Speed up WB_SYNC_NONE when a WB_SYNC_ALL occurs simultaneously

Page writebacks with WB_SYNC_NONE can take several seconds to complete 
since they wait for the transaction group to close before being 
committed. This is usually not a problem since the caller does not 
need to wait. However, if we're simultaneously doing a writeback 
with WB_SYNC_ALL (e.g via msync), the latter can block for several 
seconds (up to zfs_txg_timeout) due to the active WB_SYNC_NONE 
writeback since it needs to wait for the transaction to complete 
and the PG_writeback bit to be cleared.

This commit deals with 2 cases:

- No page writeback is active. A WB_SYNC_ALL page writeback starts 
  and even completes. But when it's about to check if the PG_writeback 
  bit has been cleared, another writeback with WB_SYNC_NONE starts. 
  The sync page writeback ends up waiting for the non-sync page 
  writeback to complete.

- A page writeback with WB_SYNC_NONE is already active when a 
  WB_SYNC_ALL writeback starts. The WB_SYNC_ALL writeback ends up 
  waiting for the WB_SYNC_NONE writeback.

The fix works by carefully keeping track of active sync/non-sync 
writebacks and committing when beneficial.

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Shaan Nobee <[email protected]>
Closes #12662
Closes #12790

1 files changed, 1 insertions, 1 deletions

diff --git a/tests/runfiles/common.run b/tests/runfiles/common.run
index b8310264a..1930dfab1 100644
--- a/tests/runfiles/common.run
+++ b/tests/runfiles/common.run
@@ -681,7 +681,7 @@ tests = ['migration_001_pos', 'migration_002_pos', 'migration_003_pos',
 tags = ['functional', 'migration']
 
 [tests/functional/mmap]
-tests = ['mmap_write_001_pos', 'mmap_read_001_pos', 'mmap_seek_001_pos']
+tests = ['mmap_write_001_pos', 'mmap_read_001_pos', 'mmap_seek_001_pos', 'mmap_sync_001_pos']
 tags = ['functional', 'mmap']
 
 [tests/functional/mount]