[PATCH] Swapless page migration: add R/W migration entries

Implement read/write migration ptes

We take the upper two swapfiles for the two types of migration ptes and define
a series of macros in swapops.h.

The VM is modified to handle the migration entries.  migration entries can
only be encountered when the page they are pointing to is locked.  This limits
the number of places one has to fix.  We also check in copy_pte_range and in
mprotect_pte_range() for migration ptes.

We check for migration ptes in do_swap_cache and call a function that will
then wait on the page lock.  This allows us to effectively stop all accesses
to apge.

Migration entries are created by try_to_unmap if called for migration and
removed by local functions in migrate.c

From: Hugh Dickins <hugh@veritas.com>

  Several times while testing swapless page migration (I've no NUMA, just
  hacking it up to migrate recklessly while running load), I've hit the
  BUG_ON(!PageLocked(p)) in migration_entry_to_page.

  This comes from an orphaned migration entry, unrelated to the current
  correctly locked migration, but hit by remove_anon_migration_ptes as it
  checks an address in each vma of the anon_vma list.

  Such an orphan may be left behind if an earlier migration raced with fork:
  copy_one_pte can duplicate a migration entry from parent to child, after
  remove_anon_migration_ptes has checked the child vma, but before it has
  removed it from the parent vma.  (If the process were later to fault on this
  orphaned entry, it would hit the same BUG from migration_entry_wait.)

  This could be fixed by locking anon_vma in copy_one_pte, but we'd rather
  not.  There's no such problem with file pages, because vma_prio_tree_add
  adds child vma after parent vma, and the page table locking at each end is
  enough to serialize.  Follow that example with anon_vma: add new vmas to the
  tail instead of the head.

  (There's no corresponding problem when inserting migration entries,
  because a missed pte will leave the page count and mapcount high, which is
  allowed for.  And there's no corresponding problem when migrating via swap,
  because a leftover swap entry will be correctly faulted.  But the swapless
  method has no refcounting of its entries.)

From: Ingo Molnar <mingo@elte.hu>

  pte_unmap_unlock() takes the pte pointer as an argument.

From: Hugh Dickins <hugh@veritas.com>

  Several times while testing swapless page migration, gcc has tried to exec
  a pointer instead of a string: smells like COW mappings are not being
  properly write-protected on fork.

  The protection in copy_one_pte looks very convincing, until at last you
  realize that the second arg to make_migration_entry is a boolean "write",
  and SWP_MIGRATION_READ is 30.

  Anyway, it's better done like in change_pte_range, using
  is_write_migration_entry and make_migration_entry_read.

From: Hugh Dickins <hugh@veritas.com>

  Remove unnecessary obfuscation from sys_swapon's range check on swap type,
  which blew up causing memory corruption once swapless migration made
  MAX_SWAPFILES no longer 2 ^ MAX_SWAPFILES_SHIFT.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Acked-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Christoph Lameter <clameter@engr.sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
From: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/linux/swap.h

@@ -28,7 +28,14 @@ static inline int current_is_kswapd(void)
  * the type/offset into the pte as 5/27 as well.
  */
 #define MAX_SWAPFILES_SHIFT	5
+#ifndef CONFIG_MIGRATION
 #define MAX_SWAPFILES		(1 << MAX_SWAPFILES_SHIFT)
+#else
+/* Use last two entries for page migration swap entries */
+#define MAX_SWAPFILES		((1 << MAX_SWAPFILES_SHIFT)-2)
+#define SWP_MIGRATION_READ	MAX_SWAPFILES
+#define SWP_MIGRATION_WRITE	(MAX_SWAPFILES + 1)
+#endif
 
