Re: [PATCH v7] zswap: replace RB tree with xarray

[Chris Li <chrisl@kernel.org>]

On Wed, Mar 20, 2024 at 3:08 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Wed, Mar 20, 2024 at 07:24:27AM +0000, Yosry Ahmed wrote:
> > [..]
> > > > > -     /* map */
> > > > > -     spin_lock(&tree->lock);
> > > > >       /*
> > > > > -      * The folio may have been dirtied again, invalidate the
> > > > > -      * possibly stale entry before inserting the new entry.
> > > > > +      * We finish initializing the entry while it's already in xarray.
> > > > > +      * This is safe because:
> > > > > +      *
> > > > > +      * 1. Concurrent stores and invalidations are excluded by folio lock.
> > > > > +      *
> > > > > +      * 2. Writeback is excluded by the entry not being on the LRU yet.
> > > > > +      *    The publishing order matters to prevent writeback from seeing
> > > > > +      *    an incoherent entry.
> > > >
> > > > As I mentioned before, writeback is also protected by the folio lock.
> > > > Concurrent writeback will find the folio in the swapcache and abort. The
> > > > fact that the entry is not on the LRU yet is just additional protection,
> > > > so I don't think the publishing order actually matters here. Right?
> > >
> > > Right. This comment is explaining why this publishing order does not
> > > matter. I think we are talking about the same thing here?
> >
> > The comment literally says "the publishing order matters.." :)
> >
> > I believe Johannes meant that we should only publish the entry to the
> > LRU once it is fully initialized, to prevent writeback from using a
> > partially initialized entry.
> >
> > What I am saying is that, even if we add a partially initialized entry
> > to the zswap LRU, writeback will skip it anyway because the folio is
> > locked in the swapcache.
> >
> > So basically I think the comment should say:
> >
> >       /*
> >        * We finish initializing the entry while it's already in the
> >        * xarray. This is safe because the folio is locked in the swap
> >        * cache, which should protect against concurrent stores,
> >        * invalidations, and writeback.
> >        */
> >
> > Johannes, what do you think?
>
> I don't think that's quite right.
>
> Writeback will bail on swapcache insert, yes, but it will access the
> entry before attempting it. If LRU publishing happened before setting
> entry->swpentry e.g., we'd have a problem, while your comment suggets
> it would be safe to rearrange the code like this.
>
> So LRU publishing order does matter.

Yes, I agree with Johannes on this one. The publish order does matter,
it is not always safe recording the publish order. I will keep the V7
comments here.

Chris