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

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Message-ID: <ZaltEwGXHyFFuI0F@casper.infradead.org>
Date: Thu, 18 Jan 2024 18:25:23 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: Chris Li <chrisl@...nel.org>, 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 08:59:55AM -0800, Yosry Ahmed wrote:
> On Thu, Jan 18, 2024 at 5:52 AM Matthew Wilcox <willy@...radead.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?

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.