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

Minchan Kim
2016-04-19T07:50:22Z

Hi Sergey,

On Mon, Apr 18, 2016 at 10:04:08AM +0900, Sergey Senozhatsky wrote:
> 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?

I guess it is remained thing after I rebased to catch any mistake.
But I'm heavily chainging this part.
Please review next version instead of this after a few days. :)

>
> > }
> > /* 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?

The reason I did out of class->lock is deadlock between page_lock
and class->lock with upcoming page migration.
However, as I said, I'm now heavily changing the part. :)

>
> 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);
> > }