Re: [PATCH] NFS: Fix nfs_netfs_issue_read() xarray locking for writeback interrupt

[Jeff Layton <jlayton@kernel.org>]

On Mon, 2024-01-29 at 12:34 -0500, David Wysochanski wrote:
> On Mon, Jan 29, 2024 at 12:15 PM David Howells <dhowells@redhat.com> wrote:
> > 
> > Dave Wysochanski <dwysocha@redhat.com> wrote:
> > 
> > > -     xas_lock(&xas);
> > > +     xas_lock_irqsave(&xas, flags);
> > >       xas_for_each(&xas, page, last) {
> > 
> > You probably want to use RCU, not xas_lock().  The pages are locked and so
> > cannot be evicted from the xarray.
> > 
> 
> I tried RCU originally and ran into a problem because NFS can schedule
> (see comment on line 328 below)
> 
> 326         xas_lock_irqsave(&xas, flags);
> 327         xas_for_each(&xas, page, last) {
> 328                 /* nfs_read_add_folio() may schedule() due to pNFS
> layout and other RPCs  */
> 329                 xas_pause(&xas);
> 330                 xas_unlock_irqrestore(&xas, flags);
> 331                 err = nfs_read_add_folio(&pgio, ctx, page_folio(page));
> 332                 if (err < 0) {
> 333                         netfs->error = err;
> 334                         goto out;
> 335                 }
> 336                 xas_lock_irqsave(&xas, flags);
> 337         }
> 338         xas_unlock_irqrestore(&xas, flags);
> 

Looking at it more closely, I think you might want to just use
xa_for_each_start(). That will do the traversal using the rcu_read_lock
under the hood, and you should be able to block on every iteration.

-- 
Jeff Layton <jlayton@kernel.org>