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From: Yosry Ahmed <yosryahmed@google.com>
Date: Fri, 19 Jan 2024 14:08:04 -0800
Message-ID: <CAJD7tkbKZqOwkDgNUQ7+XCHs_Uqb=gbCk7L=Es4sR1T3wWG+gA@mail.gmail.com>
Subject: Re: [PATCH 1/2] mm: zswap.c: add xarray tree to zswap
To: Chris Li <chriscli@google.com>
Cc: Matthew Wilcox <willy@infradead.org>, Andrew Morton <akpm@linux-foundation.org>, 
	linux-kernel@vger.kernel.org, linux-mm@kvack.org, 
	=?UTF-8?B?V2VpIFh177+8?= <weixugc@google.com>, Yu Zhao <yuzhao@google.com>, 
	Greg Thelen <gthelen@google.com>, Chun-Tse Shao <ctshao@google.com>, 
	=?UTF-8?Q?Suren_Baghdasaryan=EF=BF=BC?= <surenb@google.com>, 
	Brain Geffon <bgeffon@google.com>, Minchan Kim <minchan@kernel.org>, Michal Hocko <mhocko@suse.com>, 
	Mel Gorman <mgorman@techsingularity.net>, Huang Ying <ying.huang@intel.com>, 
	Nhat Pham <nphamcs@gmail.com>, Johannes Weiner <hannes@cmpxchg.org>, Kairui Song <kasong@tencent.com>, 
	Zhongkun He <hezhongkun.hzk@bytedance.com>, Kemeng Shi <shikemeng@huaweicloud.com>, 
	Barry Song <v-songbaohua@oppo.com>, "Liam R. Howlett" <Liam.Howlett@oracle.com>, 
	Joel Fernandes <joel@joelfernandes.org>, Chengming Zhou <zhouchengming@bytedance.com>
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On Fri, Jan 19, 2024 at 2:05=E2=80=AFPM Chris Li <chriscli@google.com> wrot=
e:
>
> Hi Yosry,
>
> On Fri, Jan 19, 2024 at 1:41=E2=80=AFPM Yosry Ahmed <yosryahmed@google.co=
m> wrote:
> > > if your code is naturally structured so that you delete the entries
> > > after freeing them, you have no reason to call xa_destroy().
> >
> > Thanks for elaborating. Based on this, I believe doing xas_for_each()
> > { zswap_free_entry(); }; xa_destroy(); is both closer to the current
> > code structure and more efficient.
> >
> Can't do that in this case though.  Because you get the RCU read lock
> on the tree.
> Other threads can still lookup the xarray (also RCU read lock) and get
> a pointer to the already freed memory.

During swapoff no one should be trying to swapin or swapout, where
would the lookups come from?