[PATCH v2 3/7] zsmalloc: support compaction

Minchan Kim
2015-03-04T05:02:43Z

This patch provides core functions for migration of zsmalloc.
Migraion policy is simple as follows.

for each size class {
while {
src_page = get zs_page from ZS_ALMOST_EMPTY
if (!src_page)
break;
dst_page = get zs_page from ZS_ALMOST_FULL
if (!dst_page)
dst_page = get zs_page from ZS_ALMOST_EMPTY
if (!dst_page)
break;
migrate(from src_page, to dst_page);
}
}

For migration, we need to identify which objects in zspage are
allocated to migrate them out. We could know it by iterating
of freed objects in a zspage because first_page of zspage keeps
free objects singly-linked list but it's not efficient.
Instead, this patch adds a tag(ie, OBJ_ALLOCATED_TAG) in header
of each object(ie, handle) so we could check whether the object
is allocated easily.

This patch adds another status bit in handle to synchronize
between user access through zs_map_object and migration.
During migration, we cannot move objects user are using due to
data coherency between old object and new object.

Signed-off-by: Minchan Kim <minchan@kernel.org>
---
include/linux/zsmalloc.h | 1 +
mm/zsmalloc.c | 377 ++++++++++++++++++++++++++++++++++++++++++++---
2 files changed, 359 insertions(+), 19 deletions(-)

diff --git a/include/linux/zsmalloc.h b/include/linux/zsmalloc.h
index 3283c6a55425..1338190b5478 100644
--- a/include/linux/zsmalloc.h
+++ b/include/linux/zsmalloc.h
@@ -47,5 +47,6 @@ void *zs_map_object(struct zs_pool *pool, unsigned long handle,
void zs_unmap_object(struct zs_pool *pool, unsigned long handle);

unsigned long zs_get_total_pages(struct zs_pool *pool);
+unsigned long zs_compact(struct zs_pool *pool);

#endif
diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index e52b1b6f5535..24a581c48aea 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -135,7 +135,26 @@
#endif
#endif
#define _PFN_BITS (MAX_PHYSMEM_BITS - PAGE_SHIFT)
-#define OBJ_INDEX_BITS (BITS_PER_LONG - _PFN_BITS)
+
+/*
+ * Memory for allocating for handle keeps object position by
+ * encoding <page, obj_idx> and the encoded value has a room
+ * in least bit(ie, look at obj_to_location).
+ * We use the bit to synchronize between object access by
+ * user and migration.
+ */
+#define HANDLE_PIN_BIT 0
+
+/*
+ * Head in allocated object should have OBJ_ALLOCATED_TAG
+ * to identify the object was allocated or not.
+ * It's okay to add the status bit in the least bit because
+ * header keeps handle which is 4byte-aligned address so we
+ * have room for two bit at least.
+ */
+#define OBJ_ALLOCATED_TAG 1
+#define OBJ_TAG_BITS 1
+#define OBJ_INDEX_BITS (BITS_PER_LONG - _PFN_BITS - OBJ_TAG_BITS)
#define OBJ_INDEX_MASK ((_AC(1, UL) << OBJ_INDEX_BITS) - 1)

#define MAX(a, b) ((a) >= (b) ? (a) : (b))
@@ -610,35 +629,35 @@ static struct page *get_next_page(struct page *page)

/*
* Encode <page, obj_idx> as a single handle value.
- * On hardware platforms with physical memory starting at 0x0 the pfn
- * could be 0 so we ensure that the handle will never be 0 by adjusting the
- * encoded obj_idx value before encoding.
+ * We use the least bit of handle for tagging.
*/
-static void *obj_location_to_handle(struct page *page, unsigned long obj_idx)
+static void *location_to_obj(struct page *page, unsigned long obj_idx)
{
- unsigned long handle;
+ unsigned long obj;

if (!page) {
BUG_ON(obj_idx);
return NULL;
}

- handle = page_to_pfn(page) << OBJ_INDEX_BITS;
- handle |= ((obj_idx + 1) & OBJ_INDEX_MASK);
+ obj = page_to_pfn(page) << OBJ_INDEX_BITS;
+ obj |= ((obj_idx) & OBJ_INDEX_MASK);
+ obj <<= OBJ_TAG_BITS;

