Reduce the stack usage of dsl_dataset_remove_clones_key

dataset_remove_clones_key does recursion, so if the recursion goes
deep it can overrun the linux kernel stack size of 8KB. I have seen
this happen in the actual deployment, and subsequently confirmed it by
running a test workload on a custom-built kernel that uses 32KB stack.

See the following stack trace as an example of the case where it would
have run over the 8KB stack kernel:

        Depth    Size   Location    (42 entries)
        -----    ----   --------
  0)    11192      72   __kmalloc+0x2e/0x240
  1)    11120     144   kmem_alloc_debug+0x20e/0x500
  2)    10976      72   dbuf_hold_impl+0x4a/0xa0
  3)    10904     120   dbuf_prefetch+0xd3/0x280
  4)    10784      80   dmu_zfetch_dofetch.isra.5+0x10f/0x180
  5)    10704     240   dmu_zfetch+0x5f7/0x10e0
  6)    10464     168   dbuf_read+0x71e/0x8f0
  7)    10296     104   dnode_hold_impl+0x1ee/0x620
  8)    10192      16   dnode_hold+0x19/0x20
  9)    10176      88   dmu_buf_hold+0x42/0x1b0
 10)    10088     144   zap_lockdir+0x48/0x730
 11)     9944     128   zap_cursor_retrieve+0x1c4/0x2f0
 12)     9816     392   dsl_dataset_remove_clones_key.isra.14+0xab/0x190
 13)     9424     392   dsl_dataset_remove_clones_key.isra.14+0x10c/0x190
 14)     9032     392   dsl_dataset_remove_clones_key.isra.14+0x10c/0x190
 15)     8640     392   dsl_dataset_remove_clones_key.isra.14+0x10c/0x190
 16)     8248     392   dsl_dataset_remove_clones_key.isra.14+0x10c/0x190
 17)     7856     392   dsl_dataset_remove_clones_key.isra.14+0x10c/0x190
 18)     7464     392   dsl_dataset_remove_clones_key.isra.14+0x10c/0x190
 19)     7072     392   dsl_dataset_remove_clones_key.isra.14+0x10c/0x190
 20)     6680     392   dsl_dataset_remove_clones_key.isra.14+0x10c/0x190
 21)     6288     392   dsl_dataset_remove_clones_key.isra.14+0x10c/0x190
 22)     5896     392   dsl_dataset_remove_clones_key.isra.14+0x10c/0x190
 23)     5504     392   dsl_dataset_remove_clones_key.isra.14+0x10c/0x190
 24)     5112     392   dsl_dataset_remove_clones_key.isra.14+0x10c/0x190
 25)     4720     392   dsl_dataset_remove_clones_key.isra.14+0x10c/0x190
 26)     4328     392   dsl_dataset_remove_clones_key.isra.14+0x10c/0x190
 27)     3936     392   dsl_dataset_remove_clones_key.isra.14+0x10c/0x190
 28)     3544     392   dsl_dataset_remove_clones_key.isra.14+0x10c/0x190
 29)     3152     392   dsl_dataset_remove_clones_key.isra.14+0x10c/0x190
 30)     2760     392   dsl_dataset_remove_clones_key.isra.14+0x10c/0x190
 31)     2368     392   dsl_dataset_remove_clones_key.isra.14+0x10c/0x190
 32)     1976     392   dsl_dataset_remove_clones_key.isra.14+0x10c/0x190
 33)     1584     392   dsl_dataset_remove_clones_key.isra.14+0x10c/0x190
 34)     1192     232   dsl_dataset_destroy_sync+0x311/0xf60
 35)      960      72   dsl_sync_task_group_sync+0x12f/0x230
 36)      888     168   dsl_pool_sync+0x48b/0x5c0
 37)      720     184   spa_sync+0x417/0xb00
 38)      536     184   txg_sync_thread+0x325/0x5b0
 39)      352      48   thread_generic_wrapper+0x7a/0x90
 40)      304     128   kthread+0xc0/0xd0
 41)      176     176   ret_from_fork+0x7c/0xb0

This change reduces the stack usage in dsl_dataset_remove_clones_key
by allocating structures in heap, not in stack.  This is not a fundamental
fix, as one can create an arbitrary large data set that runs over any
fixed size stack, but this will make the problem far less likely.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Kohsuke Kawaguchi <kk@kohsuke.org>
Closes #1726

diff --git a/module/zfs/dsl_destroy.c b/module/zfs/dsl_destroy.c
index 1fb3859ac242c1fe0408ec59363e8bcabd4ba2cf..5c80c4eee3536d9ab691d5c4b8f26deee771484e 100644 (file)
--- a/module/zfs/dsl_destroy.c
+++ b/module/zfs/dsl_destroy.c
@@ -201,8 +201,8 @@ static void
 dsl_dataset_remove_clones_key(dsl_dataset_t *ds, uint64_t mintxg, dmu_tx_t *tx)
 {
        objset_t *mos = ds->ds_dir->dd_pool->dp_meta_objset;
-       zap_cursor_t zc;
-       zap_attribute_t za;
+       zap_cursor_t *zc;
+       zap_attribute_t *za;
 
        /*
         * If it is the old version, dd_clones doesn't exist so we can't
@@ -212,13 +212,16 @@ dsl_dataset_remove_clones_key(dsl_dataset_t *ds, uint64_t mintxg, dmu_tx_t *tx)
        if (ds->ds_dir->dd_phys->dd_clones == 0)
                return;
 
-       for (zap_cursor_init(&zc, mos, ds->ds_dir->dd_phys->dd_clones);
-           zap_cursor_retrieve(&zc, &za) == 0;
-           zap_cursor_advance(&zc)) {
+       zc = kmem_alloc(sizeof (zap_cursor_t), KM_PUSHPAGE);
+       za = kmem_alloc(sizeof (zap_attribute_t), KM_PUSHPAGE);
+
+       for (zap_cursor_init(zc, mos, ds->ds_dir->dd_phys->dd_clones);
+           zap_cursor_retrieve(zc, za) == 0;
+           zap_cursor_advance(zc)) {
                dsl_dataset_t *clone;
 
                VERIFY0(dsl_dataset_hold_obj(ds->ds_dir->dd_pool,
-                   za.za_first_integer, FTAG, &clone));
+                   za->za_first_integer, FTAG, &clone));
                if (clone->ds_dir->dd_origin_txg > mintxg) {
                        dsl_deadlist_remove_key(&clone->ds_deadlist,
                            mintxg, tx);
@@ -226,7 +229,10 @@ dsl_dataset_remove_clones_key(dsl_dataset_t *ds, uint64_t mintxg, dmu_tx_t *tx)
                }
                dsl_dataset_rele(clone, FTAG);
        }
-       zap_cursor_fini(&zc);
+       zap_cursor_fini(zc);
+
+       kmem_free(za, sizeof (zap_attribute_t));
+       kmem_free(zc, sizeof (zap_cursor_t));
 }
 
 void