[PATCH] mm: Save soft-dirty bits on swapped pages

[Cyrill Gorcunov <gorcunov@gmail.com>]

Andy Lutomirski reported that in case if a page with _PAGE_SOFT_DIRTY
bit set get swapped out, the bit is getting lost and no longer
available when pte read back.

To resolve this we introduce _PTE_SWP_SOFT_DIRTY bit which is
saved in pte entry for the page being swapped out. When such page
is to be read back from a swap cache we check for bit presence
and if it's there we clear it and restore the former _PAGE_SOFT_DIRTY
bit back.

One of the problem was to find a place in pte entry where we can
save the _PTE_SWP_SOFT_DIRTY bit while page is in swap. The
_PAGE_PSE was chosen for that, it doesn't intersect with swap
entry format stored in pte.

Reported-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Matt Mackall <mpm@selenic.com>
Cc: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
---
While I've intensively tested this patch on x86-64/32 I would
really appreciate detailed review, to be sure I've not missed
places where borrowing _PAGE_PSE bit for own needs doesn't
cause any problems. Thanks!

 arch/x86/include/asm/pgtable.h       |   15 +++++++++++++++
 arch/x86/include/asm/pgtable_types.h |   13 +++++++++++++
 fs/proc/task_mmu.c                   |   23 +++++++++++++++++------
 include/linux/swapops.h              |    4 ++++
 mm/memory.c                          |    4 ++++
 mm/rmap.c                            |    6 +++++-
 6 files changed, 58 insertions(+), 7 deletions(-)

Index: linux-2.6.git/arch/x86/include/asm/pgtable.h
===================================================================
--- linux-2.6.git.orig/arch/x86/include/asm/pgtable.h
+++ linux-2.6.git/arch/x86/include/asm/pgtable.h
@@ -314,6 +314,21 @@ static inline pmd_t pmd_mksoft_dirty(pmd
 	return pmd_set_flags(pmd, _PAGE_SOFT_DIRTY);
 }
 
+static inline pte_t pte_swp_mksoft_dirty(pte_t pte)
+{
+	return pte_set_flags(pte, _PAGE_SWP_SOFT_DIRTY);
+}
+
+static inline int pte_swp_soft_dirty(pte_t pte)
+{
+	return pte_flags(pte) & _PAGE_SWP_SOFT_DIRTY;
+}
+
+static inline pte_t pte_swp_clear_soft_dirty(pte_t pte)
+{
+	return pte_clear_flags(pte, _PAGE_SWP_SOFT_DIRTY);
+}
+
 /*
  * Mask out unsupported bits in a present pgprot.  Non-present pgprots
  * can use those bits for other purposes, so leave them be.
Index: linux-2.6.git/arch/x86/include/asm/pgtable_types.h
===================================================================
--- linux-2.6.git.orig/arch/x86/include/asm/pgtable_types.h
+++ linux-2.6.git/arch/x86/include/asm/pgtable_types.h
@@ -67,6 +67,19 @@
 #define _PAGE_SOFT_DIRTY	(_AT(pteval_t, 0))
 #endif
 
+/*
+ * Tracking soft dirty bit when a page goes to a swap is tricky.
+ * We need a bit which can be stored in pte _and_ not conflict
+ * with swap entry format. On x86 bits 6 and 7 are *not* involved
+ * into swap entry computation, but bit 6 is used for nonlinear
+ * file mapping, so we borrow bit 7 for soft dirty tracking.
+ */
+#ifdef CONFIG_MEM_SOFT_DIRTY
+#define _PAGE_SWP_SOFT_DIRTY	_PAGE_PSE
+#else
+#define _PAGE_SWP_SOFT_DIRTY	(_AT(pteval_t, 0))
+#endif
+
 #if defined(CONFIG_X86_64) || defined(CONFIG_X86_PAE)
 #define _PAGE_NX	(_AT(pteval_t, 1) << _PAGE_BIT_NX)
 #else
Index: linux-2.6.git/fs/proc/task_mmu.c
===================================================================
--- linux-2.6.git.orig/fs/proc/task_mmu.c
+++ linux-2.6.git/fs/proc/task_mmu.c
@@ -730,8 +730,14 @@ static inline void clear_soft_dirty(stru
 	 * of how soft-dirty works.
 	 */
 	pte_t ptent = *pte;
-	ptent = pte_wrprotect(ptent);
-	ptent = pte_clear_flags(ptent, _PAGE_SOFT_DIRTY);
+
+	if (pte_present(ptent)) {
+		ptent = pte_wrprotect(ptent);
+		ptent = pte_clear_flags(ptent, _PAGE_SOFT_DIRTY);
+	} else if (pte_swp_soft_dirty(ptent)) {
+		ptent = pte_swp_clear_soft_dirty(ptent);
+	}
+
 	set_pte_at(vma->vm_mm, addr, pte, ptent);
 #endif
 }
@@ -752,14 +758,15 @@ static int clear_refs_pte_range(pmd_t *p
 	pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
 	for (; addr != end; pte++, addr += PAGE_SIZE) {
 		ptent = *pte;
-		if (!pte_present(ptent))
-			continue;
 
