OpenZFS 8997 - ztest assertion failure in zil_lwb_write_issue

PROBLEM
=======

When `dmu_tx_assign` is called from `zil_lwb_write_issue`, it's possible
for either `ERESTART` or `EIO` to be returned.

If `ERESTART` is returned, this will cause an assertion to fail directly
in `zil_lwb_write_issue`, where the code assumes the return value is
`EIO` if `dmu_tx_assign` returns a non-zero value. This can occur if the
SPA is suspended when `dmu_tx_assign` is called, and most often occurs
when running `zloop`.

If `EIO` is returned, this can cause assertions to fail elsewhere in the
ZIL code. For example, `zil_commit_waiter_timeout` contains the
following logic:

    lwb_t *nlwb = zil_lwb_write_issue(zilog, lwb);
    ASSERT3S(lwb->lwb_state, !=, LWB_STATE_OPENED);

In this case, if `dmu_tx_assign` returned `EIO` from within
`zil_lwb_write_issue`, the `lwb` variable passed in will not be issued
to disk. Thus, it's `lwb_state` field will remain `LWB_STATE_OPENED` and
this assertion will fail. `zil_commit_waiter_timeout` assumes that after
it calls `zil_lwb_write_issue`, the `lwb` will be issued to disk, and
doesn't handle the case where this is not true; i.e. it doesn't handle
the case where `dmu_tx_assign` returns `EIO`.

SOLUTION
========

This change modifies the `dmu_tx_assign` function such that `txg_how` is
a bitmask, rather than of the `txg_how_t` enum type. Now, the previous
`TXG_WAITED` semantics can be used via `TXG_NOTHROTTLE`, along with
specifying either `TXG_NOWAIT` or `TXG_WAIT` semantics.

Previously, when `TXG_WAITED` was specified, `TXG_NOWAIT` semantics was
automatically invoked. This was not ideal when using `TXG_WAITED` within
`zil_lwb_write_issued`, leading the problem described above. Rather, we
want to achieve the semantics of `TXG_WAIT`, while also preventing the
`tx` from being penalized via the dirty delay throttling.

With this change, `zil_lwb_write_issued` can acheive the semtantics that
it requires by passing in the value `TXG_WAIT | TXG_NOTHROTTLE` to
`dmu_tx_assign`.

Further, consumers of `dmu_tx_assign` wishing to achieve the old
`TXG_WAITED` semantics can pass in the value `TXG_NOWAIT | TXG_NOTHROTTLE`.

Authored by: Prakash Surya <prakash.surya@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed by: Andriy Gapon <avg@FreeBSD.org>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
Porting Notes:
- Additionally updated `zfs_tmpfile` to use `TXG_NOTHROTTLE`

OpenZFS-issue: https://www.illumos.org/issues/8997
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/19ea6cb0f9
Closes #7084

diff --git a/include/sys/dmu.h b/include/sys/dmu.h
index 755a90561ae5313b3e6e84d97d6c64281eca672e..5b355afb90bf4395307eb2d7e1fae356a745f97e 100644 (file)
--- a/include/sys/dmu.h
+++ b/include/sys/dmu.h
@@ -227,11 +227,14 @@ typedef enum dmu_object_type {
        DMU_OTN_ZAP_METADATA = DMU_OT(DMU_BSWAP_ZAP, B_TRUE),
 } dmu_object_type_t;
 
-typedef enum txg_how {
-       TXG_WAIT = 1,
-       TXG_NOWAIT,
-       TXG_WAITED,
-} txg_how_t;
+/*
+ * These flags are intended to be used to specify the "txg_how"
+ * parameter when calling the dmu_tx_assign() function. See the comment
+ * above dmu_tx_assign() for more details on the meaning of these flags.
+ */
+#define        TXG_NOWAIT      (0ULL)
+#define        TXG_WAIT        (1ULL<<0)
+#define        TXG_NOTHROTTLE  (1ULL<<1)
 
 void byteswap_uint64_array(void *buf, size_t size);
 void byteswap_uint32_array(void *buf, size_t size);
@@ -694,7 +697,7 @@ void dmu_tx_hold_spill(dmu_tx_t *tx, uint64_t object);
 void dmu_tx_hold_sa(dmu_tx_t *tx, struct sa_handle *hdl, boolean_t may_grow);
 void dmu_tx_hold_sa_create(dmu_tx_t *tx, int total_size);
 void dmu_tx_abort(dmu_tx_t *tx);
-int dmu_tx_assign(dmu_tx_t *tx, enum txg_how txg_how);
+int dmu_tx_assign(dmu_tx_t *tx, uint64_t txg_how);
 void dmu_tx_wait(dmu_tx_t *tx);
 void dmu_tx_commit(dmu_tx_t *tx);
 void dmu_tx_mark_netfree(dmu_tx_t *tx);

diff --git a/include/sys/dmu_tx.h b/include/sys/dmu_tx.h
index d82a79310db69339f03da1ef684671382f7d2b9d..74b7e1116382696c6582a83302667eba6718bd09 100644 (file)
--- a/include/sys/dmu_tx.h
+++ b/include/sys/dmu_tx.h
@@ -67,9 +67,6 @@ struct dmu_tx {
        /* placeholder for syncing context, doesn't need specific holds */
        boolean_t tx_anyobj;
 
