diff options
| -rw-r--r-- | module/zfs/dmu.c | 35 |
1 files changed, 24 insertions, 11 deletions
diff --git a/module/zfs/dmu.c b/module/zfs/dmu.c index fc1dcd0de..81f434380 100644 --- a/module/zfs/dmu.c +++ b/module/zfs/dmu.c @@ -1656,19 +1656,32 @@ dmu_sync(zio_t *pio, uint64_t txg, dmu_sync_cb_t *done, zgd_t *zgd) ASSERT(dr->dr_next == NULL || dr->dr_next->dr_txg < txg); /* - * Assume the on-disk data is X, the current syncing data is Y, - * and the current in-memory data is Z (currently in dmu_sync). - * X and Z are identical but Y is has been modified. Normally, - * when X and Z are the same we will perform a nopwrite but if Y - * is different we must disable nopwrite since the resulting write - * of Y to disk can free the block containing X. If we allowed a - * nopwrite to occur the block pointing to Z would reference a freed - * block. Since this is a rare case we simplify this by disabling - * nopwrite if the current dmu_sync-ing dbuf has been modified in - * a previous transaction. + * Assume the on-disk data is X, the current syncing data (in + * txg - 1) is Y, and the current in-memory data is Z (currently + * in dmu_sync). + * + * We usually want to perform a nopwrite if X and Z are the + * same. However, if Y is different (i.e. the BP is going to + * change before this write takes effect), then a nopwrite will + * be incorrect - we would override with X, which could have + * been freed when Y was written. + * + * (Note that this is not a concern when we are nop-writing from + * syncing context, because X and Y must be identical, because + * all previous txgs have been synced.) + * + * Therefore, we disable nopwrite if the current BP could change + * before this TXG. There are two ways it could change: by + * being dirty (dr_next is non-NULL), or by being freed + * (dnode_block_freed()). This behavior is verified by + * zio_done(), which VERIFYs that the override BP is identical + * to the on-disk BP. */ - if (dr->dr_next) + DB_DNODE_ENTER(db); + dn = DB_DNODE(db); + if (dr->dr_next != NULL || dnode_block_freed(dn, db->db_blkid)) zp.zp_nopwrite = B_FALSE; + DB_DNODE_EXIT(db); ASSERT(dr->dr_txg == txg); if (dr->dt.dl.dr_override_state == DR_IN_DMU_SYNC || |