From f321e4910398cf7922265d269fb17fd26f312571 Mon Sep 17 00:00:00 2001
From: Yan Zheng <zheng.yan@oracle.com>
Date: Wed, 30 Jul 2008 09:26:11 -0400
Subject: [PATCH] Btrfs: Update and fix mount -o nodatacow

To check whether a given file extent is referenced by multiple snapshots, the
checker walks down the fs tree through dead root and checks all tree blocks in
the path.

We can easily detect whether a given tree block is directly referenced by other
snapshot. We can also detect any indirect reference from other snapshot by
checking reference's generation. The checker can always detect multiple
references, but can't reliably detect cases of single reference. So btrfs may
do file data cow even there is only one reference.

Signed-off-by: Chris Mason <chris.mason@oracle.com>
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