filesystem-dax: Fix dax_layout_busy_page() livelock

In the presence of multi-order entries the typical
pagevec_lookup_entries() pattern may loop forever:

	while (index < end && pagevec_lookup_entries(&pvec, mapping, index,
				min(end - index, (pgoff_t)PAGEVEC_SIZE),
				indices)) {
		...
		for (i = 0; i < pagevec_count(&pvec); i++) {
			index = indices[i];
			...
		}
		index++; /* BUG */
	}

The loop updates 'index' for each index found and then increments to the
next possible page to continue the lookup. However, if the last entry in
the pagevec is multi-order then the next possible page index is more
than 1 page away. Fix this locally for the filesystem-dax case by
checking for dax-multi-order entries. Going forward new users of
multi-order entries need to be similarly careful, or we need a generic
way to report the page increment in the radix iterator.

Fixes: 5fac7408d828 ("mm, fs, dax: handle layout changes to pinned dax...")
Cc: <stable@vger.kernel.org>
Cc: Ross Zwisler <zwisler@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

diff --git a/fs/dax.c b/fs/dax.c
index 4becbf1..0fb270f 100644 (file)
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -666,6 +666,8 @@ struct page *dax_layout_busy_page(struct address_space *mapping)
        while (index < end && pagevec_lookup_entries(&pvec, mapping, index,
                                min(end - index, (pgoff_t)PAGEVEC_SIZE),
                                indices)) {
+               pgoff_t nr_pages = 1;
+
                for (i = 0; i < pagevec_count(&pvec); i++) {
                        struct page *pvec_ent = pvec.pages[i];
                        void *entry;
@@ -680,8 +682,15 @@ struct page *dax_layout_busy_page(struct address_space *mapping)
 
                        xa_lock_irq(&mapping->i_pages);
                        entry = get_unlocked_mapping_entry(mapping, index, NULL);
-                       if (entry)
+                       if (entry) {
                                page = dax_busy_page(entry);
+                               /*
+                                * Account for multi-order entries at
+                                * the end of the pagevec.
+                                */
+                               if (i + 1 >= pagevec_count(&pvec))
+                                       nr_pages = 1UL << dax_radix_order(entry);
+                       }
                        put_unlocked_mapping_entry(mapping, index, entry);
                        xa_unlock_irq(&mapping->i_pages);
                        if (page)
@@ -696,7 +705,7 @@ struct page *dax_layout_busy_page(struct address_space *mapping)
                 */
                pagevec_remove_exceptionals(&pvec);
                pagevec_release(&pvec);
-               index++;
+               index += nr_pages;
 
                if (page)
                        break;