linux-kernel - Re: [PATCH 0/2] RFC: zswap tree use xarray instead of RB tree

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Date: Wed, 17 Jan 2024 23:05:15 -0800
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Christopher Li <chrisl@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, Wei Xu <weixugc@...gle.com>, 
	Yu Zhao <yuzhao@...gle.com>, Greg Thelen <gthelen@...gle.com>, 
	Chun-Tse Shao <ctshao@...gle.com>, Suren Baghdasaryan <surenb@...gle.com>, 
	Brain Geffon <bgeffon@...gle.com>, Minchan Kim <minchan@...nel.org>, Michal Hocko <mhocko@...e.com>, 
	Mel Gorman <mgorman@...hsingularity.net>, Huang Ying <ying.huang@...el.com>, 
	Nhat Pham <nphamcs@...il.com>, Johannes Weiner <hannes@...xchg.org>, Kairui Song <kasong@...cent.com>, 
	Zhongkun He <hezhongkun.hzk@...edance.com>, Kemeng Shi <shikemeng@...weicloud.com>, 
	Barry Song <v-songbaohua@...o.com>, "Matthew Wilcox (Oracle)" <willy@...radead.org>, 
	"Liam R. Howlett" <Liam.Howlett@...cle.com>, Joel Fernandes <joel@...lfernandes.org>, 
	Chengming Zhou <zhouchengming@...edance.com>
Subject: Re: [PATCH 0/2] RFC: zswap tree use xarray instead of RB tree

The name changes from Chris to Christopher are confusing :D

>
> I think it makes the review easier. The code adding and removing does
> not have much overlap. Combining it to a single patch does not save
> patch size. Having the assert check would be useful for some bisecting
> to narrow down which step causing the problem. I am fine with squash
> it to one patch as well.

I think having two patches is unnecessarily noisy, and we add some
debug code in this patch that we remove in the next patch anyway.
Let's see what others think, but personally I prefer a single patch.

> >
> > >
> > > I expect to merge the zswap rb tree spin lock with the xarray
> > > lock in the follow up changes.
> >
> > Shouldn't this simply be changing uses of tree->lock to use
> > xa_{lock/unlock}? We also need to make sure we don't try to lock the
> > tree when operating on the xarray if the caller is already holding the
> > lock, but this seems to be straightforward enough to be done as part
> > of this patch or this series at least.
> >
> > Am I missing something?
>
> Currently the zswap entry refcount is protected by the zswap tree spin
> lock as well. Can't remove the tree spin lock without changing the
> refcount code. I think the zswap search entry should just return the
> entry with refcount atomic increase, inside the RCU read() or xarray
> lock. The previous zswap code does the find_and_get entry() which is
> closer to what I want.

I think this can be done in an RCU read section surrounding xa_load()
and the refcount increment. Didn't look closely to check how much
complexity this adds to manage refcounts with RCU, but I think there
should be a lot of examples all around the kernel.

IIUC, there are no performance benefits from this conversion until we
remove the tree spinlock, right?