[PATCH 4.18 133/135] filesystem-dax: Fix dax_layout_busy_page() livelock

Greg Kroah-Hartman
2018-10-16T17:16:30Z

From: Dan Williams <dan.j.williams@intel.com>

commit d7782145e1ad537df4ce74e58c50f1f732a1462d upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

--- a/fs/dax.c
+++ b/fs/dax.c
@@ -558,6 +558,8 @@ struct page *dax_layout_busy_page(struct
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;
@@ -571,8 +573,15 @@ struct page *dax_layout_busy_page(struct

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)
@@ -580,7 +589,7 @@ struct page *dax_layout_busy_page(struct
}
pagevec_remove_exceptionals(&pvec);
pagevec_release(&pvec);
- index++;
+ index += nr_pages;

if (page)
break;