin_lock(&zone->lock);
>  	if (unlikely(has_isolate_pageblock(zone) ||
>  		is_migrate_isolate(migratetype))) {
>  		migratetype = get_pfnblock_migratetype(page, pfn);
>  	}
> -	__free_one_page(page, pfn, zone, order, migratetype, FOP_NONE);
> +	__free_one_page(page, pfn, zone, order, migratetype, fop_flags);
>  	spin_unlock(&zone->lock);
>  }
>  
> @@ -1507,7 +1506,8 @@ void __meminit reserve_bootmem_region(phys_addr_t start, phys_addr_t end)
>  	}
>  }
>  
> -static void __free_pages_ok(struct page *page, unsigned int order)
> +static void __free_pages_ok(struct page *page, unsigned int order,
> +			    fop_t fop_flags)
>  {
>  	unsigned long flags;
>  	int migratetype;
> @@ -1519,7 +1519,8 @@ static void __free_pages_ok(struct page *page, unsigned int order)
>  	migratetype = get_pfnblock_migratetype(page, pfn);
>  	local_irq_save(flags);
>  	__count_vm_events(PGFREE, 1 << order);
> -	free_one_page(page_zone(page), page, pfn, order, migratetype);
> +	free_one_page(page_zone(page), page, pfn, order, migratetype,
> +		      fop_flags);
>  	local_irq_restore(flags);
>  }
>  
> @@ -1529,6 +1530,11 @@ void __free_pages_core(struct page *page, unsigned int order)
>  	struct page *p = page;
>  	unsigned int loop;
>  
> +	/*
> +	 * When initializing the memmap, init_single_page() sets the refcount
> +	 * of all pages to 1 ("allocated"/"not free"). We have to set the
> +	 * refcount of all involved pages to 0.
> +	 */
>  	prefetchw(p);
>  	for (loop = 0; loop < (nr_pages - 1); loop++, p++) {
>  		prefetchw(p + 1);
> @@ -1539,8 +1545,12 @@ void __free_pages_core(struct page *page, unsigned int order)
>  	set_page_count(p, 0);
>  
>  	atomic_long_add(nr_pages, &page_zone(page)->managed_pages);
> -	set_page_refcounted(page);
> -	__free_pages(page, order);
> +
> +	/*
> +	 * Bypass PCP and place fresh pages right to the tail, primarily
> +	 * relevant for memory onlining.
> +	 */
> +	__free_pages_ok(page, order, FOP_TO_TAIL);
>  }
>  
>  #ifdef CONFIG_NEED_MULTIPLE_NODES
> @@ -3171,7 +3181,8 @@ static void free_unref_page_commit(struct page *page, unsigned long pfn)
>  	 */
>  	if (migratetype >= MIGRATE_PCPTYPES) {
>  		if (unlikely(is_migrate_isolate(migratetype))) {
> -			free_one_page(zone, page, pfn, 0, migratetype);
> +			free_one_page(zone, page, pfn, 0, migratetype,
> +				      FOP_NONE);
>  			return;
>  		}
>  		migratetype = MIGRATE_MOVABLE;
> @@ -5063,7 +5074,7 @@ static inline void free_the_page(struct page *page, unsigned int order)
>  	if (order == 0)		/* Via pcp? */
>  		free_unref_page(page);
>  	else
> -		__free_pages_ok(page, order);
> +		__free_pages_ok(page, order, FOP_NONE);
>  }
>  
>  void __free_pages(struct page *page, unsigned int order)
> -- 
> 2.26.2
> 

-- 
Michal Hocko
SUSE Labs