Re: [PATCH v2] smaps should deal with huge zero page exactly same as normal zero page.

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Tue, Oct 28, 2014 at 11:18:38PM +0800, Fengwei Yin wrote:
> On Mon, Oct 27, 2014 at 03:17:48PM -0700, Andrew Morton wrote:
> > On Mon, 27 Oct 2014 23:02:13 +0800 Fengwei Yin <yfw.kernel@gmail.com> wrote:
> > 
> > > We could see following memory info in /proc/xxxx/smaps with THP enabled.
> > >   7bea458b3000-7fea458b3000 r--p 00000000 00:13 39989  /dev/zero
> > >   Size:           4294967296 kB
> > >   Rss:            10612736 kB
> > >   Pss:            10612736 kB
> > >   Shared_Clean:          0 kB
> > >   Shared_Dirty:          0 kB
> > >   Private_Clean:  10612736 kB
> > >   Private_Dirty:         0 kB
> > >   Referenced:     10612736 kB
> > >   Anonymous:             0 kB
> > >   AnonHugePages:  10612736 kB
> > >   Swap:                  0 kB
> > >   KernelPageSize:        4 kB
> > >   MMUPageSize:           4 kB
> > >   Locked:                0 kB
> > >   VmFlags: rd mr mw me
> > > which is wrong becuase just huge_zero_page/normal_zero_page is used for
> > > /dev/zero. Most of the value should be 0.
> > > 
> > > This patch detects huge_zero_page (original implementation just detect
> > > normal_zero_page) and avoids to update the wrong value for huge_zero_page.
> > > 
> > > ...
> > >
> > > --- a/mm/memory.c
> > > +++ b/mm/memory.c
> > > @@ -41,6 +41,7 @@
> > >  #include <linux/kernel_stat.h>
> > >  #include <linux/mm.h>
> > >  #include <linux/hugetlb.h>
> > > +#include <linux/huge_mm.h>
> > >  #include <linux/mman.h>
> > >  #include <linux/swap.h>
> > >  #include <linux/highmem.h>
> > > @@ -787,6 +788,9 @@ check_pfn:
> > >  		return NULL;
> > >  	}
> > >  
> > > +	if (is_huge_zero_pfn(pfn))
> > > +		return NULL;
> > > +
> > 
> > Why this change?
> > 
> I suppose the huge zero page should have same behavior as normal zero
> page. vm_normal_page will return NULL if the pte is for normal zero
> page. This change make it return NULL for huge zero page.
> 
> > What effect does it have upon vm_normal_page()'s many existing callers?
> This is good question. I suppose it will not impact existing caller.

vm_normal_page() is designed to handle pte. We only get there due hack
with pmd to pte cast in smaps_pte_range(). Let's try to get rid of it
instead.

Could you test the patch below? I think it's a better fix.

From 592a7e789128d92e7f5165b583558443e82c88fd Mon Sep 17 00:00:00 2001
From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Date: Tue, 28 Oct 2014 14:51:31 +0200
Subject: [PATCH] mm: fix huge zero page accounting in smaps report

As a small zero page, huge zero page should not be accounted in smaps
report as normal page.

For small pages we rely on vm_normal_page() to filter out zero page, but
vm_normal_page() is not designed to handle pmds. We only get here due
hackish cast pmd to pte in smaps_pte_range() -- pte and pmd format is
not necessary compatible on each and every architecture.

Let's add separate codepath to handle pmds. follow_trans_huge_pmd() will
detect huge zero page for us.

