xarray: Fix early termination of xas_for_each_marked

xas_for_each_marked() is using entry == NULL as a termination condition
of the iteration. When xas_for_each_marked() is used protected only by
RCU, this can however race with xas_store(xas, NULL) in the following
way:

TASK1                                   TASK2
page_cache_delete()         	        find_get_pages_range_tag()
                                          xas_for_each_marked()
                                            xas_find_marked()
                                              off = xas_find_chunk()

  xas_store(&xas, NULL)
    xas_init_marks(&xas);
    ...
    rcu_assign_pointer(*slot, NULL);
                                              entry = xa_entry(off);

And thus xas_for_each_marked() terminates prematurely possibly leading
to missed entries in the iteration (translating to missing writeback of
some pages or a similar problem).

If we find a NULL entry that has been marked, skip it (unless we're trying
to allocate an entry).

Reported-by: Jan Kara <jack@suse.cz>
CC: stable@vger.kernel.org
Fixes: ef8e5717db01 ("page cache: Convert delete_batch to XArray")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

1 files changed, 5 insertions, 1 deletions

diff --git a/include/linux/xarray.h b/include/linux/xarray.h
index a491653d8882..14c893433139 100644
--- a/include/linux/xarray.h
+++ b/include/linux/xarray.h
@@ -1648,6 +1648,7 @@ static inline void *xas_next_marked(struct xa_state *xas, unsigned long max,
 								xa_mark_t mark)
 {
 	struct xa_node *node = xas->xa_node;
+	void *entry;
 	unsigned int offset;
 
 	if (unlikely(xas_not_node(node) || node->shift))
@@ -1659,7 +1660,10 @@ static inline void *xas_next_marked(struct xa_state *xas, unsigned long max,
 		return NULL;
 	if (offset == XA_CHUNK_SIZE)
 		return xas_find_marked(xas, max, mark);
-	return xa_entry(xas->xa, node, offset);
+	entry = xa_entry(xas->xa, node, offset);
+	if (!entry)
+		return xas_find_marked(xas, max, mark);
+	return entry;
 }
 
 /*