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

[Minchan Kim <minchan@kernel.org>]

On Mon, Aug 05, 2013 at 10:38:58AM +0800, Wanpeng Li wrote:
> Hi Minchan,
> 
> On Mon, Aug 05, 2013 at 11:17:15AM +0900, Minchan Kim wrote:
> >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@amacapital.net>
> >> >Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
> >> >Cc: Pavel Emelyanov <xemul@parallels.com>
> >> >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>
> >> >Cc: Peter Zijlstra <peterz@infradead.org>
> >> >Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
> >> >---
> >> > 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.
> >
> 
> I see your discussion with Cyrill, however, pte_to_swp_entry and pte_swp_soft_dirty
> both against orig_pte, where I miss? ;-)

pte_to_swp_entry is passed orig_pte by vaule, not a pointer
so although pte_to_swp_entry clear out _PTE_SWP_SOFT_DIRTY, it does it in local-copy.
So orig_pte is never changed.

> 
> >> 
> >> > 	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)
> >> >
> >> >--
> >> >To unsubscribe, send a message with 'unsubscribe linux-mm' in
> >> >the body to majordomo@kvack.org.  For more info on Linux MM,
> >> >see: http://www.linux-mm.org/ .
> >> >Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
> >> 
> >> --
> >> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> >> the body to majordomo@kvack.org.  For more info on Linux MM,
> >> see: http://www.linux-mm.org/ .
> >> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
> >
> >-- 
> >Kind regards,
> >Minchan Kim
> 
> --
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-- 
Kind regards,
Minchan Kim

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