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MFC r358336, r358340: MFZoL: Fix txg_sync_thread hang in scan_exec_io()

Description

MFC r358336, r358340: MFZoL: Fix txg_sync_thread hang in scan_exec_io()

When scn->scn_maxinflight_bytes has not been initialized it's
possible to hang on the condition variable in scan_exec_io().
This issue was uncovered by ztest and is only possible when
deduplication is enabled through the following call path.

txg_sync_thread()
  spa_sync()
    ddt_sync_table()
      ddt_sync_entry()
        dsl_scan_ddt_entry()
          dsl_scan_scrub_cb()
            dsl_scan_enqueuei()
              scan_exec_io()
                cv_wait()

Resolve the issue by always initializing scn_maxinflight_bytes
to a reasonable minimum value. This value will be recalculated
in dsl_scan_sync() to pick up changes to zfs_scan_vdev_limit
and the addition/removal of vdevs.

Reviewed-by: Tom Caputi <tcaputi@datto.com>
Reviewed by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7098
zfsonlinux/zfs@f90a30ad1b32a971f62a540f8944e42f99b254ce

Details

Provenance
mavAuthored on
Parents
rS358599: MFC r358340: Fix patch mismerge in r358336.
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