Fix stack traverse_visitbp()

Due to  limited stack space recursive functions are frowned upon in
the Linux kernel.  However, they often are the most elegant solution
to a problem.  The following code preserves the recursive function
traverse_visitbp() but moves the local variables AND function
arguments to the heap to minimize the stack frame size.  Enough
space is initially allocated on the stack for 20 levels of recursion.
This change does ugly-up-the-code but it reduces the worst case
usage from roughly 4160 bytes to 960 bytes on x86_64 archs.

Signed-off-by: Brian Behlendorf <[email protected]>

1 files changed, 177 insertions, 98 deletions

diff --git a/module/zfs/dmu_traverse.c b/module/zfs/dmu_traverse.c
index fb102c6ca..ee7eb5f8a 100644
--- a/module/zfs/dmu_traverse.c
+++ b/module/zfs/dmu_traverse.c
@@ -59,6 +59,31 @@ typedef struct traverse_data {
 	void *td_arg;
 } traverse_data_t;
 
+typedef struct traverse_visitbp_data {
+	/* Function arguments */
+	traverse_data_t *tv_td;
+	const dnode_phys_t *tv_dnp;
+	arc_buf_t *tv_pbuf;
+	blkptr_t *tv_bp;
+	const zbookmark_t *tv_zb;
+	/* Local variables */
+	prefetch_data_t *tv_pd;
+	zbookmark_t tv_czb;
+	arc_buf_t *tv_buf;
+	boolean_t tv_hard;
+	objset_phys_t *tv_osp;
+	dnode_phys_t *tv_ldnp;
+	blkptr_t *tv_cbp;
+	uint32_t tv_flags;
+	int tv_err;
+	int tv_lasterr;
+	int tv_i;
+	int tv_epb;
+	int tv_depth;
+} traverse_visitbp_data_t;
+
+static inline int traverse_visitbp(traverse_data_t *td, const
+    dnode_phys_t *dnp, arc_buf_t *pbuf, blkptr_t *bp, const zbookmark_t *zb);
 static int traverse_dnode(traverse_data_t *td, const dnode_phys_t *dnp,
     arc_buf_t *buf, uint64_t objset, uint64_t object);
 
@@ -128,137 +153,191 @@ traverse_zil(traverse_data_t *td, zil_header_t *zh)
 	zil_free(zilog);
 }
 
-static int
-traverse_visitbp(traverse_data_t *td, const dnode_phys_t *dnp,
-    arc_buf_t *pbuf, blkptr_t *bp, const zbookmark_t *zb)
+#define TRAVERSE_VISITBP_MAX_DEPTH	20
+
+static void
+__traverse_visitbp_init(traverse_visitbp_data_t *tv,
+    traverse_data_t *td, const dnode_phys_t *dnp,
+    arc_buf_t *pbuf, blkptr_t *bp, const zbookmark_t *zb, int depth)
 {
-	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;
+	tv->tv_td = td;
+	tv->tv_dnp = dnp;
+	tv->tv_pbuf = pbuf;
+	tv->tv_bp = bp;
+	tv->tv_zb = zb;
+	tv->tv_err = 0;
+	tv->tv_lasterr = 0;
+	tv->tv_buf = NULL;
+	tv->tv_pd = td->td_pfd;
+	tv->tv_hard = td->td_flags & TRAVERSE_HARD;
+	tv->tv_flags = ARC_WAIT;
+	tv->tv_depth = depth;
+}
 
-	if (bp->blk_birth == 0) {
-		err = td->td_func(td->td_spa, NULL, NULL, pbuf, zb, dnp,
-		    td->td_arg);
-		return (err);
+static noinline int
+__traverse_visitbp(traverse_visitbp_data_t *tv)
+{
+	ASSERT3S(tv->tv_depth, <, TRAVERSE_VISITBP_MAX_DEPTH);
+
+	if (tv->tv_bp->blk_birth == 0) {
+		tv->tv_err = tv->tv_td->td_func(tv->tv_td->td_spa, NULL, NULL,
+		    tv->tv_pbuf, tv->tv_zb, tv->tv_dnp, tv->tv_td->td_arg);
+		return (tv->tv_err);
 	}
 
