Re: [PATCH v6] zswap: replace RB tree with xarray

[Chris Li]

On Sat, Mar 16, 2024 at 6:33 AM Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
>
> On Fri, Mar 15, 2024 at 06:30:37PM -0700, Yosry Ahmed wrote:
> > [..]
> > >
> > > @@ -1555,28 +1473,35 @@ bool zswap_store(struct folio *folio)
> > >  insert_entry:
> > >         entry->swpentry = swp;
> > >         entry->objcg = objcg;
> > > -       if (objcg) {
> > > -               obj_cgroup_charge_zswap(objcg, entry->length);
> > > -               /* Account before objcg ref is moved to tree */
> >
> >
> > I do not understand this comment, but it seems to care about the
> > charging happening before the entry is added to the tree. This patch
> > will move it after the tree insertion.
> >
> > Johannes, do you mind elaborating what this comment is referring to?
> > It should be clarified, updated, or removed as part of this movement.
>
> Wait, I wrote that? ^_^
>
> The thinking was this: the objcg reference acquired in this context is
> passed on to the tree. Once the entry is in the tree and the
> tree->lock released, the entry is public and the current context
> doesn't have its own pin on objcg anymore. Ergo, objcg is no longer
> safe to access from this context.
>
> This is a conservative take, though, considering the wider context:
> the swapcache itself, through folio lock, prevents invalidation; and
> reclaim/writeback cannot happen before the entry is on the LRU.
>
> After Chris's patch, the tree is no longer a serialization point for
> stores. The swapcache and the LRU are. I had asked Chris upthread to
> add an explicit comment about that. I think once he does that, the
> objcg situation should be self-evident as well.
>
> So in the next version, please just remove this now stale one-liner.

Ack.

Chris