linux-kernel - Re: [PATCH 2/2] mm: zswap: remove unnecessary tree cleanups in zswap

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Message-ID: <20240122201906.GA1567330@cmpxchg.org>
Date: Mon, 22 Jan 2024 15:19:06 -0500
From: Johannes Weiner <hannes@...xchg.org>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Nhat Pham <nphamcs@...il.com>, Chris Li <chrisl@...nel.org>,
	Chengming Zhou <zhouchengming@...edance.com>,
	Huang Ying <ying.huang@...el.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm: zswap: remove unnecessary tree cleanups in
 zswap_swapoff()

On Sat, Jan 20, 2024 at 02:40:07AM +0000, Yosry Ahmed wrote:
> During swapoff, try_to_unuse() makes sure that zswap_invalidate() is
> called for all swap entries before zswap_swapoff() is called. This means
> that all zswap entries should already be removed from the tree. Simplify
> zswap_swapoff() by removing the tree cleanup loop, and leaving an
> assertion in its place.
> 
> Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>

Acked-by: Johannes Weiner <hannes@...xchg.org>

That's a great simplification.

Removing the tree->lock made me double take, but at this point the
swapfile and its cache should be fully dead and I don't see how any of
the zswap operations that take tree->lock could race at this point.

> ---
> Chengming, Chris, I think this should make the tree split and the xarray
> conversion patches simpler (especially the former). If others agree,
> both changes can be rebased on top of this.

The resulting code is definitely simpler, but this patch is not a
completely trivial cleanup, either. If you put it before Chengming's
patch and it breaks something, it would be difficult to pull out
without affecting the tree split.