linux-kernel - Re: [PATCH v5 4/6] zsmalloc: Add a LRU to zs_pool to keep track of zspages in LRU order

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Date:   Fri, 18 Nov 2022 18:23:09 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Nhat Pham <nphamcs@...il.com>, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        ngupta@...are.org, senozhatsky@...omium.org, sjenning@...hat.com,
        ddstreet@...e.org, vitaly.wool@...sulko.com
Subject: Re: [PATCH v5 4/6] zsmalloc: Add a LRU to zs_pool to keep track of
 zspages in LRU order

On Fri, Nov 18, 2022 at 01:35:01PM -0800, Minchan Kim wrote:
> On Fri, Nov 18, 2022 at 03:05:04PM -0500, Johannes Weiner wrote:
> > On Fri, Nov 18, 2022 at 11:32:01AM -0800, Minchan Kim wrote:
> > > On Fri, Nov 18, 2022 at 10:24:05AM -0800, Nhat Pham wrote:
> > > > @@ -1444,6 +1473,11 @@ unsigned long zs_malloc(struct zs_pool *pool, size_t size, gfp_t gfp)
> > > > 
> > > >  	/* We completely set up zspage so mark them as movable */
> > > >  	SetZsPageMovable(pool, zspage);
> > > > +out:
> > > > +#ifdef CONFIG_ZPOOL
> > > > +	/* Move the zspage to front of pool's LRU */
> > > > +	move_to_front(pool, zspage);
> > > > +#endif
> > > >  	spin_unlock(&pool->lock);
> > > 
> > > Please move the move_to_front into zs_map_object with ZS_MM_WO with
> > > comment with "why we are doing only for WO case".
> > 
> > I replied to the other thread, but I disagree with this request.
> > 
> > The WO exception would be as zswap-specific as is the
> > rotate-on-alloc. It doesn't make the resulting zsmalloc code any
> 
> That's true but at least, zs_pool allocators have the accessor so
> that's fair place to have the LRU updating. I guess that's why
> you agreed that's better place. No?
> 
> I understand that's zswap-specific that the bad design keeps
> pushing smelly code into allocators and then "push to take it
> since other were already doing" with "we will take them off with
> better solution in future". I am really struggling to understand
> this concept. Johannes, Is that really how we work over a decade?

My point was that there is no difference between having zswap code in
alloc or in map. And there is a small upside to having it in alloc
because of the other backends.

But I won't fight you on it. The code isn't going to stay like this
for long anyway.