Re: [PATCH v5 08/12] zsmalloc: introduce zspage structure

Sergey Senozhatsky
2016-05-16T03:10:16Z

On (05/09/16 11:20), Minchan Kim wrote:
> We have squeezed meta data of zspage into first page's descriptor.
> So, to get meta data from subpage, we should get first page first
> of all. But it makes trouble to implment page migration feature
> of zsmalloc because any place where to get first page from subpage
> can be raced with first page migration. IOW, first page it got
> could be stale. For preventing it, I have tried several approahces
> but it made code complicated so finally, I concluded to separate
> metadata from first page. Of course, it consumes more memory. IOW,
> 16bytes per zspage on 32bit at the moment. It means we lost 1%
> at *worst case*(40B/4096B) which is not bad I think at the cost of
> maintenance.
>
> Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> Signed-off-by: Minchan Kim <minchan@kernel.org>
[..]
> @@ -153,8 +138,6 @@
> enum fullness_group {
> ZS_ALMOST_FULL,
> ZS_ALMOST_EMPTY,
> - _ZS_NR_FULLNESS_GROUPS,
> -
> ZS_EMPTY,
> ZS_FULL
> };
> @@ -203,7 +186,7 @@ static const int fullness_threshold_frac = 4;
>
> struct size_class {
> spinlock_t lock;
> - struct page *fullness_list[_ZS_NR_FULLNESS_GROUPS];
> + struct list_head fullness_list[2];

seems that it also has some cleaup bits in it.

[..]
> -static int create_handle_cache(struct zs_pool *pool)
> +static int create_cache(struct zs_pool *pool)
> {
> pool->handle_cachep = kmem_cache_create("zs_handle", ZS_HANDLE_SIZE,
> 0, 0, NULL);
> - return pool->handle_cachep ? 0 : 1;
> + if (!pool->handle_cachep)
> + return 1;
> +
> + pool->zspage_cachep = kmem_cache_create("zspage", sizeof(struct zspage),
> + 0, 0, NULL);
> + if (!pool->zspage_cachep) {
> + kmem_cache_destroy(pool->handle_cachep);
^^^^^

do you need to NULL a pool->handle_cachep here?

zs_create_pool()
if (create_cache() == 1) {
pool->zspage_cachep NULL
pool->handle_cachep !NULL already freed -> kmem_cache_destroy()
return 1;
goto err
}
err:
zs_destroy_pool()
destroy_cache() {
kmem_cache_destroy(pool->handle_cachep); !NULL and freed
kmem_cache_destroy(pool->zspage_cachep); NULL ok
}

can we also switch create_cache() to errnos? I just like a bit
better
return -ENOMEM;
else
return 0;

than

return 1;
else
return 0;

> @@ -997,44 +951,38 @@ static void init_zspage(struct size_class *class, struct page *first_page)
> off %= PAGE_SIZE;
> }
>
> - set_freeobj(first_page, (unsigned long)location_to_obj(first_page, 0));
> + set_freeobj(zspage,
> + (unsigned long)location_to_obj(zspage->first_page, 0));

static unsigned long location_to_obj()

it's already returning "(unsigned long)", so here and in several other places
this cast can be dropped.

[..]
> +static struct zspage *isolate_zspage(struct size_class *class, bool source)
> {
> + struct zspage *zspage;
> + enum fullness_group fg[2] = {ZS_ALMOST_EMPTY, ZS_ALMOST_FULL};
> + if (!source) {
> + fg[0] = ZS_ALMOST_FULL;
> + fg[1] = ZS_ALMOST_EMPTY;
> + }
> +
> + for (i = 0; i < 2; i++) {

sorry, why not "for (i = ZS_ALMOST_EMPTY; i <= ZS_ALMOST_FULL ..." ?

> + zspage = list_first_entry_or_null(&class->fullness_list[fg[i]],
> + struct zspage, list);
> + if (zspage) {
> + remove_zspage(class, zspage, fg[i]);
> + return zspage;
> }
> }
>
> - return page;
> + return zspage;
> }

-ss