mm: hugetlb_vmemmap: use nid of the head page to reallocate it

Patch series "mm: hugetlb: Skip initialization of gigantic tail struct
pages if freed by HVO", v5.

This series moves the boot time initialization of tail struct pages of a
gigantic page to later on in the boot.  Only the
HUGETLB_VMEMMAP_RESERVE_SIZE / sizeof(struct page) - 1 tail struct pages
are initialized at the start.  If HVO is successful, then no more tail
struct pages need to be initialized.  For a 1G hugepage, this series avoid
initialization of 262144 - 63 = 262081 struct pages per hugepage.

When tested on a 512G system (allocating 500 1G hugepages), the kexec-boot
times with DEFERRED_STRUCT_PAGE_INIT enabled are:

- with patches, HVO enabled: 1.32 seconds
- with patches, HVO disabled: 2.15 seconds
- without patches, HVO enabled: 3.90  seconds
- without patches, HVO disabled: 3.58 seconds

This represents an approximately 70% reduction in boot time and will
significantly reduce server downtime when using a large number of gigantic
pages.


This patch (of 4):

If tail page prep and initialization is skipped, then the "start" page
will not contain the correct nid.  Use the nid from first vmemap page.

Link: https://lkml.kernel.org/r/20230913105401.519709-1-usama.arif@bytedance.com
Link: https://lkml.kernel.org/r/20230913105401.519709-2-usama.arif@bytedance.com
Signed-off-by: Usama Arif <usama.arif@bytedance.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Fam Zheng <fam.zheng@bytedance.com>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Punit Agrawal <punit.agrawal@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/hugetlb_vmemmap.c

@@ -319,7 +319,7 @@ static int vmemmap_remap_free(unsigned long start, unsigned long end,
 		.reuse_addr	= reuse,
 		.vmemmap_pages	= &vmemmap_pages,
 	};
-	int nid = page_to_nid((struct page *)start);
+	int nid = page_to_nid((struct page *)reuse);
 	gfp_t gfp_mask = GFP_KERNEL | __GFP_NORETRY | __GFP_NOWARN;
 
 	/*