mm: thp: enable thp migration in generic path

Add thp migration's core code, including conversions between a PMD entry
and a swap entry, setting PMD migration entry, removing PMD migration
entry, and waiting on PMD migration entries.

This patch makes it possible to support thp migration.  If you fail to
allocate a destination page as a thp, you just split the source thp as
we do now, and then enter the normal page migration.  If you succeed to
allocate destination thp, you enter thp migration.  Subsequent patches
actually enable thp migration for each caller of page migration by
allowing its get_new_page() callback to allocate thps.

[zi.yan@cs.rutgers.edu: fix gcc-4.9.0 -Wmissing-braces warning]
  Link: http://lkml.kernel.org/r/A0ABA698-7486-46C3-B209-E95A9048B22C@cs.rutgers.edu
[akpm@linux-foundation.org: fix x86_64 allnoconfig warning]
Signed-off-by: Zi Yan <zi.yan@cs.rutgers.edu>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: David Nellans <dnellans@nvidia.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/huge_memory.c

@@ -1684,10 +1684,24 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
 		spin_unlock(ptl);
 		tlb_remove_page_size(tlb, pmd_page(orig_pmd), HPAGE_PMD_SIZE);
 	} else {
-		struct page *page = pmd_page(orig_pmd);
-		page_remove_rmap(page, true);
-		VM_BUG_ON_PAGE(page_mapcount(page) < 0, page);
-		VM_BUG_ON_PAGE(!PageHead(page), page);
+		struct page *page = NULL;
+		int flush_needed = 1;
+
+		if (pmd_present(orig_pmd)) {
+			page = pmd_page(orig_pmd);
+			page_remove_rmap(page, true);
+			VM_BUG_ON_PAGE(page_mapcount(page) < 0, page);
+			VM_BUG_ON_PAGE(!PageHead(page), page);
+		} else if (thp_migration_supported()) {
+			swp_entry_t entry;
+
+			VM_BUG_ON(!is_pmd_migration_entry(orig_pmd));
+			entry = pmd_to_swp_entry(orig_pmd);
+			page = pfn_to_page(swp_offset(entry));
+			flush_needed = 0;
+		} else
+			WARN_ONCE(1, "Non present huge pmd without pmd migration enabled!");
+
 		if (PageAnon(page)) {
 			zap_deposited_table(tlb->mm, pmd);
 			add_mm_counter(tlb->mm, MM_ANONPAGES, -HPAGE_PMD_NR);
@@ -1696,8 +1710,10 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
 				zap_deposited_table(tlb->mm, pmd);
 			add_mm_counter(tlb->mm, MM_FILEPAGES, -HPAGE_PMD_NR);
 		}
+
 		spin_unlock(ptl);
-		tlb_remove_page_size(tlb, page, HPAGE_PMD_SIZE);
+		if (flush_needed)
+			tlb_remove_page_size(tlb, page, HPAGE_PMD_SIZE);
 	}
 	return 1;
 }
@@ -2745,3 +2761,61 @@ static int __init split_huge_pages_debugfs(void)
 }
 late_initcall(split_huge_pages_debugfs);
 #endif
+
+#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
+void set_pmd_migration_entry(struct page_vma_mapped_walk *pvmw,
+		struct page *page)
+{
+	struct vm_area_struct *vma = pvmw->vma;
+	struct mm_struct *mm = vma->vm_mm;
+	unsigned long address = pvmw->address;
+	pmd_t pmdval;
+	swp_entry_t entry;
+
+	if (!(pvmw->pmd && !pvmw->pte))
+		return;
+
+	mmu_notifier_invalidate_range_start(mm, address,
+			address + HPAGE_PMD_SIZE);
+
+	flush_cache_range(vma, address, address + HPAGE_PMD_SIZE);
+	pmdval = *pvmw->pmd;
+	pmdp_invalidate(vma, address, pvmw->pmd);
+	if (pmd_dirty(pmdval))
+		set_page_dirty(page);
+	entry = make_migration_entry(page, pmd_write(pmdval));
+	pmdval = swp_entry_to_pmd(entry);
+	set_pmd_at(mm, address, pvmw->pmd, pmdval);
+	page_remove_rmap(page, true);
+	put_page(page);
+
+	mmu_notifier_invalidate_range_end(mm, address,
+			address + HPAGE_PMD_SIZE);
+}
+
+void remove_migration_pmd(struct page_vma_mapped_walk *pvmw, struct page *new)
+{
+	struct vm_area_struct *vma = pvmw->vma;
+	struct mm_struct *mm = vma->vm_mm;
+	unsigned long address = pvmw->address;
+	unsigned long mmun_start = address & HPAGE_PMD_MASK;
+	pmd_t pmde;
+	swp_entry_t entry;
+
+	if (!(pvmw->pmd && !pvmw->pte))
+		return;
+
+	entry = pmd_to_swp_entry(*pvmw->pmd);
+	get_page(new);
+	pmde = pmd_mkold(mk_huge_pmd(new, vma->vm_page_prot));
+	if (is_write_migration_entry(entry))
+		pmde = maybe_pmd_mkwrite(pmde, vma);
+
+	flush_cache_range(vma, mmun_start, mmun_start + HPAGE_PMD_SIZE);
+	page_add_anon_rmap(new, vma, mmun_start, true);
+	set_pmd_at(mm, mmun_start, pvmw->pmd, pmde);
+	if (vma->vm_flags & VM_LOCKED)
+		mlock_vma_page(new);
+	update_mmu_cache_pmd(vma, address, pvmw->pmd);
+}
+#endif