linux-kernel - Re: [PATCH] mm: zswap: support exclusive loads

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Message-ID: <20230530235447.GB102494@cmpxchg.org>
Date:   Tue, 30 May 2023 19:54:47 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Seth Jennings <sjenning@...hat.com>,
        Dan Streetman <ddstreet@...e.org>,
        Vitaly Wool <vitaly.wool@...sulko.com>,
        Nhat Pham <nphamcs@...il.com>,
        Domenico Cerasuolo <cerasuolodomenico@...il.com>,
        Yu Zhao <yuzhao@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: zswap: support exclusive loads

On Tue, May 30, 2023 at 09:02:51PM +0000, Yosry Ahmed wrote:
> @@ -216,8 +216,13 @@ int __frontswap_load(struct page *page)
>  
>  	/* Try loading from each implementation, until one succeeds. */
>  	ret = frontswap_ops->load(type, offset, page);
> -	if (ret == 0)
> +	if (ret == 0) {
>  		inc_frontswap_loads();
> +		if (frontswap_ops->exclusive_loads) {
> +			SetPageDirty(page);
> +			__frontswap_clear(sis, offset);
> +		}

Somewhat tangential, but is there still a point to the frontswap
layer? It seems usecases other than zswap have never materialized, at
least not in tree. Life would be a lot easier if we were to just
hardcode the zswap callbacks in the swap functions.

It's not the patch's fault, but it highlights the boiler plate the
indirection causes. ->load() already has the page and could just dirty
it directly. Instead now both layers have to handle invalidation,
which is a vector for bugs.

Can somebody think of reasons to keep it? If not, I'd take a stab at
removing it.