linux-kernel - Re: [PATCH 1/2] mm: zswap.c: add xarray tree to zswap

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Date: Thu, 18 Jan 2024 21:28:13 -0800
From: Chris Li <chrisl@...nel.org>
To: Matthew Wilcox <willy@...radead.org>
Cc: Yosry Ahmed <yosryahmed@...gle.com>, 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>, "Liam R. Howlett" <Liam.Howlett@...cle.com>, 
	Joel Fernandes <joel@...lfernandes.org>, Chengming Zhou <zhouchengming@...edance.com>
Subject: Re: [PATCH 1/2] mm: zswap.c: add xarray tree to zswap

On Thu, Jan 18, 2024 at 10:25 AM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Thu, Jan 18, 2024 at 08:59:55AM -0800, Yosry Ahmed wrote:
> > On Thu, Jan 18, 2024 at 5:52 AM Matthew Wilcox <willy@...radeadorg> 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?
>
> 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.

Chris