[PATCH v2] filesystem-dax: Fix dax_layout_busy_page() livelock

[Dan Williams]

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@xxxxxxxxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: Ross Zwisler <zwisler@xxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
Changes in v2:
* Only update nr_pages if the last entry in the pagevec is multi-order.

 fs/dax.c |   13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c
index 4becbf168b7f..0fb270f0a0ef 100644
--- 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;