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 15:05:04 -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 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
cleaner or more generic, just weird in a slightly different way.

On the other hand, it makes zsmalloc deviate from the other backends
and introduces new callchains that invalidate thousands of machine
hours of production testing of this code.