linux-kernel - Re: [PATCH v2 5/5] zsmalloc: Implement writeback mechanism for zsmalloc

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Message-ID: <Y2SO80b+byMavdqd@google.com>
Date:   Fri, 4 Nov 2022 13:02:59 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Nhat Pham <nphamcs@...il.com>, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        minchan@...nel.org, ngupta@...are.org, sjenning@...hat.com,
        ddstreet@...e.org, vitaly.wool@...sulko.com
Subject: Re: [PATCH v2 5/5] zsmalloc: Implement writeback mechanism for
 zsmalloc

On (22/11/03 12:45), Johannes Weiner wrote:
> On Wed, Nov 02, 2022 at 01:13:58PM +0900, Sergey Senozhatsky wrote:
> > On (22/10/27 11:27), Nhat Pham wrote:
> > > +
> > > +static int zs_zpool_shrink(void *pool, unsigned int pages,
> > > +			unsigned int *reclaimed)
> > > +{
> > > +	unsigned int total = 0;
> > > +	int ret = -EINVAL;
> > > +
> > > +	while (total < pages) {
> > > +		ret = zs_reclaim_page(pool, 8);
> > > +		if (ret < 0)
> > > +			break;
> > > +		total++;
> > > +	}
> > > +
> > > +	if (reclaimed)
> > > +		*reclaimed = total;
> > > +
> > > +	return ret;
> > > +}
> > 
> > The name collides with shrinker callbacks (compaction). That's a bit
> > confusing, took me some time.
> 
> Yeah this tripped us up too.
> 
> Unfortunately, this is inherited from the zpool API:

Yeah, sure. I don't ask to rename zpool API, that was just a personal
note that it took me sometime to figure out that now we have two different
shrinkers.

> > > @@ -482,6 +504,7 @@ static struct zpool_driver zs_zpool_driver = {
> > >  	.malloc_support_movable = true,
> > >  	.malloc =		  zs_zpool_malloc,
> > >  	.free =			  zs_zpool_free,
> > > +	.shrink =     zs_zpool_shrink,
> 
> There is another terminology collision around "compaction" and
> "migration": There is zs_page_migrate() which is called from physical
> page migration and compaction to switch out backing struct pages of
> the zspage. Then there is migrate_zspage() which is called from a
> *shrinker* through *zs_compact()*, and it consolidates the objects
> from partially used zspages into full ones.

Oh, yeah...

> We're collecting these issues and want to clean them up separately. It
> seems this codebase hasn't had a lot of TLC recently, which makes it
> harder to make needed changes :/ We try to clean up as we go, but some
> of these issues (like the .shrink callback) run through all backends,
> zpool and zswap, so they'd be better off in separate cleanup patches.

Totally agreed with the separate effort/patches.