Date: Wed, 26 Nov 2025 16:49:49 -0600
From: Nathan Bossart <nathandbossart@gmail.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: show size of DSAs and dshash tables in pg_dsm_registry_allocations
Message-ID: <aSeEDeznAsHR1_YF@nathan>
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On Tue, Nov 25, 2025 at 07:13:03PM -0500, Robert Haas wrote:
> In my ideal world, it would probably show partially-initialized entires
> in some distinguishable way, like with a null size.

I reevaluated this view, and I think we can do what you suggest here.
Right now, we show NULL for DSAs and dshash tables because 1) we might not
be attached to them and 2) we don't have a pointer to the dsa_area * or
dshash_table * in local memory.  If we introduce a function that looks up
the size of a DSA given its handle (transiently attaching to the control
segment if needed), we can show the size for all successfully-initialized
entries.  Then, we can make NULL mean that the entry was
partially-initialized.

Patch attached.

-- 
nathan

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	filename=v1-0001-rework-DSM-registry-view.patch

From fc5f0d4eea19df142032c8da4110b1e8aade59f0 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Wed, 26 Nov 2025 16:48:13 -0600
Subject: [PATCH v1 1/1] rework DSM registry view

---
 doc/src/sgml/system-views.sgml                |  4 +--
 src/backend/storage/ipc/dsm_registry.c        | 17 ++++-----
 src/backend/utils/mmgr/dsa.c                  | 35 +++++++++++++++++++
 src/include/utils/dsa.h                       |  1 +
 .../expected/test_dsm_registry.out            |  4 +--
 .../sql/test_dsm_registry.sql                 |  4 +--
 6 files changed, 48 insertions(+), 17 deletions(-)

diff --git a/doc/src/sgml/system-views.sgml b/doc/src/sgml/system-views.sgml
index 0e623e7fb86..3d083128a4b 100644
--- a/doc/src/sgml/system-views.sgml
+++ b/doc/src/sgml/system-views.sgml
@@ -1150,8 +1150,8 @@ AND c1.path[c2.level] = c2.path[c2.level];
        <structfield>size</structfield> <type>int8</type>
       </para>
       <para>
-       Size of the allocation in bytes.  NULL for entries of type
-       <literal>area</literal> and <literal>hash</literal>.
+       Size of the allocation in bytes.  NULL for entries that failed
+       initialization.
       </para></entry>
      </row>
     </tbody>
diff --git a/src/backend/storage/ipc/dsm_registry.c b/src/backend/storage/ipc/dsm_registry.c
index ef6533b1100..4e327f69789 100644
--- a/src/backend/storage/ipc/dsm_registry.c
+++ b/src/backend/storage/ipc/dsm_registry.c
@@ -463,17 +463,6 @@ pg_get_dsm_registry_allocations(PG_FUNCTION_ARGS)
 		Datum		vals[3];
 		bool		nulls[3] = {0};
 
-		/* Do not show partially-initialized entries. */
-		if (entry->type == DSMR_ENTRY_TYPE_DSM &&
-			entry->dsm.handle == DSM_HANDLE_INVALID)
-			continue;
-		if (entry->type == DSMR_ENTRY_TYPE_DSA &&
-			entry->dsa.handle == DSA_HANDLE_INVALID)
-			continue;
-		if (entry->type == DSMR_ENTRY_TYPE_DSH &&
-			entry->dsh.dsa_handle == DSA_HANDLE_INVALID)
-			continue;
-
 		vals[0] = CStringGetTextDatum(entry->name);
 		vals[1] = CStringGetTextDatum(DSMREntryTypeNames[entry->type]);
 
@@ -483,6 +472,12 @@ pg_get_dsm_registry_allocations(PG_FUNCTION_ARGS)
 		 */
 		if (entry->type == DSMR_ENTRY_TYPE_DSM)
 			vals[2] = Int64GetDatum(entry->dsm.size);
+		else if (entry->type == DSMR_ENTRY_TYPE_DSA &&
+				 entry->dsa.handle != DSA_HANDLE_INVALID)
+			vals[2] = Int64GetDatum(dsa_get_total_size_from_handle(entry->dsa.handle));
+		else if (entry->type == DSMR_ENTRY_TYPE_DSH &&
+				 entry->dsh.dsa_handle !=DSA_HANDLE_INVALID)
+			vals[2] = Int64GetDatum(dsa_get_total_size_from_handle(entry->dsh.dsa_handle));
 		else
 			nulls[2] = true;
 
diff --git a/src/backend/utils/mmgr/dsa.c b/src/backend/utils/mmgr/dsa.c
index be43e9351c3..030f9463712 100644
--- a/src/backend/utils/mmgr/dsa.c
+++ b/src/backend/utils/mmgr/dsa.c
@@ -1050,6 +1050,41 @@ dsa_get_total_size(dsa_area *area)
 	return size;
 }
 
