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

[Yosry Ahmed]

On Thu, Jan 18, 2024 at 9:28 PM Chris Li <chrisl@xxxxxxxxxx> wrote:
>
> On Thu, Jan 18, 2024 at 10:25 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> >
> > On Thu, Jan 18, 2024 at 08:59:55AM -0800, Yosry Ahmed wrote:
> > > On Thu, Jan 18, 2024 at 5:52 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> > > >
> > > > On Wed, Jan 17, 2024 at 10:20:29PM -0800, Yosry Ahmed wrote:
> > > > > >         /* walk the tree and free everything */
> > > > > >         spin_lock(&tree->lock);
> > > > > > +
> > > > > > +       xas_for_each(&xas, e, ULONG_MAX)
> > > > >
> > > > > Why not use xa_for_each?
> > > >
> > > > xas_for_each() is O(n) while xa_for_each() is O(n log n), as mentioned
> > > > in the fine documentation.  If you don't need to drop the lock while
> > > > in the body of the loop, always prefer xas_for_each().
> > >
> > > Thanks for pointing this out. Out of ignorance, I skipped reading the
> > > doc for this one and operated under the general assumption to use xa_*
> > > functions were possible.
> > >
> > > The doc also says we should hold either the RCU read lock or the
> > > xa_lock while iterating, we are not doing either here, no?
> >
> > I have no idea; I haven't studied the patches in detail yet.  I have
> > debugging assertions for that, so I was assuming that Chris had been
> > developing with debug options turned on.  If not, I guess the bots will
> > do it for him.
>
> It is fine now because we have the extra zswap tree spin lock. When we
> remove the zswap tree spin lock it does require RCU read lock. You are
> right I would get assertion failure.

I would imagine the assertions are that we are holding either the RCU
read lock or the xa_lock, how would holding the zswap tree lock help?