jbd2,ext4: add a shrinker to release checkpointed buffers

Current metadata buffer release logic in bdev_try_to_free_page() have
a lot of use-after-free issues when umount filesystem concurrently, and
it is difficult to fix directly because ext4 is the only user of
s_op->bdev_try_to_free_page callback and we may have to add more special
refcount or lock that is only used by ext4 into the common vfs layer,
which is unacceptable.

One better solution is remove the bdev_try_to_free_page callback, but
the real problem is we cannot easily release journal_head on the
checkpointed buffer, so try_to_free_buffers() cannot release buffers and
page under memory pressure, which is more likely to trigger
out-of-memory. So we cannot remove the callback directly before we find
another way to release journal_head.

This patch introduce a shrinker to free journal_head on the checkpointed
transaction. After the journal_head got freed, try_to_free_buffers()
could free buffer properly.

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Suggested-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20210610112440.3438139-6-yi.zhang@huawei.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

1 files changed, 101 insertions, 0 deletions

diff --git a/include/trace/events/jbd2.h b/include/trace/events/jbd2.h
index d16a32867f3a..a4dfe005983d 100644
--- a/include/trace/events/jbd2.h
+++ b/include/trace/events/jbd2.h
@@ -394,6 +394,107 @@ TRACE_EVENT(jbd2_lock_buffer_stall,
 		__entry->stall_ms)
 );
 
+DECLARE_EVENT_CLASS(jbd2_journal_shrink,
+
+	TP_PROTO(journal_t *journal, unsigned long nr_to_scan,
+		 unsigned long count),
+
+	TP_ARGS(journal, nr_to_scan, count),
+
+	TP_STRUCT__entry(
+		__field(dev_t, dev)
+		__field(unsigned long, nr_to_scan)
+		__field(unsigned long, count)
+	),
+
+	TP_fast_assign(
+		__entry->dev		= journal->j_fs_dev->bd_dev;
+		__entry->nr_to_scan	= nr_to_scan;
+		__entry->count		= count;
+	),
+
+	TP_printk("dev %d,%d nr_to_scan %lu count %lu",
+		  MAJOR(__entry->dev), MINOR(__entry->dev),
+		  __entry->nr_to_scan, __entry->count)
+);
+
+DEFINE_EVENT(jbd2_journal_shrink, jbd2_shrink_count,
+
+	TP_PROTO(journal_t *journal, unsigned long nr_to_scan, unsigned long count),
+
+	TP_ARGS(journal, nr_to_scan, count)
+);
+
+DEFINE_EVENT(jbd2_journal_shrink, jbd2_shrink_scan_enter,
+
+	TP_PROTO(journal_t *journal, unsigned long nr_to_scan, unsigned long count),
+
+	TP_ARGS(journal, nr_to_scan, count)
+);
+
+TRACE_EVENT(jbd2_shrink_scan_exit,
+
+	TP_PROTO(journal_t *journal, unsigned long nr_to_scan,
+		 unsigned long nr_shrunk, unsigned long count),
+
+	TP_ARGS(journal, nr_to_scan, nr_shrunk, count),
+
+	TP_STRUCT__entry(
+		__field(dev_t, dev)
+		__field(unsigned long, nr_to_scan)
+		__field(unsigned long, nr_shrunk)
+		__field(unsigned long, count)
+	),
+
+	TP_fast_assign(
+		__entry->dev		= journal->j_fs_dev->bd_dev;
+		__entry->nr_to_scan	= nr_to_scan;
+		__entry->nr_shrunk	= nr_shrunk;
+		__entry->count		= count;
+	),
+
+	TP_printk("dev %d,%d nr_to_scan %lu nr_shrunk %lu count %lu",
+		  MAJOR(__entry->dev), MINOR(__entry->dev),
+		  __entry->nr_to_scan, __entry->nr_shrunk,
+		  __entry->count)
+);
+
+TRACE_EVENT(jbd2_shrink_checkpoint_list,
+
+	TP_PROTO(journal_t *journal, tid_t first_tid, tid_t tid, tid_t last_tid,
+		 unsigned long nr_freed, unsigned long nr_scanned,
+		 tid_t next_tid),
+
+	TP_ARGS(journal, first_tid, tid, last_tid, nr_freed,
+		nr_scanned, next_tid),
+
+	TP_STRUCT__entry(
+		__field(dev_t, dev)
+		__field(tid_t, first_tid)
+		__field(tid_t, tid)
+		__field(tid_t, last_tid)
+		__field(unsigned long, nr_freed)
+		__field(unsigned long, nr_scanned)
+		__field(tid_t, next_tid)
+	),
+
+	TP_fast_assign(
+		__entry->dev		= journal->j_fs_dev->bd_dev;
+		__entry->first_tid	= first_tid;
+		__entry->tid		= tid;
+		__entry->last_tid	= last_tid;
+		__entry->nr_freed	= nr_freed;
+		__entry->nr_scanned	= nr_scanned;
+		__entry->next_tid	= next_tid;
+	),
+
+	TP_printk("dev %d,%d shrink transaction %u-%u(%u) freed %lu "
+		  "scanned %lu next transaction %u",
+		  MAJOR(__entry->dev), MINOR(__entry->dev),
+		  __entry->first_tid, __entry->tid, __entry->last_tid,
+		  __entry->nr_freed, __entry->nr_scanned, __entry->next_tid)
+);
+
 #endif /* _TRACE_JBD2_H */
 
 /* This part must be outside protection */