Fix lseek(SEEK_DATA/SEEK_HOLE) mmap consistency

When using lseek(2) to report data/holes memory mapped regions of
the file were ignored.  This could result in incorrect results.
To handle this zfs_holey_common() was updated to asynchronously
writeback any dirty mmap(2) regions prior to reporting holes.

Additionally, while not strictly required, the dn_struct_rwlock is
now held over the dirty check to prevent the dnode structure from
changing.  This ensures that a clean dnode can't be dirtied before
the data/hole is located.  The range lock is now also taken to
ensure the call cannot race with zfs_write().

Furthermore, the code was refactored to provide a dnode_is_dirty()
helper function which checks the dnode for any dirty records to
determine its dirtiness.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Rich Ercolani <rincebrain@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #11900
Closes #12724

module/zfs/dmu.c

@@ -2045,42 +2045,41 @@ int
 dmu_offset_next(objset_t *os, uint64_t object, boolean_t hole, uint64_t *off)
 {
 	dnode_t *dn;
-	int i, err;
-	boolean_t clean = B_TRUE;
+	int err;
 
+restart:
 	err = dnode_hold(os, object, FTAG, &dn);
 	if (err)
 		return (err);
 
-	/*
-	 * Check if dnode is dirty
-	 */
-	for (i = 0; i < TXG_SIZE; i++) {
-		if (multilist_link_active(&dn->dn_dirty_link[i])) {
-			clean = B_FALSE;
-			break;
+	rw_enter(&dn->dn_struct_rwlock, RW_READER);
+
+	if (dnode_is_dirty(dn)) {
+		/*
+		 * If the zfs_dmu_offset_next_sync module option is enabled
+		 * then strict hole reporting has been requested.  Dirty
+		 * dnodes must be synced to disk to accurately report all
+		 * holes.  When disabled (the default) dirty dnodes are
+		 * reported to not have any holes which is always safe.
+		 *
+		 * When called by zfs_holey_common() the zp->z_rangelock
+		 * is held to prevent zfs_write() and mmap writeback from
+		 * re-dirtying the dnode after txg_wait_synced().
+		 */
+		if (zfs_dmu_offset_next_sync) {
+			rw_exit(&dn->dn_struct_rwlock);
+			dnode_rele(dn, FTAG);
+			txg_wait_synced(dmu_objset_pool(os), 0);
+			goto restart;
 		}
-	}
 
-	/*
-	 * If compatibility option is on, sync any current changes before
-	 * we go trundling through the block pointers.
-	 */
-	if (!clean && zfs_dmu_offset_next_sync) {
-		clean = B_TRUE;
-		dnode_rele(dn, FTAG);
-		txg_wait_synced(dmu_objset_pool(os), 0);
-		err = dnode_hold(os, object, FTAG, &dn);
-		if (err)
-			return (err);
-	}
-
-	if (clean)
-		err = dnode_next_offset(dn,
-		    (hole ? DNODE_FIND_HOLE : 0), off, 1, 1, 0);
-	else
 		err = SET_ERROR(EBUSY);
+	} else {
+		err = dnode_next_offset(dn, DNODE_FIND_HAVELOCK |
+		    (hole ? DNODE_FIND_HOLE : 0), off, 1, 1, 0);
+	}
 
+	rw_exit(&dn->dn_struct_rwlock);
 	dnode_rele(dn, FTAG);
 
 	return (err);

module/zfs/dnode.c

@@ -1653,6 +1653,26 @@ dnode_try_claim(objset_t *os, uint64_t object, int slots)
 	    slots, NULL, NULL));
 }
 
+/*
+ * Checks if the dnode contains any uncommitted dirty records.
+ */
+boolean_t
+dnode_is_dirty(dnode_t *dn)
+{
+	mutex_enter(&dn->dn_mtx);
+
+	for (int i = 0; i < TXG_SIZE; i++) {
+		if (list_head(&dn->dn_dirty_records[i]) != NULL) {
+			mutex_exit(&dn->dn_mtx);
+			return (B_TRUE);
+		}
+	}
+
+	mutex_exit(&dn->dn_mtx);
+
+	return (B_FALSE);
+}
+
 void
 dnode_setdirty(dnode_t *dn, dmu_tx_t *tx)
 {

module/zfs/zfs_vnops.c

@@ -85,6 +85,7 @@ zfs_fsync(znode_t *zp, int syncflag, cred_t *cr)
 static int
 zfs_holey_common(znode_t *zp, ulong_t cmd, loff_t *off)
 {
+	zfs_locked_range_t *lr;
 	uint64_t noff = (uint64_t)*off; /* new offset */
 	uint64_t file_sz;
 	int error;
@@ -100,12 +101,18 @@ zfs_holey_common(znode_t *zp, ulong_t cmd, loff_t *off)
 	else
 		hole = B_FALSE;
 
+	/* Flush any mmap()'d data to disk */
+	if (zn_has_cached_data(zp))
+		zn_flush_cached_data(zp, B_FALSE);
+
+	lr = zfs_rangelock_enter(&zp->z_rangelock, 0, file_sz, RL_READER);
 	error = dmu_offset_next(ZTOZSB(zp)->z_os, zp->z_id, hole, &noff);
+	zfs_rangelock_exit(lr);
 
 	if (error == ESRCH)
 		return (SET_ERROR(ENXIO));
 
-	/* file was dirty, so fall back to using generic logic */
+	/* File was dirty, so fall back to using generic logic */
 	if (error == EBUSY) {
 		if (hole)
 			*off = file_sz;