log xattr=sa create/remove/update to ZIL

As such, there are no specific synchronous semantics defined for
the xattrs. But for xattr=on, it does log to ZIL and zil_commit() is
done, if sync=always is set on dataset. This provides sync semantics
for xattr=on with sync=always set on dataset.

For the xattr=sa implementation, it doesn't log to ZIL, so, even with
sync=always, xattrs are not guaranteed to be synced before xattr call
returns to caller. So, xattr can be lost if system crash happens, before
txg carrying xattr transaction is synced.

This change adds xattr=sa logging to ZIL on xattr create/remove/update
and xattrs are synced to ZIL (zil_commit() done) for sync=always.
This makes xattr=sa behavior similar to xattr=on.

Implementation notes:
The actual logging is fairly straight-forward and does not warrant
additional explanation.
However, it has been 14 years since we last added new TX types
to the ZIL [1], hence this is the first time we do it after the
introduction of zpool features. Therefore, here is an overview of the
feature activation and deactivation workflow:

1. The feature must be enabled. Otherwise, we don't log the new
    record type. This ensures compatibility with older software.
2. The feature is activated per-dataset, since the ZIL is per-dataset.
3. If the feature is enabled and dataset is not for zvol, any append to
    the ZIL chain will activate the feature for the dataset. Likewise
    for starting a new ZIL chain.
4. A dataset that doesn't have a ZIL chain has the feature deactivated.

We ensure (3) by activating on the first zil_commit() after the feature
was enabled. Since activating the features requires waiting for txg
sync, the first zil_commit() after enabling the feature will be slower
than usual. The downside is that this is really a conservative
approximation: even if we never append a 'TX_SETSAXATTR' to the ZIL
chain, we pay the penalty for feature activation. The upside is that the
user is in control of when we pay the penalty, i.e., upon enabling the
feature.

We ensure (4) by hooking into zil_sync(), where ZIL destroy actually
happens.

One more piece on feature activation, since it's spread across
multiple functions:

zil_commit()
  zil_process_commit_list()
    if lwb == NULL // first zil_commit since zil_open
      zil_create()
        if no log block pointer in ZIL header:
          if feature enabled and not active:
	    // CASE 1
            enable, COALESCE txg wait with dmu_tx that allocated the
	    log block
         else // log block was allocated earlier than this zil_open
          if feature enabled and not active:
	    // CASE 2
            enable, EXPLICIT txg wait
    else // already have an in-DRAM LWB
      if feature enabled and not active:
        // this happens when we enable the feature after zil_create
	// CASE 3
        enable, EXPLICIT txg wait

[1] https://github.com/illumos/illumos-gate/commit/da6c28aaf62fa55f0fdb8004aa40f88f23bf53f0

Reviewed-by: Matthew Ahrens <[email protected]>
Reviewed-by: Christian Schwarz <[email protected]>
Reviewed-by: Ahelenia Ziemiańska <[email protected]>
Reviewed-by: Ryan Moeller <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Jitendra Patidar <[email protected]>
Closes #8768 
Closes #9078 

3 files changed, 14 insertions, 3 deletions

diff --git a/include/sys/zfs_sa.h b/include/sys/zfs_sa.h
index a0c383807..6b0336997 100644
--- a/include/sys/zfs_sa.h
+++ b/include/sys/zfs_sa.h
@@ -138,7 +138,7 @@ void zfs_sa_symlink(struct znode *, char *link, int len, dmu_tx_t *);
 void zfs_sa_get_scanstamp(struct znode *, xvattr_t *);
 void zfs_sa_set_scanstamp(struct znode *, xvattr_t *, dmu_tx_t *);
 int zfs_sa_get_xattr(struct znode *);
-int zfs_sa_set_xattr(struct znode *);
+int zfs_sa_set_xattr(struct znode *, const char *, const void *, size_t);
 void zfs_sa_upgrade(struct sa_handle  *, dmu_tx_t *);
 void zfs_sa_upgrade_txholds(dmu_tx_t *, struct znode *);
 void zfs_sa_init(void);
diff --git a/include/sys/zfs_znode.h b/include/sys/zfs_znode.h
index 1bf25a77d..e20c18cc2 100644
--- a/include/sys/zfs_znode.h
+++ b/include/sys/zfs_znode.h
@@ -286,6 +286,8 @@ extern void zfs_log_acl(zilog_t *zilog, dmu_tx_t *tx, znode_t *zp,
     vsecattr_t *vsecp, zfs_fuid_info_t *fuidp);
 extern void zfs_xvattr_set(znode_t *zp, xvattr_t *xvap, dmu_tx_t *tx);
 extern void zfs_upgrade(zfsvfs_t *zfsvfs, dmu_tx_t *tx);
+extern void zfs_log_setsaxattr(zilog_t *zilog, dmu_tx_t *tx, int txtype,
+    znode_t *zp, const char *name, const void *value, size_t size);
 
 extern void zfs_znode_update_vfs(struct znode *);
 
diff --git a/include/sys/zil.h b/include/sys/zil.h
index 8e5a49da2..05e3647e6 100644
--- a/include/sys/zil.h
+++ b/include/sys/zil.h
@@ -162,7 +162,8 @@ typedef enum zil_create {
 #define	TX_MKDIR_ATTR		18	/* mkdir with attr */
 #define	TX_MKDIR_ACL_ATTR	19	/* mkdir with ACL + attrs */
 #define	TX_WRITE2		20	/* dmu_sync EALREADY write */
-#define	TX_MAX_TYPE		21	/* Max transaction type */
+#define	TX_SETSAXATTR		21	/* Set sa xattrs on file */
+#define	TX_MAX_TYPE		22	/* Max transaction type */
 
 /*
  * The transactions for mkdir, symlink, remove, rmdir, link, and rename
@@ -182,7 +183,8 @@ typedef enum zil_create {
 	(txtype) == TX_SETATTR ||	\
 	(txtype) == TX_ACL_V0 ||	\
 	(txtype) == TX_ACL ||		\
-	(txtype) == TX_WRITE2)
+	(txtype) == TX_WRITE2 ||	\
+	(txtype) == TX_SETSAXATTR)
 
 /*
  * The number of dnode slots consumed by the object is stored in the 8
@@ -337,6 +339,13 @@ typedef struct {
 
 typedef struct {
 	lr_t		lr_common;	/* common portion of log record */
+	uint64_t	lr_foid;	/* file object to change attributes */
+	uint64_t	lr_size;
+	/* xattr name and value follows */
+} lr_setsaxattr_t;
+
+typedef struct {
+	lr_t		lr_common;	/* common portion of log record */
 	uint64_t	lr_foid;	/* obj id of file */
 	uint64_t	lr_aclcnt;	/* number of acl entries */
 	/* lr_aclcnt number of ace_t entries follow this */