-       /* has this transaction already been delayed? */
-       boolean_t tx_waited;
-
        /* transaction is marked as being a "net free" of space */
        boolean_t tx_netfree;
 
@@ -79,6 +76,9 @@ struct dmu_tx {
        /* need to wait for sufficient dirty space */
        boolean_t tx_wait_dirty;
 
+       /* has this transaction already been delayed? */
+       boolean_t tx_dirty_delayed;
+
        int tx_err;
 };
 
@@ -138,7 +138,7 @@ extern dmu_tx_stats_t dmu_tx_stats;
  * These routines are defined in dmu.h, and are called by the user.
  */
 dmu_tx_t *dmu_tx_create(objset_t *dd);
-int dmu_tx_assign(dmu_tx_t *tx, txg_how_t txg_how);
+int dmu_tx_assign(dmu_tx_t *tx, uint64_t txg_how);
 void dmu_tx_commit(dmu_tx_t *tx);
 void dmu_tx_abort(dmu_tx_t *tx);
 uint64_t dmu_tx_get_txg(dmu_tx_t *tx);

diff --git a/module/zfs/dmu_tx.c b/module/zfs/dmu_tx.c
index c3cc03a691a7776e0249c63c5798a830905bfae5..6ebff267144fbdd1471308aa9b6bfcf326a1871f 100644 (file)
--- a/module/zfs/dmu_tx.c
+++ b/module/zfs/dmu_tx.c
@@ -854,7 +854,7 @@ dmu_tx_delay(dmu_tx_t *tx, uint64_t dirty)
  * decreasing performance.
  */
 static int
-dmu_tx_try_assign(dmu_tx_t *tx, txg_how_t txg_how)
+dmu_tx_try_assign(dmu_tx_t *tx, uint64_t txg_how)
 {
        spa_t *spa = tx->tx_pool->dp_spa;
 
@@ -878,13 +878,13 @@ dmu_tx_try_assign(dmu_tx_t *tx, txg_how_t txg_how)
                 * of the failuremode setting.
                 */
                if (spa_get_failmode(spa) == ZIO_FAILURE_MODE_CONTINUE &&
-                   txg_how != TXG_WAIT)
+                   !(txg_how & TXG_WAIT))
                        return (SET_ERROR(EIO));
 
                return (SET_ERROR(ERESTART));
        }
 
-       if (!tx->tx_waited &&
+       if (!tx->tx_dirty_delayed &&
            dsl_pool_need_dirty_delay(tx->tx_pool)) {
                tx->tx_wait_dirty = B_TRUE;
                DMU_TX_STAT_BUMP(dmu_tx_dirty_delay);
@@ -976,41 +976,44 @@ dmu_tx_unassign(dmu_tx_t *tx)
 }
 
 /*
- * Assign tx to a transaction group.  txg_how can be one of:
+ * Assign tx to a transaction group; txg_how is a bitmask:
  *
- * (1) TXG_WAIT.  If the current open txg is full, waits until there's
- *     a new one.  This should be used when you're not holding locks.
- *     It will only fail if we're truly out of space (or over quota).
+ * If TXG_WAIT is set and the currently open txg is full, this function
+ * will wait until there's a new txg. This should be used when no locks
+ * are being held. With this bit set, this function will only fail if
+ * we're truly out of space (or over quota).
  *
- * (2) TXG_NOWAIT.  If we can't assign into the current open txg without
- *     blocking, returns immediately with ERESTART.  This should be used
- *     whenever you're holding locks.  On an ERESTART error, the caller
- *     should drop locks, do a dmu_tx_wait(tx), and try again.
+ * If TXG_WAIT is *not* set and we can't assign into the currently open
+ * txg without blocking, this function will return immediately with
+ * ERESTART. This should be used whenever locks are being held.  On an
+ * ERESTART error, the caller should drop all locks, call dmu_tx_wait(),
+ * and try again.
  *
- * (3) TXG_WAITED.  Like TXG_NOWAIT, but indicates that dmu_tx_wait()
- *     has already been called on behalf of this operation (though
- *     most likely on a different tx).
+ * If TXG_NOTHROTTLE is set, this indicates that this tx should not be
+ * delayed due on the ZFS Write Throttle (see comments in dsl_pool.c for
+ * details on the throttle). This is used by the VFS operations, after
+ * they have already called dmu_tx_wait() (though most likely on a
+ * different tx).
  */
 int
-dmu_tx_assign(dmu_tx_t *tx, txg_how_t txg_how)
+dmu_tx_assign(dmu_tx_t *tx, uint64_t txg_how)
 {
        int err;
 
