linux-kernel - Re: [PATCH 2/2] mm: zswap.c: remove RB tree

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Date: Thu, 18 Jan 2024 21:49:02 -0800
From: Chris Li <chrisl@...nel.org>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: 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>, "Matthew Wilcox (Oracle)" <willy@...radead.org>, 
	"Liam R. Howlett" <Liam.Howlett@...cle.com>, Joel Fernandes <joel@...lfernandes.org>, 
	Chengming Zhou <zhouchengming@...edance.com>
Subject: Re: [PATCH 2/2] mm: zswap.c: remove RB tree

On Thu, Jan 18, 2024 at 11:36 AM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
> On Wed, Jan 17, 2024 at 10:35 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
> >
> > > @@ -493,45 +471,47 @@ static struct zswap_entry *zswap_search(struct zswap_tree *tree, pgoff_t offset)
> > >  static int zswap_insert(struct zswap_tree *tree, struct zswap_entry *entry,
> > >                         struct zswap_entry **dupentry)
> > >  {
> > > -       struct rb_root *root = &tree->rbroot;
> > > -       struct rb_node **link = &root->rb_node, *parent = NULL;
> > > -       struct zswap_entry *myentry, *old;
> > > -       pgoff_t myentry_offset, entry_offset = swp_offset(entry->swpentry);
> > > -
> > > -
> > > -       while (*link) {
> > > -               parent = *link;
> > > -               myentry = rb_entry(parent, struct zswap_entry, rbnode);
> > > -               myentry_offset = swp_offset(myentry->swpentry);
> > > -               if (myentry_offset > entry_offset)
> > > -                       link = &(*link)->rb_left;
> > > -               else if (myentry_offset < entry_offset)
> > > -                       link = &(*link)->rb_right;
> > > -               else {
> > > -                       old = xa_load(&tree->xarray, entry_offset);
> > > -                       BUG_ON(old != myentry);
> > > -                       *dupentry = myentry;
> > > +       struct zswap_entry *e;
> > > +       pgoff_t offset = swp_offset(entry->swpentry);
> > > +       XA_STATE(xas, &tree->xarray, offset);
> > > +
> > > +       do {
> > > +               xas_lock_irq(&xas);
> > > +               do {
> > > +                       e = xas_load(&xas);
> > > +                       if (xa_is_zero(e))
> > > +                               e = NULL;
> > > +               } while (xas_retry(&xas, e));
> > > +               if (xas_valid(&xas) && e) {
> > > +                       xas_unlock_irq(&xas);
> > > +                       *dupentry = e;
> > >                         return -EEXIST;
> > >                 }
> > > -       }
> > > -       rb_link_node(&entry->rbnode, parent, link);
> > > -       rb_insert_color(&entry->rbnode, root);
> > > -       old = xa_store(&tree->xarray, entry_offset, entry, GFP_KERNEL);
> > > -       return 0;
> > > +               xas_store(&xas, entry);
> > > +               xas_unlock_irq(&xas);
> > > +       } while (xas_nomem(&xas, GFP_KERNEL));
> > > +       return xas_error(&xas);
> >
> > I think using the xas_* APIs can be avoided here. The only reason we
> > need it is that we want to check if there's an existing entry first,
> > and return -EEXIST. However, in that case, the caller will replace it
> > anyway (and do some operations on the dupentry):
> >
> > while (zswap_rb_insert(&tree->rbroot, entry, &dupentry) == -EEXIST) {
> >         WARN_ON(1);
> >         zswap_duplicate_entry++;
> >         zswap_invalidate_entry(tree, dupentry);
> > }
> >
> > So I think we can do something like this in zswap_insert() instead:
> >
> > dupentry = xa_store(..);
> > if (WARN_ON(dupentry)) {
> >         zswap_duplicate_entry++;
> >         zswap_invalidate_entry(tree, dupentry);
> > }
>
> If this is doable, I think we can return xa_store(..) and keep the
> logic in the caller. I think there's a chance
> zswap_{search/insert/erase} may end up being very thin wrappers around
> xa_{load/store/erase}, and we may be able to remove them completely.
> Let's see how it goes.
>

See my other email about erasing an entry raced into a new entry. That
is the part I am not certain.
Anyway, when zswap adopte folio, then it might need to handle multi
entry, we will be back to using the XAS API.

Chris