v20260405_2-0012-Add-option-for-aligning-shmem-allocations.patch

From 3b815d71e898dff859acf148a7f57c4200ff66d3 Mon Sep 17 00:00:00 2001
From: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date: Sat, 21 Mar 2026 23:44:15 +0200
Subject: [PATCH v20260405 12/15] Add option for aligning shmem allocations

The buffer blocks (in the next commit) are IO-aligned. This might come
handy in other places too, so make it an explicit feature of
ShmemRequestStruct.
---
 src/backend/storage/ipc/shmem.c | 26 ++++++++++++++++----------
 src/include/storage/shmem.h     |  6 ++++++
 2 files changed, 22 insertions(+), 10 deletions(-)

diff --git a/src/backend/storage/ipc/shmem.c b/src/backend/storage/ipc/shmem.c
index bc186d6ea17..973811e545e 100644
--- a/src/backend/storage/ipc/shmem.c
+++ b/src/backend/storage/ipc/shmem.c
@@ -239,7 +239,7 @@ typedef struct ShmemAllocatorData
 
 #define ShmemIndexLock (&ShmemAllocator->index_lock)
 
-static void *ShmemAllocRaw(Size size, Size *allocated_size);
+static void *ShmemAllocRaw(Size size, Size alignment, Size *allocated_size);
 
 /* shared memory global variables */
 
@@ -400,7 +400,8 @@ ShmemGetRequestedSize(void)
 	{
 		size = add_size(size, request->options->size);
 		/* calculate alignment padding like ShmemAllocRaw() does */
-		size = CACHELINEALIGN(size);
+		size = TYPEALIGN(Max(request->options->alignment, PG_CACHE_LINE_SIZE),
+						 size);
 	}
 
 	return size;
@@ -525,7 +526,9 @@ InitShmemIndexEntry(ShmemRequest *request)
 	 * We inserted the entry to the shared memory index.  Allocate requested
 	 * amount of shared memory for it, and initialize the index entry.
 	 */
-	structPtr = ShmemAllocRaw(request->options->size, &allocated_size);
+	structPtr = ShmemAllocRaw(request->options->size,
+							  request->options->alignment,
+							  &allocated_size);
 	if (structPtr == NULL)
 	{
 		/* out of memory; remove the failed ShmemIndex entry */
@@ -754,7 +757,7 @@ ShmemAlloc(Size size)
 	void	   *newSpace;
 	Size		allocated_size;
 
-	newSpace = ShmemAllocRaw(size, &allocated_size);
+	newSpace = ShmemAllocRaw(size, 0, &allocated_size);
 	if (!newSpace)
 		ereport(ERROR,
 				(errcode(ERRCODE_OUT_OF_MEMORY),
@@ -773,7 +776,7 @@ ShmemAllocNoError(Size size)
 {
 	Size		allocated_size;
 
-	return ShmemAllocRaw(size, &allocated_size);
+	return ShmemAllocRaw(size, 0, &allocated_size);
 }
 
 /*
@@ -783,8 +786,9 @@ ShmemAllocNoError(Size size)
  * be equal to the number requested plus any padding we choose to add.
  */
 static void *
-ShmemAllocRaw(Size size, Size *allocated_size)
+ShmemAllocRaw(Size size, Size alignment, Size *allocated_size)
 {
+	Size		rawStart;
 	Size		newStart;
 	Size		newFree;
 	void	   *newSpace;
@@ -800,14 +804,15 @@ ShmemAllocRaw(Size size, Size *allocated_size)
 	 * structures out to a power-of-two size - but without this, even that
 	 * won't be sufficient.
 	 */
-	size = CACHELINEALIGN(size);
-	*allocated_size = size;
+	if (alignment < PG_CACHE_LINE_SIZE)
+		alignment = PG_CACHE_LINE_SIZE;
 
 	Assert(ShmemSegHdr != NULL);
 
 	SpinLockAcquire(&ShmemAllocator->shmem_lock);
 
-	newStart = ShmemAllocator->free_offset;
+	rawStart = ShmemAllocator->free_offset;
+	newStart = TYPEALIGN(alignment, rawStart);
 
 	newFree = newStart + size;
 	if (newFree <= ShmemSegHdr->totalsize)
@@ -821,8 +826,9 @@ ShmemAllocRaw(Size size, Size *allocated_size)
 	SpinLockRelease(&ShmemAllocator->shmem_lock);
 
 	/* note this assert is okay with newSpace == NULL */
-	Assert(newSpace == (void *) CACHELINEALIGN(newSpace));
+	Assert(newSpace == (void *) TYPEALIGN(alignment, newSpace));
 
+	*allocated_size = newFree - rawStart;
 	return newSpace;
 }
 
diff --git a/src/include/storage/shmem.h b/src/include/storage/shmem.h
index 147a6915f7e..91218db6d6e 100644
--- a/src/include/storage/shmem.h
+++ b/src/include/storage/shmem.h
@@ -51,6 +51,12 @@ typedef struct ShmemStructOpts
 	 */
 	ssize_t		size;
 
+	/*
+	 * Alignment of the starting address. If not set, defaults to cacheline
+	 * boundary.  Must be a power of two.
+	 */
+	size_t		alignment;
+
 	/*
 	 * When the shmem area is initialized or attached to, pointer to it is
 	 * stored in *ptr.  It usually points to a global variable, used to access
-- 
2.34.1