-	if (bp->blk_birth <= td->td_min_txg)
+	if (tv->tv_bp->blk_birth <= tv->tv_td->td_min_txg)
 		return (0);
 
-	if (pd && !pd->pd_exited &&
-	    ((pd->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);
+	if (tv->tv_pd && !tv->tv_pd->pd_exited &&
+	    ((tv->tv_pd->pd_flags & TRAVERSE_PREFETCH_DATA) ||
+	    BP_GET_TYPE(tv->tv_bp) == DMU_OT_DNODE ||
+	    BP_GET_LEVEL(tv->tv_bp) > 0)) {
+		mutex_enter(&tv->tv_pd->pd_mtx);
+		ASSERT(tv->tv_pd->pd_blks_fetched >= 0);
+		while (tv->tv_pd->pd_blks_fetched == 0 && !tv->tv_pd->pd_exited)
+			cv_wait(&tv->tv_pd->pd_cv, &tv->tv_pd->pd_mtx);
+		tv->tv_pd->pd_blks_fetched--;
+		cv_broadcast(&tv->tv_pd->pd_cv);
+		mutex_exit(&tv->tv_pd->pd_mtx);
 	}
 
-	if (td->td_flags & TRAVERSE_PRE) {
-		err = td->td_func(td->td_spa, NULL, bp, pbuf, zb, dnp,
-		    td->td_arg);
-		if (err == TRAVERSE_VISIT_NO_CHILDREN)
+	if (tv->tv_td->td_flags & TRAVERSE_PRE) {
+		tv->tv_err = tv->tv_td->td_func(tv->tv_td->td_spa, NULL,
+		    tv->tv_bp, tv->tv_pbuf, tv->tv_zb, tv->tv_dnp,
+		    tv->tv_td->td_arg);
+		if (tv->tv_err == TRAVERSE_VISIT_NO_CHILDREN)
 			return (0);
-		if (err)
-			return (err);
+		if (tv->tv_err)
+			return (tv->tv_err);
 	}
 
-	if (BP_GET_LEVEL(bp) > 0) {
-		uint32_t flags = ARC_WAIT;
-		int i;
-		blkptr_t *cbp;
-		int epb = BP_GET_LSIZE(bp) >> SPA_BLKPTRSHIFT;
+	if (BP_GET_LEVEL(tv->tv_bp) > 0) {
+		tv->tv_epb = BP_GET_LSIZE(tv->tv_bp) >> SPA_BLKPTRSHIFT;
 
-		err = dsl_read(NULL, td->td_spa, bp, pbuf,
-		    arc_getbuf_func, &buf,
-		    ZIO_PRIORITY_ASYNC_READ, ZIO_FLAG_CANFAIL, &flags, zb);
-		if (err)
-			return (err);
+		tv->tv_err = dsl_read(NULL, tv->tv_td->td_spa, tv->tv_bp,
+		    tv->tv_pbuf, arc_getbuf_func, &tv->tv_buf,
+		    ZIO_PRIORITY_ASYNC_READ, ZIO_FLAG_CANFAIL,
+		    &tv->tv_flags, tv->tv_zb);
+		if (tv->tv_err)
+			return (tv->tv_err);
 