- return (void *)handle;
+ return (void *)obj;
}

/*
* Decode <page, obj_idx> pair from the given object handle. We adjust the
* decoded obj_idx back to its original value since it was adjusted in
- * obj_location_to_handle().
+ * location_to_obj().
*/
-static void obj_to_location(unsigned long handle, struct page **page,
+static void obj_to_location(unsigned long obj, struct page **page,
unsigned long *obj_idx)
{
- *page = pfn_to_page(handle >> OBJ_INDEX_BITS);
- *obj_idx = (handle & OBJ_INDEX_MASK) - 1;
+ obj >>= OBJ_TAG_BITS;
+ *page = pfn_to_page(obj >> OBJ_INDEX_BITS);
+ *obj_idx = (obj & OBJ_INDEX_MASK);
}

static unsigned long handle_to_obj(unsigned long handle)
@@ -646,6 +665,11 @@ static unsigned long handle_to_obj(unsigned long handle)
return *(unsigned long *)handle;
}

+unsigned long obj_to_head(void *obj)
+{
+ return *(unsigned long *)obj;
+}
+
static unsigned long obj_idx_to_offset(struct page *page,
unsigned long obj_idx, int class_size)
{
@@ -657,6 +681,25 @@ static unsigned long obj_idx_to_offset(struct page *page,
return off + obj_idx * class_size;
}

+static inline int trypin_tag(unsigned long handle)
+{
+ unsigned long *ptr = (unsigned long *)handle;
+
+ return !test_and_set_bit_lock(HANDLE_PIN_BIT, ptr);
+}
+
+static void pin_tag(unsigned long handle)
+{
+ while (!trypin_tag(handle));
+}
+
+static void unpin_tag(unsigned long handle)
+{
+ unsigned long *ptr = (unsigned long *)handle;
+
+ clear_bit_unlock(HANDLE_PIN_BIT, ptr);
+}
+
static void reset_page(struct page *page)
{
clear_bit(PG_private, &page->flags);
@@ -718,7 +761,7 @@ static void init_zspage(struct page *first_page, struct size_class *class)
link = (struct link_free *)vaddr + off / sizeof(*link);

while ((off += class->size) < PAGE_SIZE) {
- link->next = obj_location_to_handle(page, i++);
+ link->next = location_to_obj(page, i++);
link += class->size / sizeof(*link);
}

@@ -728,7 +771,7 @@ static void init_zspage(struct page *first_page, struct size_class *class)
* page (if present)
*/
next_page = get_next_page(page);
- link->next = obj_location_to_handle(next_page, 0);
+ link->next = location_to_obj(next_page, 0);
kunmap_atomic(vaddr);
page = next_page;
off %= PAGE_SIZE;
@@ -782,7 +825,7 @@ static struct page *alloc_zspage(struct size_class *class, gfp_t flags)

init_zspage(first_page, class);

- first_page->freelist = obj_location_to_handle(first_page, 0);
+ first_page->freelist = location_to_obj(first_page, 0);
/* Maximum number of objects we can store in this zspage */
first_page->objects = class->pages_per_zspage * PAGE_SIZE / class->size;

@@ -1017,6 +1060,13 @@ static bool can_merge(struct size_class *prev, int size, int pages_per_zspage)
return true;
}

+static bool zspage_full(struct page *page)
+{
+ BUG_ON(!is_first_page(page));
+
+ return page->inuse == page->objects;
+}
+
#ifdef CONFIG_ZSMALLOC_STAT

static inline void zs_stat_inc(struct size_class *class,
@@ -1219,6 +1269,9 @@ void *zs_map_object(struct zs_pool *pool, unsigned long handle,
*/
BUG_ON(in_interrupt());

+ /* From now on, migration cannot move the object */
+ pin_tag(handle);
+
obj = handle_to_obj(handle);
obj_to_location(obj, &page, &obj_idx);
get_zspage_mapping(get_first_page(page), &class_idx, &fg);
@@ -1276,6 +1329,7 @@ void zs_unmap_object(struct zs_pool *pool, unsigned long handle)
__zs_unmap_object(area, pages, off, class->size);
}
put_cpu_var(zs_map_area);
+ unpin_tag(handle);
}
EXPORT_SYMBOL_GPL(zs_unmap_object);

@@ -1289,6 +1343,7 @@ static unsigned long obj_malloc(struct page *first_page,
unsigned long m_objidx, m_offset;
void *vaddr;

+ handle |= OBJ_ALLOCATED_TAG;
obj = (unsigned long)first_page->freelist;
obj_to_location(obj, &m_page, &m_objidx);
m_offset = obj_idx_to_offset(m_page, m_objidx, class->size);
@@ -1374,6 +1429,7 @@ static void obj_free(struct zs_pool *pool, struct size_class *class,

BUG_ON(!obj);

+ obj &= ~OBJ_ALLOCATED_TAG;
obj_to_location(obj, &f_page, &f_objidx);
first_page = get_first_page(f_page);

@@ -1403,8 +1459,8 @@ void zs_free(struct zs_pool *pool, unsigned long handle)
if (unlikely(!handle))
return;

