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

Minchan Kim
2016-05-17T01:14:19Z

On Mon, May 16, 2016 at 12:09:41PM +0900, Sergey Senozhatsky wrote:
> 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?

Thanks, Will fix.

>
> 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;
>

Hmm, of course, I can do it easily.
But zs_create_pool returns NULL without error propagation from sub
functions so I don't see any gain from returning errno from
create_cache. I don't mean I hate it but just need a justificaion
to persuade grumpy me.

>
> > @@ -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.

Yeb.

>
> [..]
> > +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 ..." ?

For source zspage, the policy is to find a fragment object from ZS_ALMOST_EMPTY.
For target zspage, the policy is to find a fragment object from ZS_ALMOST_FULL.

Do I misunderstand your question?

>
> > + 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