Re: [PATCH v3 11/16] zsmalloc: separate free_zspage from putback_zspage

Sergey Senozhatsky
2016-04-18T01:02:40Z

Hello Minchan,

On (03/30/16 16:12), Minchan Kim wrote:
[..]
> @@ -1835,23 +1827,31 @@ static void __zs_compact(struct zs_pool *pool, struct size_class *class)
> if (!migrate_zspage(pool, class, &cc))
> break;
>
> - putback_zspage(pool, class, dst_page);
> + VM_BUG_ON_PAGE(putback_zspage(pool, class,
> + dst_page) == ZS_EMPTY, dst_page);

can this VM_BUG_ON_PAGE() condition ever be true?

> }
> /* Stop if we couldn't find slot */
> if (dst_page == NULL)
> break;
> - putback_zspage(pool, class, dst_page);
> - if (putback_zspage(pool, class, src_page) == ZS_EMPTY)
> + VM_BUG_ON_PAGE(putback_zspage(pool, class,
> + dst_page) == ZS_EMPTY, dst_page);

hm... this VM_BUG_ON_PAGE(dst_page) is sort of confusing. under what
circumstances it can be true?

a minor nit, it took me some time (need some coffee I guess) to
correctly parse this macro wrapper

VM_BUG_ON_PAGE(putback_zspage(pool, class,
dst_page) == ZS_EMPTY, dst_page);

may be do it like:
fullness = putback_zspage(pool, class, dst_page);
VM_BUG_ON_PAGE(fullness == ZS_EMPTY, dst_page);

well, if we want to VM_BUG_ON_PAGE() at all. there haven't been any
problems with compaction, is there any specific reason these macros
were added?

> + if (putback_zspage(pool, class, src_page) == ZS_EMPTY) {
> pool->stats.pages_compacted += class->pages_per_zspage;
> - spin_unlock(&class->lock);
> + spin_unlock(&class->lock);
> + free_zspage(pool, class, src_page);

do we really need to free_zspage() out of class->lock?
wouldn't something like this

if (putback_zspage(pool, class, src_page) == ZS_EMPTY) {
pool->stats.pages_compacted += class->pages_per_zspage;
free_zspage(pool, class, src_page);
}
spin_unlock(&class->lock);

be simpler?

besides, free_zspage() now updates class stats out of class lock,
not critical but still.

-ss

> + } else {
> + spin_unlock(&class->lock);
> + }
> +
> cond_resched();
> spin_lock(&class->lock);
> }
>
> if (src_page)
> - putback_zspage(pool, class, src_page);
> + VM_BUG_ON_PAGE(putback_zspage(pool, class,
> + src_page) == ZS_EMPTY, src_page);
>
> spin_unlock(&class->lock);
> }