Reduce stack for traverse_visitbp() recursion

During pool import stack overflows may still occur due to the
potentially deep recursion of traverse_visitbp().  This is most
likely to occur when additional layers are added to the block
device stack such as DM multipath.  To minimize the stack usage
for this call path the following changes were made:

1) Added the keywork 'noinline' to the vdev_*_map_alloc() functions
   to prevent them from being inlined by gcc.  This reduced the
   stack usage of vdev_raidz_io_start() from 208 to 128 bytes, and
   vdev_mirror_io_start() from 144 to 128 bytes.

2) The 'saved_poolname' charater array in zfsdev_ioctl() was moved
   from the stack to the heap.  This reduced the stack usage of
   zfsdev_ioctl() from 368 to 112 bytes.

3) The major saving came from slimming down traverse_visitbp() from
   from 224 to 144 bytes.  Since this function is called recursively
   the 80 bytes saved per invokation adds up.  The following changes
   were made:

  a) The 'hard' local variable was replaced by a TD_HARD() macro.

  b) The 'pd' local variable was replaced by 'td->td_pfd' references.

  c) The zbookmark_t was moved to the heap.  This does cost us an
     additional memory allocation per recursion by that cost should
     still be minimal.  The cost could be further reduced by adding
     a dedicated zbookmark_t slab cache.

  d) The variable declarations in 'if (BP_GET_LEVEL()) { }' were
     restructured to use the minimum amount of stack.  This includes
     removing the 'cbp' local variable.

Overall for the offending use case roughly 1584 of total stack space
has been saved.  This is enough to avoid overflowing the stack on
stock kernels with 8k stacks.  See #1778 for additional details.

Signed-off-by: Brian Behlendorf <[email protected]>
Signed-off-by: Ned Bass <[email protected]>
Closes #1778

1 files changed, 41 insertions, 36 deletions

diff --git a/module/zfs/dmu_traverse.c b/module/zfs/dmu_traverse.c
index 939dfe2fa..e6d18df51 100644
--- a/module/zfs/dmu_traverse.c
+++ b/module/zfs/dmu_traverse.c
@@ -61,6 +61,8 @@ typedef struct traverse_data {
 	void *td_arg;
 } traverse_data_t;
 
+#define	TD_HARD(td)	(td->td_flags & TRAVERSE_HARD)
+
 static int traverse_dnode(traverse_data_t *td, const dnode_phys_t *dnp,
     uint64_t objset, uint64_t object);
 static void prefetch_dnode_metadata(traverse_data_t *td, const dnode_phys_t *,
@@ -208,11 +210,8 @@ static int
 traverse_visitbp(traverse_data_t *td, const dnode_phys_t *dnp,
     const blkptr_t *bp, const zbookmark_t *zb)
 {
-	zbookmark_t czb;
 	int err = 0, lasterr = 0;
 	arc_buf_t *buf = NULL;
-	prefetch_data_t *pd = td->td_pfd;
-	boolean_t hard = td->td_flags & TRAVERSE_HARD;
 	boolean_t pause = B_FALSE;
 
 	switch (resume_skip_check(td, dnp, zb)) {
@@ -234,16 +233,17 @@ traverse_visitbp(traverse_data_t *td, const dnode_phys_t *dnp,
 	if (bp->blk_birth <= td->td_min_txg)
 		return (0);
 
