[PATCHv5 19/28] mm: store mapcount for compound page separately

From: Kirill A. Shutemov
Date: Thu Apr 23 2015 - 17:08:51 EST

We're going to allow mapping of individual 4k pages of THP compound and
we need a cheap way to find out how many time the compound page is
mapped with PMD -- compound_mapcount() does this.

We use the same approach as with compound page destructor and compound
order: use space in first tail page, ->mapping this time.

page_mapcount() counts both: PTE and PMD mappings of the page.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Tested-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
---
include/linux/mm.h | 25 ++++++++++++--
include/linux/mm_types.h | 1 +
include/linux/rmap.h | 4 +--
mm/debug.c | 5 ++-
mm/huge_memory.c | 2 +-
mm/hugetlb.c | 4 +--
mm/memory.c | 2 +-
mm/migrate.c | 2 +-
mm/page_alloc.c | 14 ++++++--
mm/rmap.c | 87 +++++++++++++++++++++++++++++++++++++-----------
10 files changed, 114 insertions(+), 32 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index dad667d99304..33cb3aa647a6 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -393,6 +393,19 @@ static inline int is_vmalloc_or_module_addr(const void *x)

extern void kvfree(const void *addr);

+static inline atomic_t *compound_mapcount_ptr(struct page *page)
+{
+ return &page[1].compound_mapcount;
+}
+
+static inline int compound_mapcount(struct page *page)
+{
+ if (!PageCompound(page))
+ return 0;
+ page = compound_head(page);
+ return atomic_read(compound_mapcount_ptr(page)) + 1;
+}
+
/*
* The atomic page->_mapcount, starts from -1: so that transitions
* both from it and to it can be tracked, using atomic_inc_and_test
@@ -405,8 +418,16 @@ static inline void page_mapcount_reset(struct page *page)

static inline int page_mapcount(struct page *page)
{
+ int ret;
VM_BUG_ON_PAGE(PageSlab(page), page);
- return atomic_read(&page->_mapcount) + 1;
+ ret = atomic_read(&page->_mapcount) + 1;
+ /*
+ * Positive compound_mapcount() offsets ->_mapcount in every page by
+ * one. Let's substract it here.
+ */
+ if (compound_mapcount(page))
+ ret += compound_mapcount(page) - 1;
+ return ret;
}

static inline int page_count(struct page *page)
@@ -888,7 +909,7 @@ static inline pgoff_t page_file_index(struct page *page)
*/
static inline int page_mapped(struct page *page)
{
- return atomic_read(&(page)->_mapcount) >= 0;
+ return atomic_read(&(page)->_mapcount) + compound_mapcount(page) >= 0;
}

/*
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 126f481bb95a..c8485fe2381c 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -56,6 +56,7 @@ struct page {
* see PAGE_MAPPING_ANON below.
*/
void *s_mem; /* slab first object */
+ atomic_t compound_mapcount; /* first tail page */
};

/* Second double word */
diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index e7ecba43ae71..bb16ec73eeb7 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -181,9 +181,9 @@ void hugepage_add_anon_rmap(struct page *, struct vm_area_struct *,
void hugepage_add_new_anon_rmap(struct page *, struct vm_area_struct *,
unsigned long);

-static inline void page_dup_rmap(struct page *page)
+static inline void page_dup_rmap(struct page *page, bool compound)
{
- atomic_inc(&page->_mapcount);
+ atomic_inc(compound ? compound_mapcount_ptr(page) : &page->_mapcount);
}

