btrfs: send on fully deduped file
For fully deduped file, whose file extents are all pointing to the
same extent, btrfs backref walk can be very time consuming, long
enough to trigger softlock.
Unfortunately, btrfs send is one of the caller of such backref walk
under an O(n) loop, making the total time complexity to O(n^3) or
more.
And even worse, btrfs send will allocate memory in such loop, to
trigger OOM on system with small memory(<4G).
This test case will check if btrfs send will cause these problems.
Reporeted-by: Tsutomu Itoh <t-itoh@jp.fujitsu.com>
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>