writeback: don't access page->mapping directly in track_foreign_dirty TP

page->mapping may encode different values in it and page_mapping()
should always be used to access the mapping pointer.
track_foreign_dirty tracepoint was incorrectly accessing page->mapping
directly.  Use page_mapping() instead.  Also, add NULL checks while at
it.

Fixes: 3a8e9ac89e6a ("writeback: add tracepoints for cgroup foreign writebacks")
Reported-by: Jan Kara <jack@suse.cz>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
index 3dc9fb9e7c783e08d860e81b57829d9ce20bd0a4..3a27335fce2c77aa263a36b27fcd210565b3dc0e 100644 (file)
--- a/include/trace/events/writeback.h
+++ b/include/trace/events/writeback.h
@@ -251,9 +251,12 @@ TRACE_EVENT(track_foreign_dirty,
        ),
 
        TP_fast_assign(
+               struct address_space *mapping = page_mapping(page);
+               struct inode *inode = mapping ? mapping->host : NULL;
+
                strncpy(__entry->name,  dev_name(wb->bdi->dev), 32);
                __entry->bdi_id         = wb->bdi->id;
-               __entry->ino            = page->mapping->host->i_ino;
+               __entry->ino            = inode ? inode->i_ino : 0;
                __entry->memcg_id       = wb->memcg_css->id;
                __entry->cgroup_ino     = __trace_wb_assign_cgroup(wb);
                __entry->page_cgroup_ino = page->mem_cgroup->css.cgroup->kn->id.ino;