+ pin_tag(handle);
obj = handle_to_obj(handle);
- free_handle(pool, handle);
obj_to_location(obj, &f_page, &f_objidx);
first_page = get_first_page(f_page);

@@ -1414,18 +1470,301 @@ void zs_free(struct zs_pool *pool, unsigned long handle)
spin_lock(&class->lock);
obj_free(pool, class, obj);
fullness = fix_fullness_group(class, first_page);
- if (fullness == ZS_EMPTY)
+ if (fullness == ZS_EMPTY) {
zs_stat_dec(class, OBJ_ALLOCATED, get_maxobj_per_zspage(
class->size, class->pages_per_zspage));
+ atomic_long_sub(class->pages_per_zspage,
+ &pool->pages_allocated);
+ free_zspage(first_page);
+ }
spin_unlock(&class->lock);
+ unpin_tag(handle);
+
+ free_handle(pool, handle);
+}
+EXPORT_SYMBOL_GPL(zs_free);
+
+static void zs_object_copy(unsigned long src, unsigned long dst,
+ struct size_class *class)
+{
+ struct page *s_page, *d_page;
+ unsigned long s_objidx, d_objidx;
+ unsigned long s_off, d_off;
+ void *s_addr, *d_addr;
+ int s_size, d_size, size;
+ int written = 0;
+
+ s_size = d_size = class->size;
+
+ obj_to_location(src, &s_page, &s_objidx);
+ obj_to_location(dst, &d_page, &d_objidx);
+
+ s_off = obj_idx_to_offset(s_page, s_objidx, class->size);
+ d_off = obj_idx_to_offset(d_page, d_objidx, class->size);
+
+ if (s_off + class->size > PAGE_SIZE)
+ s_size = PAGE_SIZE - s_off;
+
+ if (d_off + class->size > PAGE_SIZE)
+ d_size = PAGE_SIZE - d_off;
+
+ s_addr = kmap_atomic(s_page);
+ d_addr = kmap_atomic(d_page);
+
+ while (1) {
+ size = min(s_size, d_size);
+ memcpy(d_addr + d_off, s_addr + s_off, size);
+ written += size;
+
+ if (written == class->size)
+ break;
+
+ if (s_off + size >= PAGE_SIZE) {
+ kunmap_atomic(d_addr);
+ kunmap_atomic(s_addr);
+ s_page = get_next_page(s_page);
+ BUG_ON(!s_page);
+ s_addr = kmap_atomic(s_page);
+ d_addr = kmap_atomic(d_page);
+ s_size = class->size - written;
+ s_off = 0;
+ } else {
+ s_off += size;
+ s_size -= size;
+ }
+
+ if (d_off + size >= PAGE_SIZE) {
+ kunmap_atomic(d_addr);
+ d_page = get_next_page(d_page);
+ BUG_ON(!d_page);
+ d_addr = kmap_atomic(d_page);
+ d_size = class->size - written;
+ d_off = 0;
+ } else {
+ d_off += size;
+ d_size -= size;
+ }
+ }
+
+ kunmap_atomic(d_addr);
+ kunmap_atomic(s_addr);
+}
+
+/*
+ * Find alloced object in zspage from index object and
+ * return handle.
+ */
+static unsigned long find_alloced_obj(struct page *page, int index,
+ struct size_class *class)
+{
+ unsigned long head;
+ int offset = 0;
+ unsigned long handle = 0;
+ void *addr = kmap_atomic(page);
+
+ if (!is_first_page(page))
+ offset = page->index;
+ offset += class->size * index;
+
+ while (offset < PAGE_SIZE) {
+ head = obj_to_head(addr + offset);
+ if (head & OBJ_ALLOCATED_TAG) {
+ handle = head & ~OBJ_ALLOCATED_TAG;
+ if (trypin_tag(handle))
+ break;
+ handle = 0;
+ }
+
+ offset += class->size;
+ index++;
+ }
+
+ kunmap_atomic(addr);
+ return handle;
+}
+
+struct zs_compact_control {
+ /* Source page for migration which could be a subpage of zspage. */
+ struct page *s_page;
+ /* Destination page for migration which should be a first page
+ * of zspage. */
+ struct page *d_page;
+ /* Starting object index within @s_page which used for live object
+ * in the subpage. */
+ int index;
+ /* how many of objects are migrated */
+ int nr_migrated;
+};
+
+static int migrate_zspage(struct zs_pool *pool, struct size_class *class,
+ struct zs_compact_control *cc)
+{
+ unsigned long used_obj, free_obj;
+ unsigned long handle;
+ struct page *s_page = cc->s_page;
+ struct page *d_page = cc->d_page;
+ unsigned long index = cc->index;
+ int nr_migrated = 0;
+ int ret = 0;
+
+ while (1) {
+ handle = find_alloced_obj(s_page, index, class);
+ if (!handle) {
+ s_page = get_next_page(s_page);
+ if (!s_page)
+ break;
+ index = 0;
+ continue;
+ }
+
+ /* Stop if there is no more space */
+ if (zspage_full(d_page)) {
+ unpin_tag(handle);
+ ret = -ENOMEM;
+ break;
+ }

