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

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Date: Thu, 18 Jan 2024 08:59:55 -0800
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Matthew Wilcox <willy@...radead.org>
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 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?