linux-kernel - Re: [PATCH 6/6] mm/zswap: zswap entry doesn't need refcount anymore

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Date: Fri, 2 Feb 2024 01:11:43 +0000
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Chengming Zhou <zhouchengming@...edance.com>
Cc: Nhat Pham <nphamcs@...il.com>, Johannes Weiner <hannes@...xchg.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org
Subject: Re: [PATCH 6/6] mm/zswap: zswap entry doesn't need refcount anymore

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@...edance.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 :)