Always wait for txg sync when umounting dataset

Currently, when unmounting a filesystem, ZFS will only wait for
a txg sync if the dataset is dirty and not readonly. However, this
can be problematic in cases where a dataset is remounted readonly
immediately before being unmounted, which often happens when the
system is being shut down. Since encrypted datasets require that
all I/O is completed before the dataset is disowned, this issue
causes problems when write I/Os leak into the txgs after the
dataset is disowned, which can happen when sync=disabled.

While looking into fixes for this issue, it was discovered that
dsl_dataset_is_dirty() does not return B_TRUE when the dataset has
been removed from the txg dirty datasets list, but has not actually
been processed yet. Furthermore, the implementation is comletely
different from dmu_objset_is_dirty(), adding to the confusion.
Rather than relying on this function, this patch forces the umount
code path (and the remount readonly code path) to always perform a
txg sync on read-write datasets and removes the function altogether.

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Tom Caputi <[email protected]>
Closes #7753
Closes #7795

1 files changed, 6 insertions, 3 deletions

diff --git a/module/zfs/zfs_vfsops.c b/module/zfs/zfs_vfsops.c
index 488eaa4f2..8ae2ef929 100644
--- a/module/zfs/zfs_vfsops.c
+++ b/module/zfs/zfs_vfsops.c
@@ -1745,10 +1745,10 @@ zfsvfs_teardown(zfsvfs_t *zfsvfs, boolean_t unmounting)
 	zfs_unregister_callbacks(zfsvfs);
 
 	/*
-	 * Evict cached data
+	 * Evict cached data. We must write out any dirty data before
+	 * disowning the dataset.
 	 */
-	if (dsl_dataset_is_dirty(dmu_objset_ds(zfsvfs->z_os)) &&
-	    !zfs_is_readonly(zfsvfs))
+	if (!zfs_is_readonly(zfsvfs))
 		txg_wait_synced(dmu_objset_pool(zfsvfs->z_os), 0);
 	dmu_objset_evict_dbufs(zfsvfs->z_os);
 
@@ -1970,6 +1970,9 @@ zfs_remount(struct super_block *sb, int *flags, zfs_mnt_t *zm)
 	if (error)
 		return (error);
 
+	if (!zfs_is_readonly(zfsvfs) && (*flags & MS_RDONLY))
+		txg_wait_synced(dmu_objset_pool(zfsvfs->z_os), 0);
+
 	zfs_unregister_callbacks(zfsvfs);
 	zfsvfs_vfs_free(zfsvfs->z_vfs);