We would need pmd_dirty() helper to do this properly. The patch adds it
to THP-enabled architectures which don't yet have one.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 arch/arm64/include/asm/pgtable.h         |   1 +
 arch/powerpc/include/asm/pgtable-ppc64.h |   1 +
 arch/sparc/include/asm/pgtable_64.h      |   7 +++
 arch/x86/include/asm/pgtable.h           |   5 ++
 fs/proc/task_mmu.c                       | 101 ++++++++++++++++++++-----------
 5 files changed, 79 insertions(+), 36 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 41a43bf26492..df22314f57cf 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -279,6 +279,7 @@ void pmdp_splitting_flush(struct vm_area_struct *vma, unsigned long address,
 #endif /* CONFIG_HAVE_RCU_TABLE_FREE */
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
 
+#define pmd_dirty(pmd)		pte_dirty(pmd_pte(pmd))
 #define pmd_young(pmd)		pte_young(pmd_pte(pmd))
 #define pmd_wrprotect(pmd)	pte_pmd(pte_wrprotect(pmd_pte(pmd)))
 #define pmd_mksplitting(pmd)	pte_pmd(pte_mkspecial(pmd_pte(pmd)))
diff --git a/arch/powerpc/include/asm/pgtable-ppc64.h b/arch/powerpc/include/asm/pgtable-ppc64.h
index ae153c40ab7c..9b4b1904efae 100644
--- a/arch/powerpc/include/asm/pgtable-ppc64.h
+++ b/arch/powerpc/include/asm/pgtable-ppc64.h
@@ -467,6 +467,7 @@ static inline pte_t *pmdp_ptep(pmd_t *pmd)
 }
 
 #define pmd_pfn(pmd)		pte_pfn(pmd_pte(pmd))
+#define pmd_dirty(pmd)		pte_dirty(pmd_pte(pmd))
 #define pmd_young(pmd)		pte_young(pmd_pte(pmd))
 #define pmd_mkold(pmd)		pte_pmd(pte_mkold(pmd_pte(pmd)))
 #define pmd_wrprotect(pmd)	pte_pmd(pte_wrprotect(pmd_pte(pmd)))
diff --git a/arch/sparc/include/asm/pgtable_64.h b/arch/sparc/include/asm/pgtable_64.h
index bfeb626085ac..90af17ee6184 100644
--- a/arch/sparc/include/asm/pgtable_64.h
+++ b/arch/sparc/include/asm/pgtable_64.h
@@ -667,6 +667,13 @@ static inline unsigned long pmd_pfn(pmd_t pmd)
 }
 
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
+static inline pmd_t pmd_dirty(pmd_t pmd)
+{
+	pte_t pte = __pte(pmd_val(pmd));
+
+	return pte_dirty(pte);
+}
+
 static inline unsigned long pmd_young(pmd_t pmd)
 {
 	pte_t pte = __pte(pmd_val(pmd));
diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index aa97a070f09f..081d6f45e006 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -99,6 +99,11 @@ static inline int pte_young(pte_t pte)
 	return pte_flags(pte) & _PAGE_ACCESSED;
 }
 
+static inline int pmd_dirty(pmd_t pmd)
+{
+	return pmd_flags(pmd) & _PAGE_DIRTY;
+}
+
 static inline int pmd_young(pmd_t pmd)
 {
 	return pmd_flags(pmd) & _PAGE_ACCESSED;
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 4e0388cffe3d..2ab200d429be 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -447,58 +447,88 @@ struct mem_size_stats {
 	u64 pss;
 };
 
+static void smaps_account(struct mem_size_stats *mss, struct page *page,
+		unsigned long size, bool young, bool dirty)
+{
+	int mapcount;
+
+	if (PageAnon(page))
+		mss->anonymous += size;
 
-static void smaps_pte_entry(pte_t ptent, unsigned long addr,
-		unsigned long ptent_size, struct mm_walk *walk)
+	mss->resident += size;
+	/* Accumulate the size in pages that have been accessed. */
+	if (young || PageReferenced(page))
+		mss->referenced += size;
+	mapcount = page_mapcount(page);
+	if (mapcount >= 2) {
+		if (dirty || PageDirty(page))
+			mss->shared_dirty += size;
+		else
+			mss->shared_clean += size;
+		mss->pss += (size << PSS_SHIFT) / mapcount;
+	} else {
+		if (dirty || PageDirty(page))
+			mss->private_dirty += size;
+		else
+			mss->private_clean += size;
+		mss->pss += (size << PSS_SHIFT);
+	}
+}
+
+
+static void smaps_pte_entry(pte_t *pte, unsigned long addr,
+		struct mm_walk *walk)
 {
 	struct mem_size_stats *mss = walk->private;
 	struct vm_area_struct *vma = mss->vma;
 	pgoff_t pgoff = linear_page_index(vma, addr);
 	struct page *page = NULL;
-	int mapcount;
 
