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[yosryahmed]

Date: Fri, 19 Jan 2024 19:23:52 +0000
From: Yosry Ahmed <yosryahmed@google.com>
To: Chris Li <chrisl@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org, Wei =?utf-8?B?WHXvv7w=?= <weixugc@google.com>,
	Yu Zhao <yuzhao@google.com>, Greg Thelen <gthelen@google.com>,
	Chun-Tse Shao <ctshao@google.com>,
	Suren =?utf-8?B?QmFnaGRhc2FyeWFu77+8?= <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>,
	"Matthew Wilcox (Oracle)" <willy@infradead.org>,
	"Liam R. Howlett" <Liam.Howlett@oracle.com>,
	Joel Fernandes <joel@joelfernandes.org>,
	Chengming Zhou <zhouchengming@bytedance.com>
Subject: Re: [PATCH 1/2] mm: zswap.c: add xarray tree to zswap
Message-ID: <ZarMHYEqXCOeKau0@google.com>
References: <20240117-zswap-xarray-v1-0-6daa86c08fae@kernel.org>
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  <CAJD7tkYEx57CPBoaN9GW4M3Mx-+jEsOMWJ02nLKSKD-MLb-WPA@mail.gmail.com>
  <CAF8kJuO5tAqwyKQK7AasWgs3Ohfc2osD9oX0m8YAkfsAZsjjyQ@mail.gmail.com>
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<CAF8kJuO5tAqwyKQK7AasWgs3Ohfc2osD9oX0m8YAkfsAZsjjyQ@mail.gmail.com>

> > > -static struct zswap_entry *zswap_rb_search(struct rb_root *root,  
> pgoff_t offset)
> > > +static struct zswap_entry *zswap_search(struct zswap_tree *tree,  
> pgoff_t offset)
> >
> > Let's change the zswap_rb_* prefixes to zswap_tree_* instead of just
> > zswap_*. Otherwise, it will be confusing to have both zswap_store and
> > zswap_insert (as well as zswap_load and zswap_search).

> How about zswap_xa_* ?

SGTM.

> >
> > [..]
> > > @@ -1790,15 +1808,21 @@ void zswap_swapon(int type)
> > >  void zswap_swapoff(int type)
> > >  {
> > >         struct zswap_tree *tree = zswap_trees[type];
> > > -       struct zswap_entry *entry, *n;
> > > +       struct zswap_entry *entry, *e, *n;
> > > +       XA_STATE(xas, tree ? &tree->xarray : NULL, 0);
> > >
> > >         if (!tree)
> > >                 return;
> > >
> > >         /* walk the tree and free everything */
> > >         spin_lock(&tree->lock);
> > > +
> > > +       xas_for_each(&xas, e, ULONG_MAX)
> >
> > Why not use xa_for_each?
> >
> > > +               zswap_invalidate_entry(tree, e);
> > > +
> > >         rbtree_postorder_for_each_entry_safe(entry, n, &tree->rbroot,  
> rbnode)
> > > -               zswap_free_entry(entry);
> >
> > Replacing zswap_free_entry() with zswap_invalidate_entry() is a
> > behavioral change that should be done separate from this series, but I
> > am wondering why it's needed. IIUC, the swapoff code should be making
> > sure there are no ongoing swapin/swapout operations, and there are no
> > pages left in zswap to writeback.
> >
> > Is it the case that swapoff may race with writeback, such that
> > writeback is holding the last remaining ref after zswap_invalidate()
> > is called, and then zswap_swapoff() is called freeing the zswap entry
> > while writeback is still accessing it?

> For the RB tree the mapping is stored in the zswap entry as RB node.
> That is different from xarray. Xarry stores the mapping outside of
> zswap entry. Just freeing the entry does not remove the mapping from
> xarray. Therefore it needs to call zswap_invalidate_entry() to remove
> the entry from the xarray. I could call zswap_erase() then free entry.
> I just think zswap_invalidate_entry() is more consistent with the rest
> of the code.

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.