+ used_obj = handle_to_obj(handle);
+ free_obj = obj_malloc(d_page, class, handle);
+ zs_object_copy(used_obj, free_obj, class);
+ index++;
+ record_obj(handle, free_obj);
+ unpin_tag(handle);
+ obj_free(pool, class, used_obj);
+ nr_migrated++;
+ }
+
+ /* Remember last position in this iteration */
+ cc->s_page = s_page;
+ cc->index = index;
+ cc->nr_migrated = nr_migrated;
+
+ return ret;
+}
+
+static struct page *alloc_target_page(struct size_class *class)
+{
+ int i;
+ struct page *page;
+
+ for (i = 0; i < _ZS_NR_FULLNESS_GROUPS; i++) {
+ page = class->fullness_list[i];
+ if (page) {
+ remove_zspage(page, class, i);
+ break;
+ }
+ }
+
+ return page;
+}
+
+static void putback_zspage(struct zs_pool *pool, struct size_class *class,
+ struct page *first_page)
+{
+ int class_idx;
+ enum fullness_group fullness;
+
+ BUG_ON(!is_first_page(first_page));
+
+ get_zspage_mapping(first_page, &class_idx, &fullness);
+ insert_zspage(first_page, class, fullness);
+ fullness = fix_fullness_group(class, first_page);
if (fullness == ZS_EMPTY) {
+ zs_stat_dec(class, OBJ_ALLOCATED, get_maxobj_per_zspage(
+ class->size, class->pages_per_zspage));
atomic_long_sub(class->pages_per_zspage,
&pool->pages_allocated);
+
free_zspage(first_page);
}
}
-EXPORT_SYMBOL_GPL(zs_free);
+
+static struct page *isolate_source_page(struct size_class *class)
+{
+ struct page *page;
+
+ page = class->fullness_list[ZS_ALMOST_EMPTY];
+ if (page)
+ remove_zspage(page, class, ZS_ALMOST_EMPTY);
+
+ return page;
+}
+
+static unsigned long __zs_compact(struct zs_pool *pool,
+ struct size_class *class)
+{
+ int nr_to_migrate;
+ struct zs_compact_control cc;
+ struct page *src_page;
+ struct page *dst_page = NULL;
+ unsigned long nr_total_migrated = 0;
+
+ cond_resched();
+
+ spin_lock(&class->lock);
+ while ((src_page = isolate_source_page(class))) {
+
+ BUG_ON(!is_first_page(src_page));
+
+ /* The goal is to migrate all live objects in source page */
+ nr_to_migrate = src_page->inuse;
+ cc.index = 0;
+ cc.s_page = src_page;
+
+ while ((dst_page = alloc_target_page(class))) {
+ cc.d_page = dst_page;
+ /*
+ * If there is no more space in dst_page, try to
+ * allocate another zspage.
+ */
+ if (!migrate_zspage(pool, class, &cc))
+ break;
+
+ putback_zspage(pool, class, dst_page);
+ nr_total_migrated += cc.nr_migrated;
+ nr_to_migrate -= cc.nr_migrated;
+ }
+
+ /* Stop if we couldn't find slot */
+ if (dst_page == NULL)
+ break;
+
+ putback_zspage(pool, class, dst_page);
+ putback_zspage(pool, class, src_page);
+ spin_unlock(&class->lock);
+ nr_total_migrated += cc.nr_migrated;
+ cond_resched();
+ spin_lock(&class->lock);
+ }
+
+ if (src_page)
+ putback_zspage(pool, class, src_page);
+
+ spin_unlock(&class->lock);
+
+ return nr_total_migrated;
+}
+
+unsigned long zs_compact(struct zs_pool *pool)
+{
+ int i;
+ unsigned long nr_migrated = 0;
+ struct size_class *class;
+
+ for (i = zs_size_classes - 1; i >= 0; i--) {
+ class = pool->size_class[i];
+ if (!class)
+ continue;
+ if (class->index != i)
+ continue;
+ nr_migrated += __zs_compact(pool, class);
+ }
+
+ synchronize_rcu();
+
+ return nr_migrated;
+}
+EXPORT_SYMBOL_GPL(zs_compact);

/**
* zs_create_pool - Creates an allocation pool to work from.
--
1.9.1