Currently memmap_init_zone_device() ends up initializing 32768 pages
when it only needs to initialize 128 given tail page reuse. That
number is worse with 1GB compound pages, 262144 instead of 128. Update
memmap_init_zone_device() to skip redundant initialization, detailed
below.
When a pgmap @vmemmap_shift is set, all pages are mapped at a given
huge page alignment and use compound pages to describe them as opposed
to a struct per 4K.
With @vmemmap_shift > 0 and when struct pages are stored in ram
(!altmap) most tail pages are reused. Consequently, the amount of
unique struct pages is a lot smaller than the total amount of struct
pages being mapped.
The altmap path is left alone since it does not support memory savings
based on compound pages devmap.
Signed-off-by: Joao Martins <joao.m.martins@xxxxxxxxxx>
---
mm/page_alloc.c | 17 ++++++++++++++++-
1 file changed, 16 insertions(+), 1 deletion(-)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index e0c1e6bb09dd..e9282d043cca 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -6653,6 +6653,21 @@ static void __ref __init_zone_device_page(struct page *page, unsigned long pfn,
}
}
+/*
+ * With compound page geometry and when struct pages are stored in ram most
+ * tail pages are reused. Consequently, the amount of unique struct pages to
+ * initialize is a lot smaller that the total amount of struct pages being
+ * mapped. This is a paired / mild layering violation with explicit knowledge
+ * of how the sparse_vmemmap internals handle compound pages in the lack
+ * of an altmap. See vmemmap_populate_compound_pages().
+ */
+static inline unsigned long compound_nr_pages(struct vmem_altmap *altmap,
+ unsigned long nr_pages)
+{
+ return is_power_of_2(sizeof(struct page)) &&
+ !altmap ? 2 * (PAGE_SIZE/sizeof(struct page)) : nr_pages;
+}
+
static void __ref memmap_init_compound(struct page *head,
unsigned long head_pfn,
unsigned long zone_idx, int nid,
@@ -6717,7 +6732,7 @@ void __ref memmap_init_zone_device(struct zone *zone,
continue;
memmap_init_compound(page, pfn, zone_idx, nid, pgmap,
- pfns_per_compound);
+ compound_nr_pages(altmap, pfns_per_compound));
}
pr_info("%s initialised %lu pages in %ums\n", __func__,
--
2.17.2
