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

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Date: Fri, 19 Jan 2024 13:41:16 -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 Fri, Jan 19, 2024 at 12:04 PM Matthew Wilcox <willy@...radead.org> 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.