 /*
  * Magic header for a swap area. The first part of the union is

include/linux/swapops.h

@@ -67,3 +67,56 @@ static inline pte_t swp_entry_to_pte(swp_entry_t entry)
 	BUG_ON(pte_file(__swp_entry_to_pte(arch_entry)));
 	return __swp_entry_to_pte(arch_entry);
 }
+
+#ifdef CONFIG_MIGRATION
+static inline swp_entry_t make_migration_entry(struct page *page, int write)
+{
+	BUG_ON(!PageLocked(page));
+	return swp_entry(write ? SWP_MIGRATION_WRITE : SWP_MIGRATION_READ,
+			page_to_pfn(page));
+}
+
+static inline int is_migration_entry(swp_entry_t entry)
+{
+	return unlikely(swp_type(entry) == SWP_MIGRATION_READ ||
+			swp_type(entry) == SWP_MIGRATION_WRITE);
+}
+
+static inline int is_write_migration_entry(swp_entry_t entry)
+{
+	return unlikely(swp_type(entry) == SWP_MIGRATION_WRITE);
+}
+
+static inline struct page *migration_entry_to_page(swp_entry_t entry)
+{
+	struct page *p = pfn_to_page(swp_offset(entry));
+	/*
+	 * Any use of migration entries may only occur while the
+	 * corresponding page is locked
+	 */
+	BUG_ON(!PageLocked(p));
+	return p;
+}
+
+static inline void make_migration_entry_read(swp_entry_t *entry)
+{
+	*entry = swp_entry(SWP_MIGRATION_READ, swp_offset(*entry));
+}
+
+extern void migration_entry_wait(struct mm_struct *mm, pmd_t *pmd,
+					unsigned long address);
+#else
+
+#define make_migration_entry(page, write) swp_entry(0, 0)
+#define is_migration_entry(swp) 0
+#define migration_entry_to_page(swp) NULL
+static inline void make_migration_entry_read(swp_entry_t *entryp) { }
+static inline void migration_entry_wait(struct mm_struct *mm, pmd_t *pmd,
+					 unsigned long address) { }
+static inline int is_write_migration_entry(swp_entry_t entry)
+{
+	return 0;
+}
+
+#endif
+

mm/memory.c

@@ -434,7 +434,9 @@ copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
 	/* pte contains position in swap or file, so copy. */
 	if (unlikely(!pte_present(pte))) {
 		if (!pte_file(pte)) {
-			swap_duplicate(pte_to_swp_entry(pte));
+			swp_entry_t entry = pte_to_swp_entry(pte);
+
+			swap_duplicate(entry);
 			/* make sure dst_mm is on swapoff's mmlist. */
 			if (unlikely(list_empty(&dst_mm->mmlist))) {
 				spin_lock(&mmlist_lock);
@@ -443,6 +445,16 @@ copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
 						 &src_mm->mmlist);
 				spin_unlock(&mmlist_lock);
 			}
+			if (is_write_migration_entry(entry) &&
+					is_cow_mapping(vm_flags)) {
+				/*
+				 * COW mappings require pages in both parent
+				 * and child to be set to read.
+				 */
+				make_migration_entry_read(&entry);
+				pte = swp_entry_to_pte(entry);
+				set_pte_at(src_mm, addr, src_pte, pte);
+			}
 		}
 		goto out_set_pte;
 	}
@@ -1879,6 +1891,10 @@ static int do_swap_page(struct mm_struct *mm, struct vm_area_struct *vma,
 		goto out;
 
 	entry = pte_to_swp_entry(orig_pte);
+	if (is_migration_entry(entry)) {
+		migration_entry_wait(mm, pmd, address);
+		goto out;
+	}
 	page = lookup_swap_cache(entry);
 	if (!page) {
  		swapin_readahead(entry, address, vma);

mm/migrate.c

@@ -15,6 +15,7 @@
 #include <linux/migrate.h>
 #include <linux/module.h>
 #include <linux/swap.h>
+#include <linux/swapops.h>
 #include <linux/pagemap.h>
 #include <linux/buffer_head.h>
 #include <linux/mm_inline.h>
@@ -23,7 +24,6 @@
 #include <linux/topology.h>
 #include <linux/cpu.h>
 #include <linux/cpuset.h>
-#include <linux/swapops.h>
 
 #include "internal.h"
 
@@ -119,6 +119,132 @@ int putback_lru_pages(struct list_head *l)
 	return count;
 }
 