 		if (cp->type == CLEAR_REFS_SOFT_DIRTY) {
 			clear_soft_dirty(vma, addr, pte);
 			continue;
 		}
 
+		if (!pte_present(ptent))
+			continue;
+
 		page = vm_normal_page(vma, addr, ptent);
 		if (!page)
 			continue;
@@ -930,8 +937,12 @@ static void pte_to_pagemap_entry(pagemap
 		flags = PM_PRESENT;
 		page = vm_normal_page(vma, addr, pte);
 	} else if (is_swap_pte(pte)) {
-		swp_entry_t entry = pte_to_swp_entry(pte);
-
+		swp_entry_t entry;
+#ifdef CONFIG_MEM_SOFT_DIRTY
+		if (pte_swp_soft_dirty(pte))
+			flags2 |= __PM_SOFT_DIRTY;
+#endif
+		entry = pte_to_swp_entry(pte);
 		frame = swp_type(entry) |
 			(swp_offset(entry) << MAX_SWAPFILES_SHIFT);
 		flags = PM_SWAP;
Index: linux-2.6.git/include/linux/swapops.h
===================================================================
--- linux-2.6.git.orig/include/linux/swapops.h
+++ linux-2.6.git/include/linux/swapops.h
@@ -67,6 +67,10 @@ static inline swp_entry_t pte_to_swp_ent
 	swp_entry_t arch_entry;
 
 	BUG_ON(pte_file(pte));
+#ifdef CONFIG_MEM_SOFT_DIRTY
+	if (pte_swp_soft_dirty(pte))
+		pte = pte_swp_clear_soft_dirty(pte);
+#endif
 	arch_entry = __pte_to_swp_entry(pte);
 	return swp_entry(__swp_type(arch_entry), __swp_offset(arch_entry));
 }
Index: linux-2.6.git/mm/memory.c
===================================================================
--- linux-2.6.git.orig/mm/memory.c
+++ linux-2.6.git/mm/memory.c
@@ -3115,6 +3115,10 @@ static int do_swap_page(struct mm_struct
 		exclusive = 1;
 	}
 	flush_icache_page(vma, page);
+#ifdef CONFIG_MEM_SOFT_DIRTY
+	if (pte_swp_soft_dirty(orig_pte))
+		pte = pte_mksoft_dirty(pte);
+#endif
 	set_pte_at(mm, address, page_table, pte);
 	if (page == swapcache)
 		do_page_add_anon_rmap(page, vma, address, exclusive);
Index: linux-2.6.git/mm/rmap.c
===================================================================
--- linux-2.6.git.orig/mm/rmap.c
+++ linux-2.6.git/mm/rmap.c
@@ -1236,6 +1236,7 @@ int try_to_unmap_one(struct page *page,
 			   swp_entry_to_pte(make_hwpoison_entry(page)));
 	} else if (PageAnon(page)) {
 		swp_entry_t entry = { .val = page_private(page) };
+		pte_t swp_pte;
 
 		if (PageSwapCache(page)) {
 			/*
@@ -1264,7 +1265,10 @@ int try_to_unmap_one(struct page *page,
 			BUG_ON(TTU_ACTION(flags) != TTU_MIGRATION);
 			entry = make_migration_entry(page, pte_write(pteval));
 		}
-		set_pte_at(mm, address, pte, swp_entry_to_pte(entry));
+		swp_pte = swp_entry_to_pte(entry);
+		if (pte_soft_dirty(pteval))
+			swp_pte = pte_swp_mksoft_dirty(swp_pte);
+		set_pte_at(mm, address, pte, swp_pte);
 		BUG_ON(pte_file(*pte));
 	} else if (IS_ENABLED(CONFIG_MIGRATION) &&
 		   (TTU_ACTION(flags) == TTU_MIGRATION)) {

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