Fix FDT rollback to not overwrite unnecessary fields

When a dedup write fails, we try to roll the DDT entry back to a known good
state. However, this also rolls the refcounts and the last-update time back to
the state they were at when we started this write. This doesn't appear to be
able to cause any refcount leaks (after the fix in 13172). This PR prevents
that from happening by only rolling back the parts of the DDT entry that have
been updated by the write so far.

Signed-off-by: Paul Dagnelie <[email protected]>
Sponsored-by: iXsystems, Inc.
Sponsored-by: Klara, Inc.

Author: Paul Dagnelie

include/sys/ddt.h

@@ -339,6 +339,8 @@ extern void ddt_bp_create(enum zio_checksum checksum, const ddt_key_t *ddk,
 
 extern void ddt_phys_extend(ddt_univ_phys_t *ddp, ddt_phys_variant_t v,
     const blkptr_t *bp);
+extern void ddt_phys_unextend(ddt_univ_phys_t *cur, ddt_univ_phys_t *orig,
+    ddt_phys_variant_t v);
 extern void ddt_phys_copy(ddt_univ_phys_t *dst, const ddt_univ_phys_t *src,
     ddt_phys_variant_t v);
 extern void ddt_phys_clear(ddt_univ_phys_t *ddp, ddt_phys_variant_t v);

module/zfs/ddt.c

@@ -731,6 +731,27 @@ ddt_phys_extend(ddt_univ_phys_t *ddp, ddt_phys_variant_t v, const blkptr_t *bp)
 	}
 }
 
+void
+ddt_phys_unextend(ddt_univ_phys_t *cur, ddt_univ_phys_t *orig,
+    ddt_phys_variant_t v)
+{
+	ASSERT3U(v, <, DDT_PHYS_NONE);
+	dva_t *cur_dvas = (v == DDT_PHYS_FLAT) ?
+	    cur->ddp_flat.ddp_dva : cur->ddp_trad[v].ddp_dva;
+	dva_t *orig_dvas = (v == DDT_PHYS_FLAT) ?
+	    orig->ddp_flat.ddp_dva : orig->ddp_trad[v].ddp_dva;
+
+	for (int d = 0; d < SPA_DVAS_PER_BP; d++)
+		cur_dvas[d] = orig_dvas[d];
+
+	if (ddt_phys_birth(orig, v) == 0) {
+		if (v == DDT_PHYS_FLAT)
+			cur->ddp_flat.ddp_phys_birth = 0;
+		else
+			cur->ddp_trad[v].ddp_phys_birth = 0;
+	}
+}
+
 void
 ddt_phys_copy(ddt_univ_phys_t *dst, const ddt_univ_phys_t *src,
     ddt_phys_variant_t v)

module/zfs/zio.c

@@ -3607,7 +3607,7 @@ zio_ddt_child_write_done(zio_t *zio)
 		 * chain. We need to revert the entry back to what it was at
 		 * the last time it was successfully extended.
 		 */
-		ddt_phys_copy(ddp, orig, v);
+		ddt_phys_unextend(ddp, orig, v);
 		ddt_phys_clear(orig, v);
 
 		ddt_exit(ddt);