-	if (pte_present(ptent)) {
-		page = vm_normal_page(vma, addr, ptent);
-	} else if (is_swap_pte(ptent)) {
-		swp_entry_t swpent = pte_to_swp_entry(ptent);
+	if (pte_present(*pte)) {
+		page = vm_normal_page(vma, addr, *pte);
+	} else if (is_swap_pte(*pte)) {
+		swp_entry_t swpent = pte_to_swp_entry(*pte);
 
 		if (!non_swap_entry(swpent))
-			mss->swap += ptent_size;
+			mss->swap += PAGE_SIZE;
 		else if (is_migration_entry(swpent))
 			page = migration_entry_to_page(swpent);
-	} else if (pte_file(ptent)) {
-		if (pte_to_pgoff(ptent) != pgoff)
-			mss->nonlinear += ptent_size;
+	} else if (pte_file(*pte)) {
+		if (pte_to_pgoff(*pte) != pgoff)
+			mss->nonlinear += PAGE_SIZE;
 	}
 
 	if (!page)
 		return;
 
-	if (PageAnon(page))
-		mss->anonymous += ptent_size;
-
 	if (page->index != pgoff)
-		mss->nonlinear += ptent_size;
+		mss->nonlinear += PAGE_SIZE;
 
-	mss->resident += ptent_size;
-	/* Accumulate the size in pages that have been accessed. */
-	if (pte_young(ptent) || PageReferenced(page))
-		mss->referenced += ptent_size;
-	mapcount = page_mapcount(page);
-	if (mapcount >= 2) {
-		if (pte_dirty(ptent) || PageDirty(page))
-			mss->shared_dirty += ptent_size;
-		else
-			mss->shared_clean += ptent_size;
-		mss->pss += (ptent_size << PSS_SHIFT) / mapcount;
-	} else {
-		if (pte_dirty(ptent) || PageDirty(page))
-			mss->private_dirty += ptent_size;
-		else
-			mss->private_clean += ptent_size;
-		mss->pss += (ptent_size << PSS_SHIFT);
-	}
+	smaps_account(mss, page, PAGE_SIZE, pte_young(*pte), pte_dirty(*pte));
+}
+
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
+		struct mm_walk *walk)
+{
+	struct mem_size_stats *mss = walk->private;
+	struct vm_area_struct *vma = mss->vma;
+	struct page *page;
+
+	/* FOLL_DUMP will return -EFAULT on huge zero page */
+	page = follow_trans_huge_pmd(vma, addr, pmd, FOLL_DUMP);
+	if (IS_ERR_OR_NULL(page))
+		return;
+	mss->anonymous_thp += HPAGE_PMD_SIZE;
+	smaps_account(mss, page, HPAGE_PMD_SIZE,
+			pmd_young(*pmd), pmd_dirty(*pmd));
 }
+#else
+static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
+		struct mm_walk *walk)
+{
+}
+#endif
 
 static int smaps_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
 			   struct mm_walk *walk)
@@ -509,9 +539,8 @@ static int smaps_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
 	spinlock_t *ptl;
 
 	if (pmd_trans_huge_lock(pmd, vma, &ptl) == 1) {
-		smaps_pte_entry(*(pte_t *)pmd, addr, HPAGE_PMD_SIZE, walk);
+		smaps_pmd_entry(pmd, addr, walk);
 		spin_unlock(ptl);
-		mss->anonymous_thp += HPAGE_PMD_SIZE;
 		return 0;
 	}
 
@@ -524,7 +553,7 @@ static int smaps_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
 	 */
 	pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
 	for (; addr != end; pte++, addr += PAGE_SIZE)
-		smaps_pte_entry(*pte, addr, PAGE_SIZE, walk);
+		smaps_pte_entry(pte, addr, walk);
 	pte_unmap_unlock(pte - 1, ptl);
 	cond_resched();
 	return 0;
-- 
 Kirill A. Shutemov

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