/*
diff --git a/mm/debug.c b/mm/debug.c
index 9dfcd77e7354..4a82f639b964 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -80,9 +80,12 @@ static void dump_flags(unsigned long flags,
void dump_page_badflags(struct page *page, const char *reason,
unsigned long badflags)
{
- pr_emerg("page:%p count:%d mapcount:%d mapping:%p index:%#lx\n",
+ pr_emerg("page:%p count:%d mapcount:%d mapping:%p index:%#lx",
page, atomic_read(&page->_count), page_mapcount(page),
page->mapping, page->index);
+ if (PageCompound(page))
+ pr_cont(" compound_mapcount: %d", compound_mapcount(page));
+ pr_cont("\n");
BUILD_BUG_ON(ARRAY_SIZE(pageflag_names) != __NR_PAGEFLAGS);
dump_flags(page->flags, pageflag_names, ARRAY_SIZE(pageflag_names));
if (reason)
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 23181f836b62..06adbe3f2100 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -892,7 +892,7 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
src_page = pmd_page(pmd);
VM_BUG_ON_PAGE(!PageHead(src_page), src_page);
get_page(src_page);
- page_dup_rmap(src_page);
+ page_dup_rmap(src_page, true);
add_mm_counter(dst_mm, MM_ANONPAGES, HPAGE_PMD_NR);

pmdp_set_wrprotect(src_mm, addr, src_pmd);
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index f27d4edada3a..94d70a16395e 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2715,7 +2715,7 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
entry = huge_ptep_get(src_pte);
ptepage = pte_page(entry);
get_page(ptepage);
- page_dup_rmap(ptepage);
+ page_dup_rmap(ptepage, true);
set_huge_pte_at(dst, addr, dst_pte, entry);
}
spin_unlock(src_ptl);
@@ -3176,7 +3176,7 @@ retry:
ClearPagePrivate(page);
hugepage_add_new_anon_rmap(page, vma, address);
} else
- page_dup_rmap(page);
+ page_dup_rmap(page, true);
new_pte = make_huge_pte(vma, page, ((vma->vm_flags & VM_WRITE)
&& (vma->vm_flags & VM_SHARED)));
set_huge_pte_at(mm, address, ptep, new_pte);
diff --git a/mm/memory.c b/mm/memory.c
index 1bad3766b00c..0b295f7094b1 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -864,7 +864,7 @@ copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
page = vm_normal_page(vma, addr, pte);
if (page) {
get_page(page);
- page_dup_rmap(page);
+ page_dup_rmap(page, false);
if (PageAnon(page))
rss[MM_ANONPAGES]++;
else
diff --git a/mm/migrate.c b/mm/migrate.c
index 9a380238a4d0..b51e88c9dba2 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -164,7 +164,7 @@ static int remove_migration_pte(struct page *new, struct vm_area_struct *vma,
if (PageAnon(new))
hugepage_add_anon_rmap(new, vma, addr);
else
- page_dup_rmap(new);
+ page_dup_rmap(new, false);
} else if (PageAnon(new))
page_add_anon_rmap(new, vma, addr, false);
else
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index df2e25424b71..ac331be78308 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -378,6 +378,7 @@ void prep_compound_page(struct page *page, unsigned long order)
smp_wmb();
__SetPageTail(p);
}
+ atomic_set(compound_mapcount_ptr(page), -1);
}

static inline void prep_zero_page(struct page *page, unsigned int order,
@@ -656,7 +657,7 @@ static inline int free_pages_check(struct page *page)
const char *bad_reason = NULL;
unsigned long bad_flags = 0;

- if (unlikely(page_mapcount(page)))
+ if (unlikely(atomic_read(&page->_mapcount) != -1))
bad_reason = "nonzero mapcount";
if (unlikely(page->mapping != NULL))
bad_reason = "non-NULL mapping";
@@ -765,7 +766,14 @@ static void free_one_page(struct zone *zone,

static int free_tail_pages_check(struct page *head_page, struct page *page)
{
- if (page->mapping != TAIL_MAPPING) {
+ /* mapping in first tail page is used for compound_mapcount() */
+ if (page - head_page == 1) {
+ if (unlikely(compound_mapcount(page))) {
+ bad_page(page, "nonzero compound_mapcount", 0);
+ page->mapping = NULL;
+ return 1;
+ }
+ } else if (page->mapping != TAIL_MAPPING) {
bad_page(page, "corrupted mapping in tail page", 0);
page->mapping = NULL;
return 1;
@@ -940,7 +948,7 @@ static inline int check_new_page(struct page *page)
const char *bad_reason = NULL;
unsigned long bad_flags = 0;

