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

[David Wysochanski <dwysocha@xxxxxxxxxx>]

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);