[PATCHv4 06/24] mm: store mapcount for compound page separately

From: Kirill A. Shutemov
Date: Wed Mar 04 2015 - 11:38:53 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>
---
include/linux/mm.h | 16 ++++++++++++++--
include/linux/mm_types.h | 1 +
include/linux/rmap.h | 4 ++--
mm/debug.c | 5 ++++-
mm/huge_memory.c | 23 ++++++++++++++---------
mm/hugetlb.c | 4 ++--
mm/memory.c | 4 ++--
mm/migrate.c | 2 +-
mm/page_alloc.c | 7 ++++++-
mm/rmap.c | 47 +++++++++++++++++++++++++++++++++++++++--------
10 files changed, 85 insertions(+), 28 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 1aea94e837a0..b64dfe352d71 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -472,6 +472,18 @@ static inline struct page *compound_head_fast(struct page *page)
return page->first_page;
return page;
}
+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
@@ -486,7 +498,7 @@ static inline void page_mapcount_reset(struct page *page)
static inline int page_mapcount(struct page *page)
{
VM_BUG_ON_PAGE(PageSlab(page), page);
- return atomic_read(&page->_mapcount) + 1;
+ return atomic_read(&page->_mapcount) + compound_mapcount(page) + 1;
}

static inline int page_count(struct page *page)
@@ -1081,7 +1093,7 @@ 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 590630eb59ba..aefbc95148c4 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 ebe50ceacea6..ec63d3f20ca3 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -190,9 +190,9 @@ 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 3eb3ac2fcee7..13d2b8146ef9 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -83,9 +83,12 @@ 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 38c6b72cbe80..0c83451679f8 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -890,7 +890,7 @@ 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);
@@ -1787,8 +1787,8 @@ static void __split_huge_page_refcount(struct page *page,
struct page *page_tail = page + i;

/* tail_page->_mapcount cannot change */
- BUG_ON(page_mapcount(page_tail) < 0);
- tail_count += page_mapcount(page_tail);
+ BUG_ON(atomic_read(&page_tail->_mapcount) + 1 < 0);
+ tail_count += atomic_read(&page_tail->_mapcount) + 1;
/* check for overflow */
BUG_ON(tail_count < 0);
BUG_ON(atomic_read(&page_tail->_count) != 0);
@@ -1805,8 +1805,7 @@ static void __split_huge_page_refcount(struct page *page,
* atomic_set() here would be safe on all archs (and
* not only on x86), it's safer to use atomic_add().
*/
- atomic_add(page_mapcount(page) + page_mapcount(page_tail) + 1,
- &page_tail->_count);
+ atomic_add(page_mapcount(page_tail) + 1, &page_tail->_count);

/* after clearing PageTail the gup refcount can be released */
smp_mb__after_atomic();
@@ -1843,15 +1842,18 @@ static void __split_huge_page_refcount(struct page *page,
* status is achieved setting a reserved bit in the
* pmd, not by clearing the present bit.
*/
- page_tail->_mapcount = page->_mapcount;
+ atomic_set(&page_tail->_mapcount, compound_mapcount(page) - 1);

- BUG_ON(page_tail->mapping);
- page_tail->mapping = page->mapping;
+ /* ->mapping in first tail page is compound_mapcount */
+ if (i != 1) {
+ BUG_ON(page_tail->mapping);
+ page_tail->mapping = page->mapping;
+ BUG_ON(!PageAnon(page_tail));
+ }

page_tail->index = page->index + i;
page_cpupid_xchg_last(page_tail, page_cpupid_last(page));

- BUG_ON(!PageAnon(page_tail));
BUG_ON(!PageUptodate(page_tail));
BUG_ON(!PageDirty(page_tail));
BUG_ON(!PageSwapBacked(page_tail));
@@ -1861,6 +1863,9 @@ static void __split_huge_page_refcount(struct page *page,
atomic_sub(tail_count, &page->_count);
BUG_ON(atomic_read(&page->_count) <= 0);