-	if (pd && !pd->pd_exited &&
-	    ((pd->pd_flags & TRAVERSE_PREFETCH_DATA) ||
+	if (td->td_pfd && !td->td_pfd->pd_exited &&
+	    ((td->td_pfd->pd_flags & TRAVERSE_PREFETCH_DATA) ||
 	    BP_GET_TYPE(bp) == DMU_OT_DNODE || BP_GET_LEVEL(bp) > 0)) {
-		mutex_enter(&pd->pd_mtx);
-		ASSERT(pd->pd_blks_fetched >= 0);
-		while (pd->pd_blks_fetched == 0 && !pd->pd_exited)
-			cv_wait(&pd->pd_cv, &pd->pd_mtx);
-		pd->pd_blks_fetched--;
-		cv_broadcast(&pd->pd_cv);
-		mutex_exit(&pd->pd_mtx);
+		mutex_enter(&td->td_pfd->pd_mtx);
+		ASSERT(td->td_pfd->pd_blks_fetched >= 0);
+		while (td->td_pfd->pd_blks_fetched == 0 &&
+		    !td->td_pfd->pd_exited)
+			cv_wait(&td->td_pfd->pd_cv, &td->td_pfd->pd_mtx);
+		td->td_pfd->pd_blks_fetched--;
+		cv_broadcast(&td->td_pfd->pd_cv);
+		mutex_exit(&td->td_pfd->pd_mtx);
 	}
 
 	if (td->td_flags & TRAVERSE_PRE) {
@@ -259,39 +259,45 @@ traverse_visitbp(traverse_data_t *td, const dnode_phys_t *dnp,
 
 	if (BP_GET_LEVEL(bp) > 0) {
 		uint32_t flags = ARC_WAIT;
-		int i;
-		blkptr_t *cbp;
-		int epb = BP_GET_LSIZE(bp) >> SPA_BLKPTRSHIFT;
+		int32_t i;
+		int32_t epb = BP_GET_LSIZE(bp) >> SPA_BLKPTRSHIFT;
+		zbookmark_t *czb;
 
 		err = arc_read(NULL, td->td_spa, bp, arc_getbuf_func, &buf,
 		    ZIO_PRIORITY_ASYNC_READ, ZIO_FLAG_CANFAIL, &flags, zb);
 		if (err != 0)
 			return (err);
-		cbp = buf->b_data;
+
+		czb = kmem_alloc(sizeof (zbookmark_t), KM_PUSHPAGE);
 
 		for (i = 0; i < epb; i++) {
-			SET_BOOKMARK(&czb, zb->zb_objset, zb->zb_object,
+			SET_BOOKMARK(czb, zb->zb_objset, zb->zb_object,
 			    zb->zb_level - 1,
 			    zb->zb_blkid * epb + i);
-			traverse_prefetch_metadata(td, &cbp[i], &czb);
+			traverse_prefetch_metadata(td,
+			    &((blkptr_t *)buf->b_data)[i], czb);
 		}
 
 		/* recursively visitbp() blocks below this */
 		for (i = 0; i < epb; i++) {
-			SET_BOOKMARK(&czb, zb->zb_objset, zb->zb_object,
+			SET_BOOKMARK(czb, zb->zb_objset, zb->zb_object,
 			    zb->zb_level - 1,
 			    zb->zb_blkid * epb + i);
-			err = traverse_visitbp(td, dnp, &cbp[i], &czb);
+			err = traverse_visitbp(td, dnp,
+			    &((blkptr_t *)buf->b_data)[i], czb);
 			if (err != 0) {
-				if (!hard)
+				if (!TD_HARD(td))
 					break;
 				lasterr = err;
 			}
 		}
+
+		kmem_free(czb, sizeof (zbookmark_t));
+
 	} else if (BP_GET_TYPE(bp) == DMU_OT_DNODE) {
 		uint32_t flags = ARC_WAIT;
-		int i;
-		int epb = BP_GET_LSIZE(bp) >> DNODE_SHIFT;
+		int32_t i;
+		int32_t epb = BP_GET_LSIZE(bp) >> DNODE_SHIFT;
 
 		err = arc_read(NULL, td->td_spa, bp, arc_getbuf_func, &buf,
 		    ZIO_PRIORITY_ASYNC_READ, ZIO_FLAG_CANFAIL, &flags, zb);
@@ -309,7 +315,7 @@ traverse_visitbp(traverse_data_t *td, const dnode_phys_t *dnp,
 			err = traverse_dnode(td, &dnp[i], zb->zb_objset,
 			    zb->zb_blkid * epb + i);
 			if (err != 0) {
-				if (!hard)
+				if (!TD_HARD(td))
 					break;
 				lasterr = err;
 			}
@@ -337,7 +343,7 @@ traverse_visitbp(traverse_data_t *td, const dnode_phys_t *dnp,
 
