Fix txg_sync_thread deadlock

Update two kmem_alloc()'s in dbuf_dirty() to use KM_PUSHPAGE.
Because these functions are called from txg_sync_thread we
must ensure they don't reenter the zfs filesystem code via
the .writepage callback.  This would result in a deadlock.

This deadlock is rare and has only been observed once under
an abusive mmap() write workload.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

diff --git a/module/zfs/dbuf.c b/module/zfs/dbuf.c
index 1ebac4e..e166c81 100644 (file)
--- a/module/zfs/dbuf.c
+++ b/module/zfs/dbuf.c
@@ -1095,7 +1095,7 @@ dbuf_dirty(dmu_buf_impl_t *db, dmu_tx_t *tx)
                dn->dn_dirtyctx =
                    (dmu_tx_is_syncing(tx) ? DN_DIRTY_SYNC : DN_DIRTY_OPEN);
                ASSERT(dn->dn_dirtyctx_firstset == NULL);
-               dn->dn_dirtyctx_firstset = kmem_alloc(1, KM_SLEEP);
+               dn->dn_dirtyctx_firstset = kmem_alloc(1, KM_PUSHPAGE);
        }
        mutex_exit(&dn->dn_mtx);
 
@@ -1172,7 +1172,7 @@ dbuf_dirty(dmu_buf_impl_t *db, dmu_tx_t *tx)
         * to make a copy of it so that the changes we make in this
         * transaction group won't leak out when we sync the older txg.
         */
-       dr = kmem_zalloc(sizeof (dbuf_dirty_record_t), KM_SLEEP);
+       dr = kmem_zalloc(sizeof (dbuf_dirty_record_t), KM_PUSHPAGE);
        list_link_init(&dr->dr_dirty_node);
        if (db->db_level == 0) {
                void *data_old = db->db_buf;