On Mon, Jan 29, 2024 at 12:44 PM Jeff Layton <jlayton@xxxxxxxxxx> wrote:
>
> On Mon, 2024-01-29 at 12:34 -0500, David Wysochanski wrote:
> > On Mon, Jan 29, 2024 at 12:15 PM David Howells <dhowells@xxxxxxxxxx> wrote:
> > >
> > > Dave Wysochanski <dwysocha@xxxxxxxxxx> 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.
>
Thanks Jeff. Yes, I agree after looking at this further, this is a
good approach, and much cleaner. I'll work on a v2 patch (actually
with xa_for_each_range as you suggested off list) and send after
a bit of testing -- so far, so good.
FWIW, my original usage of RCU was outside the whole loop.
I ran into problems due to nfs_read_add_folio().
