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From: Yosry Ahmed <yosryahmed@google.com>
Date: Fri, 19 Jan 2024 13:41:16 -0800
Message-ID: <CAJD7tkYTU10W4WX3HXzs3t4hKRLZ0saX0Nd8RDcaoYj58bzbsw@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 Fri, Jan 19, 2024 at 12:04=E2=80=AFPM Matthew Wilcox <willy@infradead.or=
g> wrote:
>
> On Fri, Jan 19, 2024 at 11:29:42AM -0800, Yosry Ahmed wrote:
> > I see, but it's not clear to me if the xarray is being properly
> > cleaned up in this case.
> >
> > Do we have to call xa_destroy() anyway to make sure everything is
> > cleaned up in the xarray? In that case, we can just do that after the
> > loop.
>
> You do not need to call xa_destroy().  xa_destroy() exists for two
> patterns: first, that you're storing values, not pointers in the tree,
> and you can just delete the tree without leaking memory.  second, that
> you do xas_for_each() { kfree(p); }; xa_destroy();  that's more
> efficient than xas_for_each() { kfree(p); xas_store(NULL); } as it
> batches the freeing of the nodes to the end.
>
> 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.