- if (unlikely(page_mapcount(page)))
+ if (unlikely(atomic_read(&page->_mapcount) != -1))
bad_reason = "nonzero mapcount";
if (unlikely(page->mapping != NULL))
bad_reason = "non-NULL mapping";
diff --git a/mm/rmap.c b/mm/rmap.c
index 1636a96e5f71..047953145710 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1030,7 +1030,7 @@ static void __page_check_anon_rmap(struct page *page,
* over the call to page_add_new_anon_rmap.
*/
BUG_ON(page_anon_vma(page)->root != vma->anon_vma->root);
- BUG_ON(page->index != linear_page_index(vma, address));
+ BUG_ON(page_to_pgoff(page) != linear_page_index(vma, address));
#endif
}

@@ -1059,9 +1059,26 @@ void page_add_anon_rmap(struct page *page,
void do_page_add_anon_rmap(struct page *page,
struct vm_area_struct *vma, unsigned long address, int flags)
{
- int first = atomic_inc_and_test(&page->_mapcount);
+ bool compound = flags & RMAP_COMPOUND;
+ bool first;
+
+ if (PageTransCompound(page)) {
+ VM_BUG_ON_PAGE(!PageLocked(page), page);
+ if (compound) {
+ VM_BUG_ON_PAGE(!PageTransHuge(page), page);
+ first = atomic_inc_and_test(compound_mapcount_ptr(page));
+ } else {
+ /* Anon THP always mapped first with PMD */
+ first = 0;
+ VM_BUG_ON_PAGE(!page_mapcount(page), page);
+ atomic_inc(&page->_mapcount);
+ }
+ } else {
+ VM_BUG_ON_PAGE(compound, page);
+ first = atomic_inc_and_test(&page->_mapcount);
+ }
+
if (first) {
- bool compound = flags & RMAP_COMPOUND;
int nr = compound ? hpage_nr_pages(page) : 1;
/*
* We use the irq-unsafe __{inc|mod}_zone_page_stat because
@@ -1070,9 +1087,17 @@ void do_page_add_anon_rmap(struct page *page,
* disabled.
*/
if (compound) {
+ int i;
VM_BUG_ON_PAGE(!PageTransHuge(page), page);
__inc_zone_page_state(page,
NR_ANON_TRANSPARENT_HUGEPAGES);
+ /*
+ * While compound_mapcount() is positive we keep *one*
+ * mapcount reference in all subpages. It's required
+ * for atomic removal from rmap.
+ */
+ for (i = 0; i < nr; i++)
+ atomic_set(&page[i]._mapcount, 0);
}
__mod_zone_page_state(page_zone(page), NR_ANON_PAGES, nr);
}
@@ -1080,6 +1105,7 @@ void do_page_add_anon_rmap(struct page *page,
return;

VM_BUG_ON_PAGE(!PageLocked(page), page);
+
/* address might be in next vma when migration races vma_adjust */
if (first)
__page_set_anon_rmap(page, vma, address,
@@ -1105,10 +1131,25 @@ void page_add_new_anon_rmap(struct page *page,

VM_BUG_ON_VMA(address < vma->vm_start || address >= vma->vm_end, vma);
SetPageSwapBacked(page);
- atomic_set(&page->_mapcount, 0); /* increment count (starts at -1) */
if (compound) {
+ int i;
+
VM_BUG_ON_PAGE(!PageTransHuge(page), page);
+ /* increment count (starts at -1) */
+ atomic_set(compound_mapcount_ptr(page), 0);
__inc_zone_page_state(page, NR_ANON_TRANSPARENT_HUGEPAGES);
+ /*
+ * While compound_mapcount() is positive we keep *one* mapcount
+ * reference in all subpages. It's required for atomic removal
+ * from rmap.
+ */
+ for (i = 0; i < nr; i++)
+ atomic_set(&page[i]._mapcount, 0);
+ } else {
+ /* Anon THP always mapped first with PMD */
+ VM_BUG_ON_PAGE(PageTransCompound(page), page);
+ /* increment count (starts at -1) */
+ atomic_set(&page->_mapcount, 0);
}
__mod_zone_page_state(page_zone(page), NR_ANON_PAGES, nr);
__page_set_anon_rmap(page, vma, address, 1);
@@ -1138,12 +1179,15 @@ static void page_remove_file_rmap(struct page *page)

memcg = mem_cgroup_begin_page_stat(page);

