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

Michal Hocko
2020-10-02T13:41:19Z

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@suse.cz>
> Reviewed-by: Oscar Salvador <osalvador@suse.de>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> Cc: Mel Gorman <mgorman@techsingularity.net>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Dave Hansen <dave.hansen@intel.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Wei Yang <richard.weiyang@linux.alibaba.com>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Mike Rapoport <rppt@kernel.org>
> Cc: "K. Y. Srinivasan" <kys@microsoft.com>
> Cc: Haiyang Zhang <haiyangz@microsoft.com>
> Cc: Stephen Hemminger <sthemmin@microsoft.com>
> Cc: Wei Liu <wei.liu@kernel.org>
> Signed-off-by: David Hildenbrand <david@redhat.com>

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

> ---
> 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