Fix regression in dmu_buf_will_fill()

Direct I/O implementation added condition to call dbuf_undirty()
only in case of block cloning.  But the condition is not right if
the block is no longer dirty in this TXG, but still in DB_NOFILL
state.  It resulted in block not reverting to DB_UNCACHED and
following NULL de-reference on attempt to access absent db_data.

While there, add assertions for db_data to make debugging easier.

Reviewed-by: Brian Atkinson <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Alexander Motin <[email protected]>
Sponsored by:	iXsystems, Inc.
Closes openzfs#16829

module/os/freebsd/zfs/dmu_os.c

@@ -103,6 +103,7 @@ dmu_write_pages(objset_t *os, uint64_t object, uint64_t offset, uint64_t size,
 			    db->db_offset + bufoff);
 			thiscpy = MIN(PAGESIZE, tocpy - copied);
 			va = zfs_map_page(*ma, &sf);
+			ASSERT(db->db_data != NULL);
 			memcpy((char *)db->db_data + bufoff, va, thiscpy);
 			zfs_unmap_page(sf);
 			ma += 1;
@@ -172,6 +173,7 @@ dmu_read_pages(objset_t *os, uint64_t object, vm_page_t *ma, int count,
 		ASSERT3U(db->db_size, >, PAGE_SIZE);
 		bufoff = IDX_TO_OFF(m->pindex) % db->db_size;
 		va = zfs_map_page(m, &sf);
+		ASSERT(db->db_data != NULL);
 		memcpy(va, (char *)db->db_data + bufoff, PAGESIZE);
 		zfs_unmap_page(sf);
 		vm_page_valid(m);
@@ -211,8 +213,10 @@ dmu_read_pages(objset_t *os, uint64_t object, vm_page_t *ma, int count,
 		 */
 		tocpy = MIN(db->db_size - bufoff, PAGESIZE - pgoff);
 		ASSERT3S(tocpy, >=, 0);
-		if (m != bogus_page)
+		if (m != bogus_page) {
+			ASSERT(db->db_data != NULL);
 			memcpy(va + pgoff, (char *)db->db_data + bufoff, tocpy);
+		}
 
 		pgoff += tocpy;
 		ASSERT3S(pgoff, >=, 0);
@@ -290,6 +294,7 @@ dmu_read_pages(objset_t *os, uint64_t object, vm_page_t *ma, int count,
 		bufoff = IDX_TO_OFF(m->pindex) % db->db_size;
 		tocpy = MIN(db->db_size - bufoff, PAGESIZE);
 		va = zfs_map_page(m, &sf);
+		ASSERT(db->db_data != NULL);
 		memcpy(va, (char *)db->db_data + bufoff, tocpy);
 		if (tocpy < PAGESIZE) {
 			ASSERT3S(i, ==, *rahead - 1);

module/zfs/dbuf.c

@@ -2916,7 +2916,7 @@ dmu_buf_will_fill(dmu_buf_t *db_fake, dmu_tx_t *tx, boolean_t canfail)
 		 * pending clone and mark the block as uncached. This will be
 		 * as if the clone was never done.
 		 */
-		if (dr && dr->dt.dl.dr_brtwrite) {
+		if (db->db_state == DB_NOFILL) {
 			VERIFY(!dbuf_undirty(db, tx));
 			db->db_state = DB_UNCACHED;
 		}

module/zfs/dmu.c

@@ -1221,6 +1221,7 @@ dmu_read_impl(dnode_t *dn, uint64_t offset, uint64_t size,
 			bufoff = offset - db->db_offset;
 			tocpy = MIN(db->db_size - bufoff, size);
 
+			ASSERT(db->db_data != NULL);
 			(void) memcpy(buf, (char *)db->db_data + bufoff, tocpy);
 
 			offset += tocpy;
@@ -1278,6 +1279,7 @@ dmu_write_impl(dmu_buf_t **dbp, int numbufs, uint64_t offset, uint64_t size,
 		else
 			dmu_buf_will_dirty(db, tx);
 
+		ASSERT(db->db_data != NULL);
 		(void) memcpy((char *)db->db_data + bufoff, buf, tocpy);
 
 		if (tocpy == db->db_size)
@@ -1426,6 +1428,7 @@ dmu_read_uio_dnode(dnode_t *dn, zfs_uio_t *uio, uint64_t size)
 		bufoff = zfs_uio_offset(uio) - db->db_offset;
 		tocpy = MIN(db->db_size - bufoff, size);
 
+		ASSERT(db->db_data != NULL);
 		err = zfs_uio_fault_move((char *)db->db_data + bufoff, tocpy,
 		    UIO_READ, uio);
 
@@ -1550,6 +1553,7 @@ dmu_write_uio_dnode(dnode_t *dn, zfs_uio_t *uio, uint64_t size, dmu_tx_t *tx)
 		else
 			dmu_buf_will_dirty(db, tx);
 
+		ASSERT(db->db_data != NULL);
 		err = zfs_uio_fault_move((char *)db->db_data + bufoff,
 		    tocpy, UIO_WRITE, uio);