        ASSERT(tx->tx_txg == 0);
-       ASSERT(txg_how == TXG_WAIT || txg_how == TXG_NOWAIT ||
-           txg_how == TXG_WAITED);
+       ASSERT0(txg_how & ~(TXG_WAIT | TXG_NOTHROTTLE));
        ASSERT(!dsl_pool_sync_context(tx->tx_pool));
 
-       if (txg_how == TXG_WAITED)
-               tx->tx_waited = B_TRUE;
-
        /* If we might wait, we must not hold the config lock. */
-       ASSERT(txg_how != TXG_WAIT || !dsl_pool_config_held(tx->tx_pool));
+       IMPLY((txg_how & TXG_WAIT), !dsl_pool_config_held(tx->tx_pool));
+
+       if ((txg_how & TXG_NOTHROTTLE))
+               tx->tx_dirty_delayed = B_TRUE;
 
        while ((err = dmu_tx_try_assign(tx, txg_how)) != 0) {
                dmu_tx_unassign(tx);
 
-               if (err != ERESTART || txg_how != TXG_WAIT)
+               if (err != ERESTART || !(txg_how & TXG_WAIT))
                        return (err);
 
                dmu_tx_wait(tx);
@@ -1054,12 +1057,12 @@ dmu_tx_wait(dmu_tx_t *tx)
                tx->tx_wait_dirty = B_FALSE;
 
                /*
-                * Note: setting tx_waited only has effect if the caller
-                * used TX_WAIT.  Otherwise they are going to destroy
-                * this tx and try again.  The common case, zfs_write(),
-                * uses TX_WAIT.
+                * Note: setting tx_dirty_delayed only has effect if the
+                * caller used TX_WAIT.  Otherwise they are going to
+                * destroy this tx and try again.  The common case,
+                * zfs_write(), uses TX_WAIT.
                 */
-               tx->tx_waited = B_TRUE;
+               tx->tx_dirty_delayed = B_TRUE;
        } else if (spa_suspended(spa) || tx->tx_lasttried_txg == 0) {
                /*
                 * If the pool is suspended we need to wait until it

diff --git a/module/zfs/zfs_vnops.c b/module/zfs/zfs_vnops.c
index 34ea751c32071dbc6e92c6ef16cb41c123b61d92..4805f89772961a8667a2a8ae57ad509467a1e9a2 100644 (file)
--- a/module/zfs/zfs_vnops.c
+++ b/module/zfs/zfs_vnops.c
@@ -129,7 +129,7 @@
  *
  *     If dmu_tx_assign() returns ERESTART and zfsvfs->z_assign is TXG_NOWAIT,
  *     then drop all locks, call dmu_tx_wait(), and try again.  On subsequent
- *     calls to dmu_tx_assign(), pass TXG_WAITED rather than TXG_NOWAIT,
+ *     calls to dmu_tx_assign(), pass TXG_NOTHROTTLE in addition to TXG_NOWAIT,
  *     to indicate that this operation has already called dmu_tx_wait().
  *     This will ensure that we don't retry forever, waiting a short bit
  *     each time.
@@ -154,7 +154,7 @@
  *     rw_enter(...);                  // grab any other locks you need
  *     tx = dmu_tx_create(...);        // get DMU tx
  *     dmu_tx_hold_*();                // hold each object you might modify
- *     error = dmu_tx_assign(tx, waited ? TXG_WAITED : TXG_NOWAIT);
+ *     error = dmu_tx_assign(tx, (waited ? TXG_NOTHROTTLE : 0) | TXG_NOWAIT);
  *     if (error) {
  *             rw_exit(...);           // drop locks
  *             zfs_dirent_unlock(dl);  // unlock directory entry
@@ -1427,7 +1427,8 @@ top:
                        dmu_tx_hold_write(tx, DMU_NEW_OBJECT,
                            0, acl_ids.z_aclp->z_acl_bytes);
                }
-               error = dmu_tx_assign(tx, waited ? TXG_WAITED : TXG_NOWAIT);
+               error = dmu_tx_assign(tx,
+                   (waited ? TXG_NOTHROTTLE : 0) | TXG_NOWAIT);
                if (error) {
                        zfs_dirent_unlock(dl);
                        if (error == ERESTART) {
@@ -1602,7 +1603,7 @@ top:
                dmu_tx_hold_write(tx, DMU_NEW_OBJECT,
                    0, acl_ids.z_aclp->z_acl_bytes);
        }
-       error = dmu_tx_assign(tx, waited ? TXG_WAITED : TXG_NOWAIT);
+       error = dmu_tx_assign(tx, (waited ? TXG_NOTHROTTLE : 0) | TXG_NOWAIT);
        if (error) {
                if (error == ERESTART) {
                        waited = B_TRUE;
@@ -1775,7 +1776,7 @@ top:
         */
        dmu_tx_mark_netfree(tx);
 
