Re: [PATCH 2/2] mm: zswap: remove unnecessary tree cleanups in zswap_swapoff()

[Johannes Weiner <hannes@cmpxchg.org>]

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@google.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.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.