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References: <20240117-zswap-xarray-v1-0-6daa86c08fae@kernel.org>
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From: Yosry Ahmed <yosryahmed@google.com>
Date: Thu, 18 Jan 2024 08:59:55 -0800
Message-ID: <CAJD7tkapY4nx=uAXuyQhJN=rz7QPj6p9OJzpPwZGr_7+7Ywotw@mail.gmail.com>
Subject: Re: [PATCH 1/2] mm: zswap.c: add xarray tree to zswap
To: Matthew Wilcox <willy@infradead.org>
Cc: Chris Li <chrisl@kernel.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 Thu, Jan 18, 2024 at 5:52=E2=80=AFAM Matthew Wilcox <willy@infradead.org=
> wrote:
>
> On Wed, Jan 17, 2024 at 10:20:29PM -0800, Yosry Ahmed wrote:
> > >         /* walk the tree and free everything */
> > >         spin_lock(&tree->lock);
> > > +
> > > +       xas_for_each(&xas, e, ULONG_MAX)
> >
> > Why not use xa_for_each?
>
> xas_for_each() is O(n) while xa_for_each() is O(n log n), as mentioned
> in the fine documentation.  If you don't need to drop the lock while
> in the body of the loop, always prefer xas_for_each().

Thanks for pointing this out. Out of ignorance, I skipped reading the
doc for this one and operated under the general assumption to use xa_*
functions were possible.

The doc also says we should hold either the RCU read lock or the
xa_lock while iterating, we are not doing either here, no?