Received-SPF: pass (google.com: domain of linux-kernel+bounces-49050-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45d1:ec00::1 as permitted sender) client-ip=2604:1380:45d1:ec00::1;
Date: Fri, 2 Feb 2024 01:11:43 +0000
In-Reply-To: <20240201-b4-zswap-invalidate-entry-v1-6-56ed496b6e55@bytedance.com>
Precedence: bulk
Mime-Version: 1.0
References: <20240201-b4-zswap-invalidate-entry-v1-0-56ed496b6e55@bytedance.com>
 <20240201-b4-zswap-invalidate-entry-v1-6-56ed496b6e55@bytedance.com>
Message-ID: <ZbxBT98Ctjvd0XPG@google.com>
Subject: Re: [PATCH 6/6] mm/zswap: zswap entry doesn't need refcount anymore
From: Yosry Ahmed <yosryahmed@google.com>
To: Chengming Zhou <zhouchengming@bytedance.com>
Cc: Nhat Pham <nphamcs@gmail.com>, Johannes Weiner <hannes@cmpxchg.org>, 
	Andrew Morton <akpm@linux-foundation.org>, linux-kernel@vger.kernel.org, 
	linux-mm@kvack.org
Content-Type: text/plain; charset="us-ascii"

On Thu, Feb 01, 2024 at 03:49:06PM +0000, Chengming Zhou wrote:
> Since we don't need to leave zswap entry on the zswap tree anymore,
> we should remove it from tree once we find it from the tree.
> 
> Then after using it, we can directly free it, no concurrent path
> can find it from tree. Only the shrinker can see it from lru list,
> which will also double check under tree lock, so no race problem.
> 
> So we don't need refcount in zswap entry anymore and don't need to
> take the spinlock for the second time to invalidate it.
> 
> The side effect is that zswap_entry_free() maybe not happen in tree
> spinlock, but it's ok since nothing need to be protected by the lock.
> 
> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>

This looks like a great simplification, and a good motivation to only
support exclusive loads. Everything is more straightforward because
every tree lookup implies a removal and exclusive ownership.

Let's see if removing support for non-exclusive loads is agreeable first
though :)