-       error = dmu_tx_assign(tx, waited ? TXG_WAITED : TXG_NOWAIT);
+       error = dmu_tx_assign(tx, (waited ? TXG_NOTHROTTLE : 0) | TXG_NOWAIT);
        if (error) {
                zfs_dirent_unlock(dl);
                if (error == ERESTART) {
@@ -2017,7 +2018,7 @@ top:
        dmu_tx_hold_sa_create(tx, acl_ids.z_aclp->z_acl_bytes +
            ZFS_SA_BASE_ATTR_SIZE);
 
-       error = dmu_tx_assign(tx, waited ? TXG_WAITED : TXG_NOWAIT);
+       error = dmu_tx_assign(tx, (waited ? TXG_NOTHROTTLE : 0) | TXG_NOWAIT);
        if (error) {
                zfs_dirent_unlock(dl);
                if (error == ERESTART) {
@@ -2156,7 +2157,7 @@ top:
        zfs_sa_upgrade_txholds(tx, zp);
        zfs_sa_upgrade_txholds(tx, dzp);
        dmu_tx_mark_netfree(tx);
-       error = dmu_tx_assign(tx, waited ? TXG_WAITED : TXG_NOWAIT);
+       error = dmu_tx_assign(tx, (waited ? TXG_NOTHROTTLE : 0) | TXG_NOWAIT);
        if (error) {
                rw_exit(&zp->z_parent_lock);
                rw_exit(&zp->z_name_lock);
@@ -3623,7 +3624,7 @@ top:
 
        zfs_sa_upgrade_txholds(tx, szp);
        dmu_tx_hold_zap(tx, zfsvfs->z_unlinkedobj, FALSE, NULL);
-       error = dmu_tx_assign(tx, waited ? TXG_WAITED : TXG_NOWAIT);
+       error = dmu_tx_assign(tx, (waited ? TXG_NOTHROTTLE : 0) | TXG_NOWAIT);
        if (error) {
                if (zl != NULL)
                        zfs_rename_unlock(&zl);
@@ -3815,7 +3816,7 @@ top:
        }
        if (fuid_dirtied)
                zfs_fuid_txhold(zfsvfs, tx);
-       error = dmu_tx_assign(tx, waited ? TXG_WAITED : TXG_NOWAIT);
+       error = dmu_tx_assign(tx, (waited ? TXG_NOTHROTTLE : 0) | TXG_NOWAIT);
        if (error) {
                zfs_dirent_unlock(dl);
                if (error == ERESTART) {
@@ -4041,7 +4042,7 @@ top:
 
        zfs_sa_upgrade_txholds(tx, szp);
        zfs_sa_upgrade_txholds(tx, dzp);
-       error = dmu_tx_assign(tx, waited ? TXG_WAITED : TXG_NOWAIT);
+       error = dmu_tx_assign(tx, (waited ? TXG_NOTHROTTLE : 0) | TXG_NOWAIT);
        if (error) {
                zfs_dirent_unlock(dl);
                if (error == ERESTART) {

diff --git a/module/zfs/zil.c b/module/zfs/zil.c
index 645b1d4d80bacb5375d7ff09a010a4629f40dea6..a2bbdcb9aa5d72cbd0b007723b152499f0390e82 100644 (file)
--- a/module/zfs/zil.c
+++ b/module/zfs/zil.c
@@ -1009,7 +1009,15 @@ zil_lwb_write_start(zilog_t *zilog, lwb_t *lwb)
         * to clean up in the event of allocation failure or I/O failure.
         */
        tx = dmu_tx_create(zilog->zl_os);
-       VERIFY(dmu_tx_assign(tx, TXG_WAIT) == 0);
+
+       /*
+        * Since we are not going to create any new dirty data, and we
+        * can even help with clearing the existing dirty data, we
+        * should not be subject to the dirty data based delays. We
+        * use TXG_NOTHROTTLE to bypass the delay mechanism.
+        */
+       VERIFY0(dmu_tx_assign(tx, TXG_WAIT | TXG_NOTHROTTLE));
+
        dsl_dataset_dirty(dmu_objset_ds(zilog->zl_os), tx);
        txg = dmu_tx_get_txg(tx);