+/*
+ * Same as dsa_get_total_size(), but accepts a DSA handle.  The area must have
+ * been created with dsa_create (not dsa_create_in_place).
+ */
+size_t
+dsa_get_total_size_from_handle(dsa_handle handle)
+{
+	size_t		size;
+	bool		already_attached;
+	dsm_segment *segment;
+	dsa_area_control *control;
+
+	already_attached = dsa_is_attached(handle);
+	if (already_attached)
+		segment = dsm_find_mapping(handle);
+	else
+		segment = dsm_attach(handle);
+
+	if (segment == NULL)
+		ereport(ERROR,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("could not attach to dynamic shared area")));
+
+	control = (dsa_area_control *) dsm_segment_address(segment);
+
+	LWLockAcquire(&control->lock, LW_SHARED);
+	size = control->total_segment_size;
+	LWLockRelease(&control->lock);
+
+	if (!already_attached)
+		dsm_detach(segment);
+
+	return size;
+}
+
 /*
  * Aggressively free all spare memory in the hope of returning DSM segments to
  * the operating system.
diff --git a/src/include/utils/dsa.h b/src/include/utils/dsa.h
index f2104dacbfc..42449ff22de 100644
--- a/src/include/utils/dsa.h
+++ b/src/include/utils/dsa.h
@@ -161,6 +161,7 @@ extern dsa_pointer dsa_allocate_extended(dsa_area *area, size_t size, int flags)
 extern void dsa_free(dsa_area *area, dsa_pointer dp);
 extern void *dsa_get_address(dsa_area *area, dsa_pointer dp);
 extern size_t dsa_get_total_size(dsa_area *area);
+extern size_t dsa_get_total_size_from_handle(dsa_handle handle);
 extern void dsa_trim(dsa_area *area);
 extern void dsa_dump(dsa_area *area);
 
diff --git a/src/test/modules/test_dsm_registry/expected/test_dsm_registry.out b/src/test/modules/test_dsm_registry/expected/test_dsm_registry.out
index ca8abbb377e..75d9eda0756 100644
--- a/src/test/modules/test_dsm_registry/expected/test_dsm_registry.out
+++ b/src/test/modules/test_dsm_registry/expected/test_dsm_registry.out
@@ -1,4 +1,4 @@
-SELECT name, type, size IS DISTINCT FROM 0 AS size
+SELECT name, type, size > 0 AS size
 FROM pg_dsm_registry_allocations
 WHERE name like 'test_dsm_registry%' ORDER BY name;
  name | type | size 
@@ -32,7 +32,7 @@ SELECT get_val_in_hash('test');
 (1 row)
 
 \c
-SELECT name, type, size IS DISTINCT FROM 0 AS size
+SELECT name, type, size > 0 AS size
 FROM pg_dsm_registry_allocations
 WHERE name like 'test_dsm_registry%' ORDER BY name;
           name          |  type   | size 
diff --git a/src/test/modules/test_dsm_registry/sql/test_dsm_registry.sql b/src/test/modules/test_dsm_registry/sql/test_dsm_registry.sql
index 965a3f1ebb6..1b9ee3ebf18 100644
--- a/src/test/modules/test_dsm_registry/sql/test_dsm_registry.sql
+++ b/src/test/modules/test_dsm_registry/sql/test_dsm_registry.sql
@@ -1,4 +1,4 @@
-SELECT name, type, size IS DISTINCT FROM 0 AS size
+SELECT name, type, size > 0 AS size
 FROM pg_dsm_registry_allocations
 WHERE name like 'test_dsm_registry%' ORDER BY name;
 CREATE EXTENSION test_dsm_registry;
@@ -8,6 +8,6 @@ SELECT set_val_in_hash('test', '1414');
 SELECT get_val_in_shmem();
 SELECT get_val_in_hash('test');
 \c
-SELECT name, type, size IS DISTINCT FROM 0 AS size
+SELECT name, type, size > 0 AS size
 FROM pg_dsm_registry_allocations
 WHERE name like 'test_dsm_registry%' ORDER BY name;
-- 
2.39.5 (Apple Git-154)


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