+static inline int is_swap_pte(pte_t pte)
+{
+	return !pte_none(pte) && !pte_present(pte) && !pte_file(pte);
+}
+
+/*
+ * Restore a potential migration pte to a working pte entry
+ */
+static void remove_migration_pte(struct vm_area_struct *vma, unsigned long addr,
+		struct page *old, struct page *new)
+{
+	struct mm_struct *mm = vma->vm_mm;
+	swp_entry_t entry;
+ 	pgd_t *pgd;
+ 	pud_t *pud;
+ 	pmd_t *pmd;
+	pte_t *ptep, pte;
+ 	spinlock_t *ptl;
+
+ 	pgd = pgd_offset(mm, addr);
+	if (!pgd_present(*pgd))
+                return;
+
+	pud = pud_offset(pgd, addr);
+	if (!pud_present(*pud))
+                return;
+
+	pmd = pmd_offset(pud, addr);
+	if (!pmd_present(*pmd))
+		return;
+
+	ptep = pte_offset_map(pmd, addr);
+
+	if (!is_swap_pte(*ptep)) {
+		pte_unmap(ptep);
+ 		return;
+ 	}
+
+ 	ptl = pte_lockptr(mm, pmd);
+ 	spin_lock(ptl);
+	pte = *ptep;
+	if (!is_swap_pte(pte))
+		goto out;
+
+	entry = pte_to_swp_entry(pte);
+
+	if (!is_migration_entry(entry) || migration_entry_to_page(entry) != old)
+		goto out;
+
+	inc_mm_counter(mm, anon_rss);
+	get_page(new);
+	pte = pte_mkold(mk_pte(new, vma->vm_page_prot));
+	if (is_write_migration_entry(entry))
+		pte = pte_mkwrite(pte);
+	set_pte_at(mm, addr, ptep, pte);
+	page_add_anon_rmap(new, vma, addr);
+out:
+	pte_unmap_unlock(ptep, ptl);
+}
+
+/*
+ * Get rid of all migration entries and replace them by
+ * references to the indicated page.
+ *
+ * Must hold mmap_sem lock on at least one of the vmas containing
+ * the page so that the anon_vma cannot vanish.
+ */
+static void remove_migration_ptes(struct page *old, struct page *new)
+{
+	struct anon_vma *anon_vma;
+	struct vm_area_struct *vma;
+	unsigned long mapping;
+
+	mapping = (unsigned long)new->mapping;
+
+	if (!mapping || (mapping & PAGE_MAPPING_ANON) == 0)
+		return;
+
+	/*
+	 * We hold the mmap_sem lock. So no need to call page_lock_anon_vma.
+	 */
+	anon_vma = (struct anon_vma *) (mapping - PAGE_MAPPING_ANON);
+	spin_lock(&anon_vma->lock);
+
+	list_for_each_entry(vma, &anon_vma->head, anon_vma_node)
+		remove_migration_pte(vma, page_address_in_vma(new, vma),
+					old, new);
+
+	spin_unlock(&anon_vma->lock);
+}
+
+/*
+ * Something used the pte of a page under migration. We need to
+ * get to the page and wait until migration is finished.
+ * When we return from this function the fault will be retried.
+ *
+ * This function is called from do_swap_page().
+ */
+void migration_entry_wait(struct mm_struct *mm, pmd_t *pmd,
+				unsigned long address)
+{
+	pte_t *ptep, pte;
+	spinlock_t *ptl;
+	swp_entry_t entry;
+	struct page *page;
+
+	ptep = pte_offset_map_lock(mm, pmd, address, &ptl);
+	pte = *ptep;
+	if (!is_swap_pte(pte))
+		goto out;
+
+	entry = pte_to_swp_entry(pte);
+	if (!is_migration_entry(entry))
+		goto out;
+
+	page = migration_entry_to_page(entry);
+
+	get_page(page);
+	pte_unmap_unlock(ptep, ptl);
+	wait_on_page_locked(page);
+	put_page(page);
+	return;
+out:
+	pte_unmap_unlock(ptep, ptl);
+}
+
 /*
  * swapout a single page
  * page is locked upon entry, unlocked on exit

mm/mprotect.c

@@ -19,7 +19,8 @@
 #include <linux/mempolicy.h>
 #include <linux/personality.h>
 #include <linux/syscalls.h>
-
+#include <linux/swap.h>
+#include <linux/swapops.h>
 #include <asm/uaccess.h>
 #include <asm/pgtable.h>
 #include <asm/cacheflush.h>
@@ -28,12 +29,13 @@
 static void change_pte_range(struct mm_struct *mm, pmd_t *pmd,
 		unsigned long addr, unsigned long end, pgprot_t newprot)
 {
-	pte_t *pte;
+	pte_t *pte, oldpte;
 	spinlock_t *ptl;
 
 	pte = pte_offset_map_lock(mm, pmd, addr, &ptl);
 	do {
-		if (pte_present(*pte)) {
+		oldpte = *pte;
+		if (pte_present(oldpte)) {
 			pte_t ptent;
 
 			/* Avoid an SMP race with hardware updated dirty/clean
@@ -43,7 +45,22 @@ static void change_pte_range(struct mm_struct *mm, pmd_t *pmd,
 			ptent = pte_modify(ptep_get_and_clear(mm, addr, pte), newprot);
 			set_pte_at(mm, addr, pte, ptent);
 			lazy_mmu_prot_update(ptent);
+#ifdef CONFIG_MIGRATION
+		} else if (!pte_file(oldpte)) {
+			swp_entry_t entry = pte_to_swp_entry(oldpte);
+
+			if (is_write_migration_entry(entry)) {
+				/*
+				 * A protection check is difficult so
+				 * just be safe and disable write
+				 */
+				make_migration_entry_read(&entry);
+				set_pte_at(mm, addr, pte,
+					swp_entry_to_pte(entry));
+			}
+#endif
 		}
+
 	} while (pte++, addr += PAGE_SIZE, addr != end);
 	pte_unmap_unlock(pte - 1, ptl);
 }

