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

From: YiPing Xu
Date: Thu Mar 17 2016 - 08:10:25 EST

On 2016/3/15 14:51, Minchan Kim wrote:
On Tue, Mar 15, 2016 at 03:40:53PM +0900, Sergey Senozhatsky wrote:
On (03/11/16 16:30), Minchan Kim wrote:
-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)

this looks big; 5 params, function "returning" both page and offset...
any chance to split it in two steps, perhaps?

Yes, it's rather ugly but I don't have a good idea.
Feel free to suggest if you have a better idea.

besides, it is more intuitive (at least to me) when 'offset'
shortened to 'offt', not 'ofs'.

the purpose to get 'obj_page' and 'ofs_in_page' is to map the page and get the meta-data pointer in the page, so, we can finish this in a single function.

just like this, and maybe we could have a better function name

static unsigned long *map_handle(struct size_class *class,
struct page *first_page, unsigned long obj_idx)
struct page *cursor = first_page;
unsigned long offset = obj_idx * class->size;
int nr_page = offset >> PAGE_SHIFT;
unsigned long offset_in_page = offset & ~PAGE_MASK;
void *addr;
int i;

if (class->huge) {
VM_BUG_ON_PAGE(!is_first_page(page), page);
return &page_private(page);

for (i = 0; i < nr_page; i++)
cursor = get_next_page(cursor);

addr = kmap_atomic(cursor);

return addr + offset_in_page;

static void unmap_handle(unsigned long *addr)
if (class->huge) {

kunmap_atomic(addr & ~PAGE_MASK);

all functions called "objidx_to_page_and_ofs" could use it like this, for example:

static unsigned long handle_from_obj(struct size_class *class,
struct page *first_page, int obj_idx)
unsigned long *head = map_handle(class, first_page, obj_idx);

if (*head & OBJ_ALLOCATED_TAG)
handle = *head & ~OBJ_ALLOCATED_TAG;


return handle;

'freeze_zspage', u'nfreeze_zspage' use it in the same way.

but in 'obj_malloc', we still have to get the page to get obj.

obj = location_to_obj(m_page, obj);

Indeed. I will change it to get_page_and_offset instead of
abbreviation if we cannot refactor it more.


- 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;

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