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Subject[PATCH] zsmalloc: keep first object offset in struct page
Date
In early draft of zspage migration, we couldn't use page._mapcount
because it was used for storing movable flag so we added runtime
calculation to get first object offset in a page but it causes rather
many instruction and even bug.

Since then, we don't use page._mapcount as page flag any more so now
there is no problem to use the field to store first object offset.

Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Minchan Kim <minchan@kernel.org>
---
mm/zsmalloc.c | 44 ++++++++++++++++----------------------------
1 file changed, 16 insertions(+), 28 deletions(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 6a58edc9a015..4b70fcbfb69b 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -512,6 +512,16 @@ static inline struct page *get_first_page(struct zspage *zspage)
return first_page;
}

+static inline int get_first_obj_offset(struct page *page)
+{
+ return page->units;
+}
+
+static inline void set_first_obj_offset(struct page *page, int offset)
+{
+ page->units = offset;
+}
+
static inline unsigned int get_freeobj(struct zspage *zspage)
{
return zspage->freeobj;
@@ -872,31 +882,6 @@ static struct page *get_next_page(struct page *page)
return page->freelist;
}

-/* Get byte offset of first object in the @page */
-static int get_first_obj_offset(struct size_class *class,
- struct page *first_page, struct page *page)
-{
- int pos;
- int page_idx = 0;
- int ofs = 0;
- struct page *cursor = first_page;
-
- if (first_page == page)
- goto out;
-
- while (page != cursor) {
- page_idx++;
- cursor = get_next_page(cursor);
- }
-
- pos = class->objs_per_zspage * class->size *
- page_idx / class->pages_per_zspage;
-
- ofs = (pos + class->size) % PAGE_SIZE;
-out:
- return ofs;
-}
-
/**
* obj_to_location - get (<page>, <obj_idx>) from encoded object value
* @page: page object resides in zspage
@@ -966,6 +951,7 @@ static void reset_page(struct page *page)
clear_bit(PG_private, &page->flags);
clear_bit(PG_private_2, &page->flags);
set_page_private(page, 0);
+ page_mapcount_reset(page);
ClearPageHugeObject(page);
page->freelist = NULL;
}
@@ -1064,6 +1050,8 @@ static void init_zspage(struct size_class *class, struct zspage *zspage)
struct link_free *link;
void *vaddr;

+ set_first_obj_offset(page, off);
+
vaddr = kmap_atomic(page);
link = (struct link_free *)vaddr + off / sizeof(*link);

@@ -1762,9 +1750,8 @@ static unsigned long find_alloced_obj(struct size_class *class,
int offset = 0;
unsigned long handle = 0;
void *addr = kmap_atomic(page);
- struct zspage *zspage = get_zspage(page);

- offset = get_first_obj_offset(class, get_first_page(zspage), page);
+ offset = get_first_obj_offset(page);
offset += class->size * index;

while (offset < PAGE_SIZE) {
@@ -1976,6 +1963,7 @@ static void replace_sub_page(struct size_class *class, struct zspage *zspage,
} while ((page = get_next_page(page)) != NULL);

create_page_chain(class, zspage, pages);
+ set_first_obj_offset(newpage, get_first_obj_offset(oldpage));
if (unlikely(PageHugeObject(oldpage)))
newpage->index = oldpage->index;
__SetPageMovable(newpage, page_mapping(oldpage));
@@ -2062,7 +2050,7 @@ int zs_page_migrate(struct address_space *mapping, struct page *newpage,
get_zspage_mapping(zspage, &class_idx, &fullness);
pool = mapping->private_data;
class = pool->size_class[class_idx];
- offset = get_first_obj_offset(class, get_first_page(zspage), page);
+ offset = get_first_obj_offset(page);

spin_lock(&class->lock);
if (!get_zspage_inuse(zspage)) {
--
1.9.1
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