mm/rmap.c

@@ -103,7 +103,7 @@ int anon_vma_prepare(struct vm_area_struct *vma)
 		spin_lock(&mm->page_table_lock);
 		if (likely(!vma->anon_vma)) {
 			vma->anon_vma = anon_vma;
-			list_add(&vma->anon_vma_node, &anon_vma->head);
+			list_add_tail(&vma->anon_vma_node, &anon_vma->head);
 			allocated = NULL;
 		}
 		spin_unlock(&mm->page_table_lock);
@@ -127,7 +127,7 @@ void __anon_vma_link(struct vm_area_struct *vma)
 	struct anon_vma *anon_vma = vma->anon_vma;
 
 	if (anon_vma) {
-		list_add(&vma->anon_vma_node, &anon_vma->head);
+		list_add_tail(&vma->anon_vma_node, &anon_vma->head);
 		validate_anon_vma(vma);
 	}
 }
@@ -138,7 +138,7 @@ void anon_vma_link(struct vm_area_struct *vma)
 
 	if (anon_vma) {
 		spin_lock(&anon_vma->lock);
-		list_add(&vma->anon_vma_node, &anon_vma->head);
+		list_add_tail(&vma->anon_vma_node, &anon_vma->head);
 		validate_anon_vma(vma);
 		spin_unlock(&anon_vma->lock);
 	}
@@ -620,17 +620,27 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
 
 	if (PageAnon(page)) {
 		swp_entry_t entry = { .val = page_private(page) };
-		/*
-		 * Store the swap location in the pte.
-		 * See handle_pte_fault() ...
-		 */
-		BUG_ON(!PageSwapCache(page));
-		swap_duplicate(entry);
-		if (list_empty(&mm->mmlist)) {
-			spin_lock(&mmlist_lock);
-			if (list_empty(&mm->mmlist))
-				list_add(&mm->mmlist, &init_mm.mmlist);
-			spin_unlock(&mmlist_lock);
+
+		if (PageSwapCache(page)) {
+			/*
+			 * Store the swap location in the pte.
+			 * See handle_pte_fault() ...
+			 */
+			swap_duplicate(entry);
+			if (list_empty(&mm->mmlist)) {
+				spin_lock(&mmlist_lock);
+				if (list_empty(&mm->mmlist))
+					list_add(&mm->mmlist, &init_mm.mmlist);
+				spin_unlock(&mmlist_lock);
+			}
+		} else {
+			/*
+			 * Store the pfn of the page in a special migration
+			 * pte. do_swap_page() will wait until the migration
+			 * pte is removed and then restart fault handling.
+			 */
+			BUG_ON(!migration);
+			entry = make_migration_entry(page, pte_write(pteval));
 		}
 		set_pte_at(mm, address, pte, swp_entry_to_pte(entry));
 		BUG_ON(pte_file(*pte));

mm/swapfile.c

@@ -395,6 +395,9 @@ void free_swap_and_cache(swp_entry_t entry)
 	struct swap_info_struct * p;
 	struct page *page = NULL;
 
+	if (is_migration_entry(entry))
+		return;
+
 	p = swap_info_get(entry);
 	if (p) {
 		if (swap_entry_free(p, swp_offset(entry)) == 1) {
@@ -1400,19 +1403,7 @@ asmlinkage long sys_swapon(const char __user * specialfile, int swap_flags)
 		if (!(p->flags & SWP_USED))
 			break;
 	error = -EPERM;
-	/*
-	 * Test if adding another swap device is possible. There are
-	 * two limiting factors: 1) the number of bits for the swap
-	 * type swp_entry_t definition and 2) the number of bits for
-	 * the swap type in the swap ptes as defined by the different
-	 * architectures. To honor both limitations a swap entry
-	 * with swap offset 0 and swap type ~0UL is created, encoded
-	 * to a swap pte, decoded to a swp_entry_t again and finally
-	 * the swap type part is extracted. This will mask all bits
-	 * from the initial ~0UL that can't be encoded in either the
-	 * swp_entry_t or the architecture definition of a swap pte.
-	 */
-	if (type > swp_type(pte_to_swp_entry(swp_entry_to_pte(swp_entry(~0UL,0))))) {
+	if (type >= MAX_SWAPFILES) {
 		spin_unlock(&swap_lock);
 		goto out;
 	}
@@ -1702,6 +1693,9 @@ int swap_duplicate(swp_entry_t entry)
 	unsigned long offset, type;
 	int result = 0;
 
+	if (is_migration_entry(entry))
+		return 1;
+
 	type = swp_type(entry);
 	if (type >= nr_swapfiles)
 		goto bad_file;