mm/hugetlb: do pre-HVO for bootmem allocated pages

For large systems, the overhead of vmemmap pages for hugetlb is
substantial.  It's about 1.5% of memory, which is about 45G for a 3T
system.  If you want to configure most of that system for hugetlb (e.g. 
to use as backing memory for VMs), there is a chance of running out of
memory on boot, even though you know that the 45G will become available
later.

To avoid this scenario, and since it's a waste to first allocate and then
free that 45G during boot, do pre-HVO for hugetlb bootmem allocated pages
('gigantic' pages).

pre-HVO is done by adding functions that are called from
sparse_init_nid_early and sparse_init_nid_late.  The first is called
before memmap allocation, so it takes care of allocating memmap HVO-style.
The second verifies that all bootmem pages look good, specifically it
checks that they do not intersect with multiple zones.  This can only be
done from sparse_init_nid_late path, when zones have been initialized.

The hugetlb page size must be aligned to the section size, and aligned to
the size of memory described by the number of page structures contained in
one PMD (since pre-HVO is not prepared to split PMDs).  This should be
true for most 'gigantic' pages, it is for 1G pages on x86, where both of
these alignment requirements are 128M.

This will only have an effect if hugetlb_bootmem_alloc was called early in
boot.  If not, it won't do anything, and HVO for bootmem hugetlb pages
works as before.

Link: https://lkml.kernel.org/r/20250228182928.2645936-20-fvdl@google.com
Signed-off-by: Frank van der Linden <fvdl@google.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Joao Martins <joao.m.martins@oracle.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Roman Gushchin (Cruise) <roman.gushchin@linux.dev>
Cc: Usama Arif <usamaarif642@gmail.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 143 insertions, 0 deletions

diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index be6b33ecbc8e..9a99dfa3c495 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -743,6 +743,149 @@ void hugetlb_vmemmap_optimize_bootmem_folios(struct hstate *h, struct list_head
 	__hugetlb_vmemmap_optimize_folios(h, folio_list, true);
 }
 
+#ifdef CONFIG_SPARSEMEM_VMEMMAP_PREINIT
+
+/* Return true of a bootmem allocated HugeTLB page should be pre-HVO-ed */
+static bool vmemmap_should_optimize_bootmem_page(struct huge_bootmem_page *m)
+{
+	unsigned long section_size, psize, pmd_vmemmap_size;
+	phys_addr_t paddr;
+
+	if (!READ_ONCE(vmemmap_optimize_enabled))
+		return false;
+
+	if (!hugetlb_vmemmap_optimizable(m->hstate))
+		return false;
+
+	psize = huge_page_size(m->hstate);
+	paddr = virt_to_phys(m);
+
+	/*
+	 * Pre-HVO only works if the bootmem huge page
+	 * is aligned to the section size.
+	 */
+	section_size = (1UL << PA_SECTION_SHIFT);
+	if (!IS_ALIGNED(paddr, section_size) ||
+	    !IS_ALIGNED(psize, section_size))
+		return false;
+
+	/*
+	 * The pre-HVO code does not deal with splitting PMDS,
+	 * so the bootmem page must be aligned to the number
+	 * of base pages that can be mapped with one vmemmap PMD.
+	 */
+	pmd_vmemmap_size = (PMD_SIZE / (sizeof(struct page))) << PAGE_SHIFT;
+	if (!IS_ALIGNED(paddr, pmd_vmemmap_size) ||
+	    !IS_ALIGNED(psize, pmd_vmemmap_size))
+		return false;
+
+	return true;
+}
+
+/*
+ * Initialize memmap section for a gigantic page, HVO-style.
+ */
+void __init hugetlb_vmemmap_init_early(int nid)
+{
+	unsigned long psize, paddr, section_size;
+	unsigned long ns, i, pnum, pfn, nr_pages;
+	unsigned long start, end;
+	struct huge_bootmem_page *m = NULL;
+	void *map;
+
+	/*
+	 * Noting to do if bootmem pages were not allocated
+	 * early in boot, or if HVO wasn't enabled in the
+	 * first place.
+	 */
+	if (!hugetlb_bootmem_allocated())
+		return;
+
+	if (!READ_ONCE(vmemmap_optimize_enabled))
+		return;
+
+	section_size = (1UL << PA_SECTION_SHIFT);
+
+	list_for_each_entry(m, &huge_boot_pages[nid], list) {
+		if (!vmemmap_should_optimize_bootmem_page(m))
+			continue;
+
+		nr_pages = pages_per_huge_page(m->hstate);
+		psize = nr_pages << PAGE_SHIFT;
+		paddr = virt_to_phys(m);
+		pfn = PHYS_PFN(paddr);
+		map = pfn_to_page(pfn);
+		start = (unsigned long)map;
+		end = start + nr_pages * sizeof(struct page);
+
+		if (vmemmap_populate_hvo(start, end, nid,
+					HUGETLB_VMEMMAP_RESERVE_SIZE) < 0)
+			continue;
+
+		memmap_boot_pages_add(HUGETLB_VMEMMAP_RESERVE_SIZE / PAGE_SIZE);
+
+		pnum = pfn_to_section_nr(pfn);
+		ns = psize / section_size;
+
+		for (i = 0; i < ns; i++) {
+			sparse_init_early_section(nid, map, pnum,
+					SECTION_IS_VMEMMAP_PREINIT);
+			map += section_map_size();
+			pnum++;
+		}
+
+		m->flags |= HUGE_BOOTMEM_HVO;
+	}
+}
+
+void __init hugetlb_vmemmap_init_late(int nid)
+{
+	struct huge_bootmem_page *m, *tm;
+	unsigned long phys, nr_pages, start, end;
+	unsigned long pfn, nr_mmap;
+	struct hstate *h;
+	void *map;
+
+	if (!hugetlb_bootmem_allocated())
+		return;
+
+	if (!READ_ONCE(vmemmap_optimize_enabled))
+		return;
+
+	list_for_each_entry_safe(m, tm, &huge_boot_pages[nid], list) {
+		if (!(m->flags & HUGE_BOOTMEM_HVO))
+			continue;
+
+		phys = virt_to_phys(m);
+		h = m->hstate;
+		pfn = PHYS_PFN(phys);
+		nr_pages = pages_per_huge_page(h);
+
+		if (!hugetlb_bootmem_page_zones_valid(nid, m)) {
+			/*
+			 * Oops, the hugetlb page spans multiple zones.
+			 * Remove it from the list, and undo HVO.
+			 */
+			list_del(&m->list);
+
+			map = pfn_to_page(pfn);
+
+			start = (unsigned long)map;
+			end = start + nr_pages * sizeof(struct page);
+
+			vmemmap_undo_hvo(start, end, nid,
+					 HUGETLB_VMEMMAP_RESERVE_SIZE);
+			nr_mmap = end - start - HUGETLB_VMEMMAP_RESERVE_SIZE;
+			memmap_boot_pages_add(DIV_ROUND_UP(nr_mmap, PAGE_SIZE));
+
+			memblock_phys_free(phys, huge_page_size(h));
+			continue;
+		} else
+			m->flags |= HUGE_BOOTMEM_ZONES_VALID;
+	}
+}
+#endif
+
 static const struct ctl_table hugetlb_vmemmap_sysctls[] = {
 	{
 		.procname	= "hugetlb_optimize_vmemmap",