mm: thp: batch-collapse PMD with set_ptes()

Refactor __split_huge_pmd_locked() so that a present PMD can be collapsed
to PTEs in a single batch using set_ptes().

This should improve performance a little bit, but the real motivation is
to remove the need for the arm64 backend to have to fold the contpte
entries.  Instead, since the ptes are set as a batch, the contpte blocks
can be initially set up pre-folded (once the arm64 contpte support is
added in the next few patches).  This leads to noticeable performance
improvement during split.

Link: https://lkml.kernel.org/r/20240215103205.2607016-3-ryan.roberts@arm.comSigned-off-by: Ryan Roberts <ryan.roberts@arm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Barry Song <21cnbao@gmail.com>
Cc: Borislav Petkov (AMD) <bp@alien8.de>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Morse <james.morse@arm.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will@kernel.org>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/huge_memory.c

@@ -2579,15 +2579,16 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 
 	pte = pte_offset_map(&_pmd, haddr);
 	VM_BUG_ON(!pte);
-	for (i = 0, addr = haddr; i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE) {
-		pte_t entry;
+
 	/*
-		 * Note that NUMA hinting access restrictions are not
-		 * transferred to avoid any possibility of altering
-		 * permissions across VMAs.
+	 * Note that NUMA hinting access restrictions are not transferred to
+	 * avoid any possibility of altering permissions across VMAs.
 	 */
 	if (freeze || pmd_migration) {
+		for (i = 0, addr = haddr; i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE) {
+			pte_t entry;
 			swp_entry_t swp_entry;
+
 			if (write)
 				swp_entry = make_writable_migration_entry(
 							page_to_pfn(page + i));
@@ -2606,8 +2607,14 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 				entry = pte_swp_mksoft_dirty(entry);
 			if (uffd_wp)
 				entry = pte_swp_mkuffd_wp(entry);
+
+			VM_WARN_ON(!pte_none(ptep_get(pte + i)));
+			set_pte_at(mm, addr, pte + i, entry);
+		}
 	} else {
-			entry = mk_pte(page + i, READ_ONCE(vma->vm_page_prot));
+		pte_t entry;
+
+		entry = mk_pte(page, READ_ONCE(vma->vm_page_prot));
 		if (write)
 			entry = pte_mkwrite(entry, vma);
 		if (!young)
@@ -2619,12 +2626,13 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 			entry = pte_mksoft_dirty(entry);
 		if (uffd_wp)
 			entry = pte_mkuffd_wp(entry);
+
+		for (i = 0; i < HPAGE_PMD_NR; i++)
+			VM_WARN_ON(!pte_none(ptep_get(pte + i)));
+
+		set_ptes(mm, haddr, pte, entry, HPAGE_PMD_NR);
 	}
-		VM_BUG_ON(!pte_none(ptep_get(pte)));
-		set_pte_at(mm, addr, pte, entry);
-		pte++;
-	}
-	pte_unmap(pte - 1);
+	pte_unmap(pte);
 
 	if (!pmd_migration)
 		folio_remove_rmap_pmd(folio, page, vma);