[PATCH v1 11/19] zsmalloc: squeeze freelist into page->mapping

From: Minchan Kim
Date: Fri Mar 11 2016 - 02:32:41 EST

Zsmalloc stores first free object's position into first_page->freelist
in each zspage. If we change it with object index from first_page
instead of location, we could squeeze it into page->mapping because
the number of bit we need to store offset is at most 11bit.

Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
---
mm/zsmalloc.c | 159 +++++++++++++++++++++++++++++++++++-----------------------
1 file changed, 96 insertions(+), 63 deletions(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 954e8758a78d..e23cd3b2dd71 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -18,9 +18,7 @@
* Usage of struct page fields:
* page->private: points to the first component (0-order) page
* page->index (union with page->freelist): offset of the first object
- * starting in this page. For the first page, this is
- * always 0, so we use this field (aka freelist) to point
- * to the first free object in zspage.
+ * starting in this page.
* page->lru: links together all component pages (except the first page)
* of a zspage
*
@@ -29,9 +27,6 @@
* page->private: refers to the component page after the first page
* If the page is first_page for huge object, it stores handle.
* Look at size_class->huge.
- * page->freelist: points to the first free object in zspage.
- * Free objects are linked together using in-place
- * metadata.
* page->lru: links together first pages of various zspages.
* Basically forming list of zspages in a fullness group.
* page->mapping: override by struct zs_meta
@@ -131,6 +126,7 @@
/* each chunk includes extra space to keep handle */
#define ZS_MAX_ALLOC_SIZE PAGE_SIZE

+#define FREEOBJ_BITS 11
#define CLASS_BITS 8
#define CLASS_MASK ((1 << CLASS_BITS) - 1)
#define FULLNESS_BITS 2
@@ -228,17 +224,17 @@ struct size_class {

/*
* Placed within free objects to form a singly linked list.
- * For every zspage, first_page->freelist gives head of this list.
+ * For every zspage, first_page->freeobj gives head of this list.
*
* This must be power of 2 and less than or equal to ZS_ALIGN
*/
struct link_free {
union {
/*
- * Position of next free chunk (encodes <PFN, obj_idx>)
+ * free object list
* It's valid for non-allocated object
*/
- void *next;
+ unsigned long next;
/*
* Handle of allocated object.
*/
@@ -270,6 +266,7 @@ struct zs_pool {
};

struct zs_meta {
+ unsigned long freeobj:FREEOBJ_BITS;
unsigned long class:CLASS_BITS;
unsigned long fullness:FULLNESS_BITS;
unsigned long inuse:INUSE_BITS;
@@ -447,6 +444,26 @@ static void mod_zspage_inuse(struct page *first_page, int val)
m->inuse += val;
}

+static void set_freeobj(struct page *first_page, int idx)
+{
+ struct zs_meta *m;
+
+ VM_BUG_ON_PAGE(!is_first_page(first_page), first_page);
+
+ m = (struct zs_meta *)&first_page->mapping;
+ m->freeobj = idx;
+}
+
+static unsigned long get_freeobj(struct page *first_page)
+{
+ struct zs_meta *m;
+
+ VM_BUG_ON_PAGE(!is_first_page(first_page), first_page);
+
+ m = (struct zs_meta *)&first_page->mapping;
+ return m->freeobj;
+}
+
static void get_zspage_mapping(struct page *first_page,
unsigned int *class_idx,
enum fullness_group *fullness)
@@ -832,30 +849,33 @@ static struct page *get_next_page(struct page *page)
return next;
}

-/*
- * Encode <page, obj_idx> as a single handle value.
- * We use the least bit of handle for tagging.
- */
-static void *location_to_obj(struct page *page, unsigned long obj_idx)
+static void objidx_to_page_and_ofs(struct size_class *class,
+ struct page *first_page,
+ unsigned long obj_idx,
+ struct page **obj_page,
+ unsigned long *ofs_in_page)
{
- unsigned long obj;
+ int i;
+ unsigned long ofs;
+ struct page *cursor;
+ int nr_page;

- if (!page) {
- VM_BUG_ON(obj_idx);
- return NULL;
- }
+ ofs = obj_idx * class->size;
+ cursor = first_page;
+ nr_page = ofs >> PAGE_SHIFT;