 		/* recursively visitbp() blocks below this */
-		cbp = buf->b_data;
-		for (i = 0; i < epb; i++, cbp++) {
-			SET_BOOKMARK(&czb, zb->zb_objset, zb->zb_object,
-			    zb->zb_level - 1,
-			    zb->zb_blkid * epb + i);
-			err = traverse_visitbp(td, dnp, buf, cbp, &czb);
-			if (err) {
-				if (!hard)
+		tv->tv_cbp = tv->tv_buf->b_data;
+		for (tv->tv_i = 0; tv->tv_i < tv->tv_epb;
+		     tv->tv_i++, tv->tv_cbp++) {
+			SET_BOOKMARK(&tv->tv_czb, tv->tv_zb->zb_objset,
+			    tv->tv_zb->zb_object, tv->tv_zb->zb_level - 1,
+			    tv->tv_zb->zb_blkid * tv->tv_epb + tv->tv_i);
+			__traverse_visitbp_init(tv + 1, tv->tv_td,
+			    tv->tv_dnp, tv->tv_buf, tv->tv_cbp,
+			    &tv->tv_czb, tv->tv_depth + 1);
+			tv->tv_err = __traverse_visitbp(tv + 1);
+			if (tv->tv_err) {
+				if (!tv->tv_hard)
 					break;
-				lasterr = err;
+				tv->tv_lasterr = tv->tv_err;
 			}
 		}
-	} else if (BP_GET_TYPE(bp) == DMU_OT_DNODE) {
-		uint32_t flags = ARC_WAIT;
-		int i;
-		int epb = BP_GET_LSIZE(bp) >> DNODE_SHIFT;
-
-		err = dsl_read(NULL, td->td_spa, bp, pbuf,
-		    arc_getbuf_func, &buf,
-		    ZIO_PRIORITY_ASYNC_READ, ZIO_FLAG_CANFAIL, &flags, zb);
-		if (err)
-			return (err);
+	} else if (BP_GET_TYPE(tv->tv_bp) == DMU_OT_DNODE) {
+		tv->tv_epb = BP_GET_LSIZE(tv->tv_bp) >> DNODE_SHIFT;
+
+		tv->tv_err = dsl_read(NULL, tv->tv_td->td_spa, tv->tv_bp,
+		    tv->tv_pbuf, arc_getbuf_func, &tv->tv_buf,
+		    ZIO_PRIORITY_ASYNC_READ, ZIO_FLAG_CANFAIL,
+		    &tv->tv_flags, tv->tv_zb);
+		if (tv->tv_err)
+			return (tv->tv_err);
 
 		/* recursively visitbp() blocks below this */
-		dnp = buf->b_data;
-		for (i = 0; i < epb; i++, dnp++) {
-			err = traverse_dnode(td, dnp, buf, zb->zb_objset,
-			    zb->zb_blkid * epb + i);
-			if (err) {
-				if (!hard)
+		tv->tv_dnp = tv->tv_buf->b_data;
+		for (tv->tv_i = 0; tv->tv_i < tv->tv_epb;
+		     tv->tv_i++, tv->tv_dnp++) {
+			tv->tv_err = traverse_dnode(tv->tv_td, tv->tv_dnp,
+			    tv->tv_buf, tv->tv_zb->zb_objset,
+			    tv->tv_zb->zb_blkid * tv->tv_epb + tv->tv_i);
+			if (tv->tv_err) {
+				if (!tv->tv_hard)
 					break;
-				lasterr = err;
+				tv->tv_lasterr = tv->tv_err;
 			}
 		}
-	} else if (BP_GET_TYPE(bp) == DMU_OT_OBJSET) {
-		uint32_t flags = ARC_WAIT;
-		objset_phys_t *osp;
-		dnode_phys_t *dnp;
-
-		err = dsl_read_nolock(NULL, td->td_spa, bp,
-		    arc_getbuf_func, &buf,
-		    ZIO_PRIORITY_ASYNC_READ, ZIO_FLAG_CANFAIL, &flags, zb);
-		if (err)
-			return (err);
-
-		osp = buf->b_data;
-		dnp = &osp->os_meta_dnode;
-		err = traverse_dnode(td, dnp, buf, zb->zb_objset,
-		    DMU_META_DNODE_OBJECT);
-		if (err && hard) {
-			lasterr = err;
-			err = 0;
+	} else if (BP_GET_TYPE(tv->tv_bp) == DMU_OT_OBJSET) {
+
+		tv->tv_err = dsl_read_nolock(NULL, tv->tv_td->td_spa,
+		    tv->tv_bp, arc_getbuf_func, &tv->tv_buf,
+		    ZIO_PRIORITY_ASYNC_READ, ZIO_FLAG_CANFAIL,
+		    &tv->tv_flags, tv->tv_zb);
+		if (tv->tv_err)
+			return (tv->tv_err);
+
+		tv->tv_osp = tv->tv_buf->b_data;
+		tv->tv_ldnp = &tv->tv_osp->os_meta_dnode;
+		tv->tv_err = traverse_dnode(tv->tv_td, tv->tv_ldnp, tv->tv_buf,
+		    tv->tv_zb->zb_objset, DMU_META_DNODE_OBJECT);
+		if (tv->tv_err && tv->tv_hard) {
+			tv->tv_lasterr = tv->tv_err;
+			tv->tv_err = 0;
 		}
-		if (err == 0 && arc_buf_size(buf) >= sizeof (objset_phys_t)) {
-			dnp = &osp->os_userused_dnode;
-			err = traverse_dnode(td, dnp, buf, zb->zb_objset,
+		if (tv->tv_err == 0 &&
+		    arc_buf_size(tv->tv_buf) >= sizeof (objset_phys_t)) {
+			tv->tv_ldnp = &tv->tv_osp->os_userused_dnode;
+			tv->tv_err = traverse_dnode(tv->tv_td, tv->tv_ldnp,
+			    tv->tv_buf, tv->tv_zb->zb_objset,
 			    DMU_USERUSED_OBJECT);
 		}
-		if (err && hard) {
-			lasterr = err;
-			err = 0;
+		if (tv->tv_err && tv->tv_hard) {
+			tv->tv_lasterr = tv->tv_err;
+			tv->tv_err = 0;
 		}
-		if (err == 0 && arc_buf_size(buf) >= sizeof (objset_phys_t)) {
-			dnp = &osp->os_groupused_dnode;
-			err = traverse_dnode(td, dnp, buf, zb->zb_objset,
+		if (tv->tv_err == 0 &&
+		    arc_buf_size(tv->tv_buf) >= sizeof (objset_phys_t)) {
+			tv->tv_ldnp = &tv->tv_osp->os_groupused_dnode;
+			tv->tv_err = traverse_dnode(tv->tv_td, tv->tv_ldnp,
+			    tv->tv_buf, tv->tv_zb->zb_objset,
 			    DMU_GROUPUSED_OBJECT);
 		}
 	}
 