- /* page still mapped by someone else? */
- if (!atomic_add_negative(-1, &page->_mapcount))
+ /* Hugepages are not counted in NR_FILE_MAPPED for now. */
+ if (unlikely(PageHuge(page))) {
+ /* hugetlb pages are always mapped with pmds */
+ atomic_dec(compound_mapcount_ptr(page));
goto out;
+ }

- /* Hugepages are not counted in NR_FILE_MAPPED for now. */
- if (unlikely(PageHuge(page)))
+ /* page still mapped by someone else? */
+ if (!atomic_add_negative(-1, &page->_mapcount))
goto out;

/*
@@ -1168,8 +1212,6 @@ out:
*/
void page_remove_rmap(struct page *page, bool compound)
{
- int nr = compound ? hpage_nr_pages(page) : 1;
-
if (!PageAnon(page)) {
VM_BUG_ON_PAGE(compound && !PageHuge(page), page);
page_remove_file_rmap(page);
@@ -1177,8 +1219,20 @@ void page_remove_rmap(struct page *page, bool compound)
}

/* page still mapped by someone else? */
- if (!atomic_add_negative(-1, &page->_mapcount))
+ if (compound) {
+ int i;
+
+ VM_BUG_ON_PAGE(!PageTransHuge(page), page);
+ if (!atomic_add_negative(-1, compound_mapcount_ptr(page)))
+ return;
+ __dec_zone_page_state(page, NR_ANON_TRANSPARENT_HUGEPAGES);
+ for (i = 0; i < hpage_nr_pages(page); i++)
+ page_remove_rmap(page + i, false);
return;
+ } else {
+ if (!atomic_add_negative(-1, &page->_mapcount))
+ return;
+ }

/* Hugepages are not counted in NR_ANON_PAGES for now. */
if (unlikely(PageHuge(page)))
@@ -1189,12 +1243,7 @@ void page_remove_rmap(struct page *page, bool compound)
* these counters are not modified in interrupt context, and
* pte lock(a spinlock) is held, which implies preemption disabled.
*/
- if (compound) {
- VM_BUG_ON_PAGE(!PageTransHuge(page), page);
- __dec_zone_page_state(page, NR_ANON_TRANSPARENT_HUGEPAGES);
- }
-
- __mod_zone_page_state(page_zone(page), NR_ANON_PAGES, -nr);
+ __dec_zone_page_state(page, NR_ANON_PAGES);

if (unlikely(PageMlocked(page)))
clear_page_mlock(page);
@@ -1635,7 +1684,7 @@ void hugepage_add_anon_rmap(struct page *page,
BUG_ON(!PageLocked(page));
BUG_ON(!anon_vma);
/* address might be in next vma when migration races vma_adjust */
- first = atomic_inc_and_test(&page->_mapcount);
+ first = atomic_inc_and_test(compound_mapcount_ptr(page));
if (first)
__hugepage_set_anon_rmap(page, vma, address, 0);
}
@@ -1644,7 +1693,7 @@ void hugepage_add_new_anon_rmap(struct page *page,
struct vm_area_struct *vma, unsigned long address)
{
BUG_ON(address < vma->vm_start || address >= vma->vm_end);
- atomic_set(&page->_mapcount, 0);
+ atomic_set(compound_mapcount_ptr(page), 0);
__hugepage_set_anon_rmap(page, vma, address, 1);
}
#endif /* CONFIG_HUGETLB_PAGE */
--
2.1.4

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