- obj = page_to_pfn(page) << OBJ_INDEX_BITS;
- obj |= ((obj_idx) & OBJ_INDEX_MASK);
- obj <<= OBJ_TAG_BITS;
+ *ofs_in_page = ofs & ~PAGE_MASK;
+
+ for (i = 0; i < nr_page; i++)
+ cursor = get_next_page(cursor);

- return (void *)obj;
+ *obj_page = cursor;
}

-/*
- * 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
- * location_to_obj().
+/**
+ * obj_to_location - get (<page>, <obj_idx>) from encoded object value
+ * @page: page object resides in zspage
+ * @obj_idx: object index
*/
static void obj_to_location(unsigned long obj, struct page **page,
unsigned long *obj_idx)
@@ -865,6 +885,23 @@ static void obj_to_location(unsigned long obj, struct page **page,
*obj_idx = (obj & OBJ_INDEX_MASK);
}

+/**
+ * location_to_obj - get obj value encoded from (<page>, <obj_idx>)
+ * @page: page object resides in zspage
+ * @obj_idx: object index
+ */
+static unsigned long location_to_obj(struct page *page,
+ unsigned long obj_idx)
+{
+ unsigned long obj;
+
+ obj = page_to_pfn(page) << OBJ_INDEX_BITS;
+ obj |= obj_idx & OBJ_INDEX_MASK;
+ obj <<= OBJ_TAG_BITS;
+
+ return obj;
+}
+
static unsigned long handle_to_obj(unsigned long handle)
{
return *(unsigned long *)handle;
@@ -880,17 +917,6 @@ static unsigned long obj_to_head(struct size_class *class, struct page *page,
return *(unsigned long *)obj;
}

-static unsigned long obj_idx_to_offset(struct page *page,
- unsigned long obj_idx, int class_size)
-{
- unsigned long off = 0;
-
- if (!is_first_page(page))
- off = page->index;
-
- return off + obj_idx * class_size;
-}
-
static inline int trypin_tag(unsigned long handle)
{
unsigned long *ptr = (unsigned long *)handle;
@@ -916,7 +942,6 @@ static void reset_page(struct page *page)
clear_bit(PG_private_2, &page->flags);
set_page_private(page, 0);
page->mapping = NULL;
- page->freelist = NULL;
page_mapcount_reset(page);
}

@@ -948,6 +973,7 @@ static void free_zspage(struct page *first_page)
/* Initialize a newly allocated zspage */
static void init_zspage(struct size_class *class, struct page *first_page)
{
+ int freeobj = 1;
unsigned long off = 0;
struct page *page = first_page;

@@ -956,14 +982,11 @@ static void init_zspage(struct size_class *class, struct page *first_page)
while (page) {
struct page *next_page;
struct link_free *link;
- unsigned int i = 1;
void *vaddr;

/*
* page->index stores offset of first object starting
- * in the page. For the first page, this is always 0,
- * so we use first_page->index (aka ->freelist) to store
- * head of corresponding zspage's freelist.
+ * in the page.
*/
if (page != first_page)
page->index = off;
@@ -972,7 +995,7 @@ static void init_zspage(struct size_class *class, struct page *first_page)
link = (struct link_free *)vaddr + off / sizeof(*link);

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

@@ -982,11 +1005,21 @@ static void init_zspage(struct size_class *class, struct page *first_page)
* page (if present)
*/
next_page = get_next_page(page);
- link->next = location_to_obj(next_page, 0);
+ if (next_page) {
+ link->next = freeobj++ << OBJ_ALLOCATED_TAG;
+ } else {
+ /*
+ * Reset OBJ_ALLOCATED_TAG bit to last link for
+ * migration to know it is allocated object or not.
+ */
+ link->next = -1 << OBJ_ALLOCATED_TAG;
+ }
kunmap_atomic(vaddr);
page = next_page;
off %= PAGE_SIZE;
}
+
+ set_freeobj(first_page, 0);
}

/*
@@ -1036,7 +1069,6 @@ static struct page *alloc_zspage(struct size_class *class, gfp_t flags)

init_zspage(class, first_page);

