Re: [PATCH v4 00/16] Overhaul multi-page lookups for THP

[Matthew Wilcox <willy@infradead.org>]

On Wed, Nov 25, 2020 at 04:11:57PM -0800, Hugh Dickins wrote:
> The little fix definitely needed was shown by generic/083: each
> fsstress waiting for page lock, happens even without forcing huge
> pages. See below...

Huh ... I need to look into why my xfstests run is skipping generic/083:

0006 generic/083 3s ... run fstests generic/083 at 2020-11-26 12:11:52
0006 [not run] this test requires a valid $SCRATCH_MNT and unique 
0006 Ran: generic/083
0006 Not run: generic/083

> >                         if (!unfalloc || !PageUptodate(page)) {
> >                                 if (page_mapping(page) != mapping) {
> >                                         /* Page was replaced by swap: retry */
> >                                         unlock_page(page);
> > -                                       index--;
> > +                                       put_page(page);
> >                                         break;
> >                                 }
> >                                 VM_BUG_ON_PAGE(PageWriteback(page), page);
> > -                               if (shmem_punch_compound(page, start, end))
> > -                                       truncate_inode_page(mapping, page);
> > -                               else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
> > -                                       /* Wipe the page and don't get stuck */
> > -                                       clear_highpage(page);
> > -                                       flush_dcache_page(page);
> > -                                       set_page_dirty(page);
> > -                                       if (index <
> > -                                           round_up(start, HPAGE_PMD_NR))
> > -                                               start = index + 1;
> > -                               }
> > +                               index = truncate_inode_partial_page(mapping,
> > +                                               page, lstart, lend);
> > +                               if (index > end)
> > +                                       end = indices[i] - 1;
> >                         }
> > -                       unlock_page(page);
> 
> The fix needed is here: instead of deleting that unlock_page(page)
> line, it needs to be } else { unlock_page(page); }

It also needs a put_page(page);

That's now taken care of by truncate_inode_partial_page(), so if we're
not calling that, we need to put the page as well.  ie this:

+++ b/mm/shmem.c
@@ -954,6 +954,9 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, loff_t lend,
                                                page, lstart, lend);
                                if (index > end)
                                        end = indices[i] - 1;
+                       } else {
+                               unlock_page(page);
+                               put_page(page);
                        }
                }
                index = indices[i - 1] + 1;

> >                 }
> > +               index = indices[i - 1] + 1;
> >                 pagevec_remove_exceptionals(&pvec);
> > -               pagevec_release(&pvec);
> > -               index++;
> > +               pagevec_reinit(&pvec);