+ page->_mapcount = *compound_mapcount_ptr(page);
+ page[1].mapping = page->mapping;
+
__mod_zone_page_state(zone, NR_ANON_TRANSPARENT_HUGEPAGES, -1);

ClearPageCompound(page);
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index ebb7329301c4..2aa2a850d002 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2606,7 +2606,7 @@ 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);
@@ -3065,7 +3065,7 @@ 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 f4fd3e078b51..c6e1a557f609 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -873,7 +873,7 @@ 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
@@ -3033,7 +3033,7 @@ static int do_shared_fault(struct mm_struct *mm, struct vm_area_struct *vma,
* pinned by vma->vm_file's reference. We rely on unlock_page()'s
* release semantics to prevent the compiler from undoing this copying.
*/
- mapping = fault_page->mapping;
+ mapping = compound_head(fault_page)->mapping;
unlock_page(fault_page);
if ((dirtied || vma->vm_ops->page_mkwrite) && mapping) {
/*
diff --git a/mm/migrate.c b/mm/migrate.c
index 0d2b3110277a..01449826b914 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 f939cb10e10d..730f6ad2ab10 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -369,6 +369,7 @@ void prep_compound_page(struct page *page, unsigned long order)

set_compound_page_dtor(page, free_compound_page);
set_compound_order(page, order);
+ atomic_set(compound_mapcount_ptr(page), -1);
__SetPageHead(page);
for (i = 1; i < nr_pages; i++) {
struct page *p = page + i;
@@ -658,7 +659,9 @@ static inline int free_pages_check(struct page *page)

if (unlikely(page_mapcount(page)))
bad_reason = "nonzero mapcount";
- if (unlikely(page->mapping != NULL))
+ if (unlikely(compound_mapcount(page)))
+ bad_reason = "nonzero compound_mapcount";
+ if (unlikely(page->mapping != NULL) && !PageTail(page))
bad_reason = "non-NULL mapping";
if (unlikely(atomic_read(&page->_count) != 0))
bad_reason = "nonzero _count";
@@ -800,6 +803,8 @@ static bool free_pages_prepare(struct page *page, unsigned int order)
}
if (bad)
return false;
+ if (order)
+ page[1].mapping = NULL;

reset_page_owner(page, order);

diff --git a/mm/rmap.c b/mm/rmap.c
index f67e83be75e4..333938475831 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1025,7 +1025,7 @@ 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
}

@@ -1054,9 +1054,26 @@ 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(compound_head(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
@@ -1074,7 +1091,8 @@ void do_page_add_anon_rmap(struct page *page,
if (unlikely(PageKsm(page)))
return;

- VM_BUG_ON_PAGE(!PageLocked(page), page);
+ VM_BUG_ON_PAGE(!PageLocked(compound_head(page)), page);
+
/* address might be in next vma when migration races vma_adjust */
if (first)
__page_set_anon_rmap(page, vma, address,
@@ -1100,10 +1118,16 @@ 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) {
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);
+ } 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);
@@ -1172,7 +1196,9 @@ void page_remove_rmap(struct page *page, bool compound)
}

/* page still mapped by someone else? */
- if (!atomic_add_negative(-1, &page->_mapcount))
+ if (!atomic_add_negative(-1, compound ?
+ compound_mapcount_ptr(page) :
+ &page->_mapcount))
return;

/* Hugepages are not counted in NR_ANON_PAGES for now. */
@@ -1185,8 +1211,13 @@ void page_remove_rmap(struct page *page, bool compound)
* pte lock(a spinlock) is held, which implies preemption disabled.
*/
if (compound) {
+ int i;
VM_BUG_ON_PAGE(!PageTransHuge(page), page);
__dec_zone_page_state(page, NR_ANON_TRANSPARENT_HUGEPAGES);
+ /* The page can be mapped with ptes */
+ for (i = 0; i < HPAGE_PMD_NR; i++)
+ if (page_mapcount(page + i))
+ nr--;
}

__mod_zone_page_state(page_zone(page), NR_ANON_PAGES, -nr);
@@ -1630,7 +1661,7 @@ 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);
}
@@ -1639,7 +1670,7 @@ 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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