- first_page->freelist = location_to_obj(first_page, 0);
error = 0; /* Success */

cleanup:
@@ -1318,7 +1350,7 @@ void *zs_map_object(struct zs_pool *pool, unsigned long handle,
obj_to_location(obj, &page, &obj_idx);
get_zspage_mapping(get_first_page(page), &class_idx, &fg);
class = pool->size_class[class_idx];
- off = obj_idx_to_offset(page, obj_idx, class->size);
+ off = (class->size * obj_idx) & ~PAGE_MASK;

area = &get_cpu_var(zs_map_area);
area->vm_mm = mm;
@@ -1357,7 +1389,7 @@ void zs_unmap_object(struct zs_pool *pool, unsigned long handle)
obj_to_location(obj, &page, &obj_idx);
get_zspage_mapping(get_first_page(page), &class_idx, &fg);
class = pool->size_class[class_idx];
- off = obj_idx_to_offset(page, obj_idx, class->size);
+ off = (class->size * obj_idx) & ~PAGE_MASK;

area = this_cpu_ptr(&zs_map_area);
if (off + class->size <= PAGE_SIZE)
@@ -1383,17 +1415,17 @@ static unsigned long obj_malloc(struct size_class *class,
struct link_free *link;

struct page *m_page;
- unsigned long m_objidx, m_offset;
+ unsigned long 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);
+ obj = get_freeobj(first_page);
+ objidx_to_page_and_ofs(class, first_page, obj,
+ &m_page, &m_offset);

vaddr = kmap_atomic(m_page);
link = (struct link_free *)vaddr + m_offset / sizeof(*link);
- first_page->freelist = link->next;
+ set_freeobj(first_page, link->next >> OBJ_ALLOCATED_TAG);
if (!class->huge)
/* record handle in the header of allocated chunk */
link->handle = handle;
@@ -1404,6 +1436,8 @@ static unsigned long obj_malloc(struct size_class *class,
mod_zspage_inuse(first_page, 1);
zs_stat_inc(class, OBJ_USED, 1);

+ obj = location_to_obj(m_page, obj);
+
return obj;
}

@@ -1473,19 +1507,17 @@ static void obj_free(struct size_class *class, unsigned long obj)

obj &= ~OBJ_ALLOCATED_TAG;
obj_to_location(obj, &f_page, &f_objidx);
+ f_offset = (class->size * f_objidx) & ~PAGE_MASK;
first_page = get_first_page(f_page);
-
- f_offset = obj_idx_to_offset(f_page, f_objidx, class->size);
-
vaddr = kmap_atomic(f_page);

/* Insert this object in containing zspage's freelist */
link = (struct link_free *)(vaddr + f_offset);
- link->next = first_page->freelist;
+ link->next = get_freeobj(first_page) << OBJ_ALLOCATED_TAG;
if (class->huge)
set_page_private(first_page, 0);
kunmap_atomic(vaddr);
- first_page->freelist = (void *)obj;
+ set_freeobj(first_page, f_objidx);
mod_zspage_inuse(first_page, -1);
zs_stat_dec(class, OBJ_USED, 1);
}
@@ -1541,8 +1573,8 @@ static void zs_object_copy(struct size_class *class, unsigned long dst,
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);
+ s_off = (class->size * s_objidx) & ~PAGE_MASK;
+ d_off = (class->size * d_objidx) & ~PAGE_MASK;

if (s_off + class->size > PAGE_SIZE)
s_size = PAGE_SIZE - s_off;
@@ -2033,9 +2065,10 @@ static int __init zs_init(void)
goto notifier_fail;

/*
- * A zspage's class index, fullness group, inuse object count are
- * encoded in its (first)page->mapping so sizeof(struct zs_meta)
- * should be less than sizeof(page->mapping(i.e., unsigned long)).
+ * A zspage's a free object index, class index, fullness group,
+ * inuse object count are encoded in its (first)page->mapping
+ * so sizeof(struct zs_meta) should be less than
+ * sizeof(page->mapping(i.e., unsigned long)).
*/
BUILD_BUG_ON(sizeof(struct zs_meta) > sizeof(unsigned long));

--
1.9.1