Re: [patch 1/2] [PATCH] mm: Save soft-dirty bits on swapped pages

[Minchan Kim]

Hello Wanpeng,

On Mon, Aug 05, 2013 at 09:48:29AM +0800, Wanpeng Li wrote:
> On Wed, Jul 31, 2013 at 12:41:55AM +0400, Cyrill Gorcunov wrote:
> >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@xxxxxxxxxxxxxx>
> >Signed-off-by: Cyrill Gorcunov <gorcunov@xxxxxxxxxx>
> >Cc: Pavel Emelyanov <xemul@xxxxxxxxxxxxx>
> >Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> >Cc: Matt Mackall <mpm@xxxxxxxxxxx>
> >Cc: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx>
> >Cc: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
> >Cc: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxx>
> >Cc: Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx>
> >Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> >Cc: "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
> >---
> > arch/x86/include/asm/pgtable.h       |   15 +++++++++++++++
> > arch/x86/include/asm/pgtable_types.h |   13 +++++++++++++
> > fs/proc/task_mmu.c                   |   21 +++++++++++++++------
> > include/asm-generic/pgtable.h        |   15 +++++++++++++++
> > include/linux/swapops.h              |    2 ++
> > mm/memory.c                          |    2 ++
> > mm/rmap.c                            |    6 +++++-
> > mm/swapfile.c                        |   19 +++++++++++++++++--
> > 8 files changed, 84 insertions(+), 9 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 (is_swap_pte(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 +937,10 @@ 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;
> >+		if (pte_swp_soft_dirty(pte))
> >+			flags2 |= __PM_SOFT_DIRTY;
> >+		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/asm-generic/pgtable.h
> >===================================================================
> >--- linux-2.6.git.orig/include/asm-generic/pgtable.h
> >+++ linux-2.6.git/include/asm-generic/pgtable.h
> >@@ -417,6 +417,21 @@ static inline pmd_t pmd_mksoft_dirty(pmd
> > {
> > 	return pmd;
> > }
> >+
> >+static inline pte_t pte_swp_mksoft_dirty(pte_t pte)
> >+{
> >+	return pte;
> >+}
> >+
> >+static inline int pte_swp_soft_dirty(pte_t pte)
> >+{
> >+	return 0;
> >+}
> >+
> >+static inline pte_t pte_swp_clear_soft_dirty(pte_t pte)
> >+{
> >+	return pte;
> >+}
> > #endif
> >
> > #ifndef __HAVE_PFNMAP_TRACKING
> >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 +67,8 @@ static inline swp_entry_t pte_to_swp_ent
> > 	swp_entry_t arch_entry;
> >
> > 	BUG_ON(pte_file(pte));
> >+	if (pte_swp_soft_dirty(pte))
> >+		pte = pte_swp_clear_soft_dirty(pte);
> > 	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 +3115,8 @@ static int do_swap_page(struct mm_struct
> > 		exclusive = 1;
> > 	}
> > 	flush_icache_page(vma, page);
> >+	if (pte_swp_soft_dirty(orig_pte))
> >+		pte = pte_mksoft_dirty(pte);
> 
> entry = pte_to_swp_entry(orig_pte);
> orig_pte's _PTE_SWP_SOFT_DIRTY bit has already been cleared. 

You seem to walk same way with me.
Please look at my stupid questions in this thread.

> 
> > 	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)) {
> >Index: linux-2.6.git/mm/swapfile.c
> >===================================================================
> >--- linux-2.6.git.orig/mm/swapfile.c
> >+++ linux-2.6.git/mm/swapfile.c
> >@@ -866,6 +866,21 @@ unsigned int count_swap_pages(int type,
> > }
> > #endif /* CONFIG_HIBERNATION */
> >
> >+static inline int maybe_same_pte(pte_t pte, pte_t swp_pte)
> >+{
> >+#ifdef CONFIG_MEM_SOFT_DIRTY
> >+	/*
> >+	 * When pte keeps soft dirty bit the pte generated
> >+	 * from swap entry does not has it, still it's same
> >+	 * pte from logical point of view.
> >+	 */
> >+	pte_t swp_pte_dirty = pte_swp_mksoft_dirty(swp_pte);
> >+	return pte_same(pte, swp_pte) || pte_same(pte, swp_pte_dirty);
> >+#else
> >+	return pte_same(pte, swp_pte);
> >+#endif
> >+}
> >+
> > /*
> >  * No need to decide whether this PTE shares the swap entry with others,
> >  * just let do_wp_page work it out if a write is requested later - to
> >@@ -892,7 +907,7 @@ static int unuse_pte(struct vm_area_stru
> > 	}
> >
> > 	pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
> >-	if (unlikely(!pte_same(*pte, swp_entry_to_pte(entry)))) {
> >+	if (unlikely(!maybe_same_pte(*pte, swp_entry_to_pte(entry)))) {
> > 		mem_cgroup_cancel_charge_swapin(memcg);
> > 		ret = 0;
> > 		goto out;
> >@@ -947,7 +962,7 @@ static int unuse_pte_range(struct vm_are
> > 		 * swapoff spends a _lot_ of time in this loop!
> > 		 * Test inline before going to call unuse_pte.
> > 		 */
> >-		if (unlikely(pte_same(*pte, swp_pte))) {
> >+		if (unlikely(maybe_same_pte(*pte, swp_pte))) {
> > 			pte_unmap(pte);
> > 			ret = unuse_pte(vma, pmd, addr, entry, page);
> > 			if (ret)
> >
> >--
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> 
> --
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-- 
Kind regards,
Minchan Kim
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