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Subject[block/for-next] 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.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Jan Kara <jack@suse.cz>
Fixes: 3a8e9ac89e6a ("writeback: add tracepoints for cgroup foreign writebacks")
---
include/trace/events/writeback.h | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
index 3dc9fb9e7c78..3a27335fce2c 100644
--- 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;
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