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From: Yosry Ahmed <yosryahmed@google.com>
Date: Fri, 19 Jan 2024 11:30:25 -0800
Message-ID: <CAJD7tkZRR2wwCyu-99+3y7FFOKm7omBZrrj-G5_tb9vOivpjWQ@mail.gmail.com>
Subject: Re: [PATCH 1/2] mm: zswap.c: add xarray tree to zswap
To: Chris Li <chrisl@kernel.org>
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 Thu, Jan 18, 2024 at 9:28=E2=80=AFPM Chris Li <chrisl@kernel.org> wrote:
>
> On Thu, Jan 18, 2024 at 10:25=E2=80=AFAM Matthew Wilcox <willy@infradead.=
org> wrote:
> >
> > On Thu, Jan 18, 2024 at 08:59:55AM -0800, Yosry Ahmed wrote:
> > > On Thu, Jan 18, 2024 at 5:52=E2=80=AFAM Matthew Wilcox <willy@infrade=
ad.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 mentio=
ned
> > > > in the fine documentation.  If you don't need to drop the lock whil=
e
> > > > 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?
> >
> > I have no idea; I haven't studied the patches in detail yet.  I have
> > debugging assertions for that, so I was assuming that Chris had been
> > developing with debug options turned on.  If not, I guess the bots will
> > do it for him.
>
> It is fine now because we have the extra zswap tree spin lock. When we
> remove the zswap tree spin lock it does require RCU read lock. You are
> right I would get assertion failure.

I would imagine the assertions are that we are holding either the RCU
read lock or the xa_lock, how would holding the zswap tree lock help?