-	if (buf)
-		(void) arc_buf_remove_ref(buf, &buf);
+	if (tv->tv_buf)
+		(void) arc_buf_remove_ref(tv->tv_buf, &tv->tv_buf);
 
-	if (err == 0 && lasterr == 0 && (td->td_flags & TRAVERSE_POST)) {
-		err = td->td_func(td->td_spa, NULL, bp, pbuf, zb, dnp,
-		    td->td_arg);
+	if (tv->tv_err == 0 && tv->tv_lasterr == 0 &&
+	    (tv->tv_td->td_flags & TRAVERSE_POST)) {
+		tv->tv_err = tv->tv_td->td_func(tv->tv_td->td_spa, NULL,
+		    tv->tv_bp, tv->tv_pbuf, tv->tv_zb, tv->tv_dnp,
+		    tv->tv_td->td_arg);
 	}
 
-	return (err != 0 ? err : lasterr);
+	return (tv->tv_err != 0 ? tv->tv_err : tv->tv_lasterr);
+}
+
+/*
+ * Due to  limited stack space recursive functions are frowned upon in
+ * the Linux kernel.  However, they often are the most elegant solution
+ * to a problem.  The following code preserves the recursive function
+ * traverse_visitbp() but moves the local variables AND function
+ * arguments to the heap to minimize the stack frame size.  Enough
+ * space is initially allocated on the stack for 16 levels of recursion.
+ * This change does ugly-up-the-code but it reduces the worst case
+ * usage from roughly 2496 bytes to 576 bytes on x86_64 archs.
+ */
+static int
+traverse_visitbp(traverse_data_t *td, const dnode_phys_t *dnp,
+    arc_buf_t *pbuf, blkptr_t *bp, const zbookmark_t *zb)
+{
+	traverse_visitbp_data_t *tv;
+	int error;
+
+	tv = kmem_zalloc(sizeof(traverse_visitbp_data_t) *
+	    TRAVERSE_VISITBP_MAX_DEPTH, KM_SLEEP);
+	__traverse_visitbp_init(tv, td, dnp, pbuf, bp, zb, 0);
+
+	error = __traverse_visitbp(tv);
+
+	kmem_free(tv, sizeof(traverse_visitbp_data_t) *
+	    TRAVERSE_VISITBP_MAX_DEPTH);
+
+	return (error);
 }
 
 static int