 		err = traverse_dnode(td, dnp, zb->zb_objset,
 		    DMU_META_DNODE_OBJECT);
-		if (err && hard) {
+		if (err && TD_HARD(td)) {
 			lasterr = err;
 			err = 0;
 		}
@@ -346,7 +352,7 @@ traverse_visitbp(traverse_data_t *td, const dnode_phys_t *dnp,
 			err = traverse_dnode(td, dnp, zb->zb_objset,
 			    DMU_USERUSED_OBJECT);
 		}
-		if (err && hard) {
+		if (err && TD_HARD(td)) {
 			lasterr = err;
 			err = 0;
 		}
@@ -369,7 +375,7 @@ post:
 
 	if (pause && td->td_resume != NULL) {
 		ASSERT3U(err, ==, ERESTART);
-		ASSERT(!hard);
+		ASSERT(!TD_HARD(td));
 		traverse_pause(td, zb);
 	}
 
@@ -400,13 +406,12 @@ traverse_dnode(traverse_data_t *td, const dnode_phys_t *dnp,
 {
 	int j, err = 0, lasterr = 0;
 	zbookmark_t czb;
-	boolean_t hard = (td->td_flags & TRAVERSE_HARD);
 
 	for (j = 0; j < dnp->dn_nblkptr; j++) {
 		SET_BOOKMARK(&czb, objset, object, dnp->dn_nlevels - 1, j);
 		err = traverse_visitbp(td, dnp, &dnp->dn_blkptr[j], &czb);
 		if (err != 0) {
-			if (!hard)
+			if (!TD_HARD(td))
 				break;
 			lasterr = err;
 		}
@@ -416,7 +421,7 @@ traverse_dnode(traverse_data_t *td, const dnode_phys_t *dnp,
 		SET_BOOKMARK(&czb, objset, object, 0, DMU_SPILL_BLKID);
 		err = traverse_visitbp(td, dnp, &dnp->dn_spill, &czb);
 		if (err != 0) {
-			if (!hard)
+			if (!TD_HARD(td))
 				return (err);
 			lasterr = err;
 		}
@@ -498,9 +503,9 @@ traverse_impl(spa_t *spa, dsl_dataset_t *ds, uint64_t objset, blkptr_t *rootbp,
 	 */
 	ASSERT(resume == NULL || !(flags & TRAVERSE_PREFETCH_DATA));
 
-	td = kmem_alloc(sizeof(traverse_data_t), KM_PUSHPAGE);
-	pd = kmem_zalloc(sizeof(prefetch_data_t), KM_PUSHPAGE);
-	czb = kmem_alloc(sizeof(zbookmark_t), KM_PUSHPAGE);
+	td = kmem_alloc(sizeof (traverse_data_t), KM_PUSHPAGE);
+	pd = kmem_zalloc(sizeof (prefetch_data_t), KM_PUSHPAGE);
+	czb = kmem_alloc(sizeof (zbookmark_t), KM_PUSHPAGE);
 
 	td->td_spa = spa;
 	td->td_objset = objset;
@@ -554,9 +559,9 @@ traverse_impl(spa_t *spa, dsl_dataset_t *ds, uint64_t objset, blkptr_t *rootbp,
 	mutex_destroy(&pd->pd_mtx);
 	cv_destroy(&pd->pd_cv);
 
-	kmem_free(czb, sizeof(zbookmark_t));
-	kmem_free(pd, sizeof(struct prefetch_data));
-	kmem_free(td, sizeof(struct traverse_data));
+	kmem_free(czb, sizeof (zbookmark_t));
+	kmem_free(pd, sizeof (struct prefetch_data));
+	kmem_free(td, sizeof (struct traverse_data));
 
 	return (err);
 }