Introduce PG_IO_ALIGN_SIZE and align all I/O buffers.

In order to have the option to use O_DIRECT/FILE_FLAG_NO_BUFFERING in a
later commit, we need the addresses of user space buffers to be well
aligned.  The exact requirements vary by OS and file system (typically
sectors and/or memory pages).  The address alignment size is set to
4096, which is enough for currently known systems: it matches modern
sectors and common memory page size.  There is no standard governing
O_DIRECT's requirements so we might eventually have to reconsider this
with more information from the field or future systems.

Aligning I/O buffers on memory pages is also known to improve regular
buffered I/O performance.

Three classes of I/O buffers for regular data pages are adjusted:
(1) Heap buffers are now allocated with the new palloc_aligned() or
MemoryContextAllocAligned() functions introduced by commit 439f6175.
(2) Stack buffers now use a new struct PGIOAlignedBlock to respect
PG_IO_ALIGN_SIZE, if possible with this compiler.  (3) The buffer
pool is also aligned in shared memory.

WAL buffers were already aligned on XLOG_BLCKSZ.  It's possible for
XLOG_BLCKSZ to be configured smaller than PG_IO_ALIGNED_SIZE and thus
for O_DIRECT WAL writes to fail to be well aligned, but that's a
pre-existing condition and will be addressed by a later commit.

BufFiles are not yet addressed (there's no current plan to use O_DIRECT
for those, but they could potentially get some incidental speedup even
in plain buffered I/O operations through better alignment).

If we can't align stack objects suitably using the compiler extensions
we know about, we disable the use of O_DIRECT by setting PG_O_DIRECT to
0.  This avoids the need to consider systems that have O_DIRECT but
can't align stack objects the way we want; such systems could in theory
be supported with more work but we don't currently know of any such
machines, so it's easier to pretend there is no O_DIRECT support
instead.  That's an existing and tested class of system.

Add assertions that all buffers passed into smgrread(), smgrwrite() and
smgrextend() are correctly aligned, unless PG_O_DIRECT is 0 (= stack
alignment tricks may be unavailable) or the block size has been set too
small to allow arrays of buffers to be all aligned.

Author: Thomas Munro <thomas.munro@gmail.com>
Author: Andres Freund <andres@anarazel.de>
Reviewed-by: Justin Pryzby <pryzby@telsasoft.com>
Discussion: https://postgr.es/m/CA+hUKGK1X532hYqJ_MzFWt0n1zt8trz980D79WbjwnT-yYLZpg@mail.gmail.com

src/backend/access/gist/gistbuild.c

@@ -415,7 +415,7 @@ gist_indexsortbuild(GISTBuildState *state)
 	 * Write an empty page as a placeholder for the root page. It will be
 	 * replaced with the real root page at the end.
 	 */
-	page = palloc0(BLCKSZ);
+	page = palloc_aligned(BLCKSZ, PG_IO_ALIGN_SIZE, MCXT_ALLOC_ZERO);
 	smgrextend(RelationGetSmgr(state->indexrel), MAIN_FORKNUM, GIST_ROOT_BLKNO,
 			   page, true);
 	state->pages_allocated++;
@@ -509,7 +509,8 @@ gist_indexsortbuild_levelstate_add(GISTBuildState *state,
 			levelstate->current_page++;
 
 		if (levelstate->pages[levelstate->current_page] == NULL)
-			levelstate->pages[levelstate->current_page] = palloc(BLCKSZ);
+			levelstate->pages[levelstate->current_page] =
+				palloc_aligned(BLCKSZ, PG_IO_ALIGN_SIZE, 0);
 
 		newPage = levelstate->pages[levelstate->current_page];
 		gistinitpage(newPage, old_page_flags);
@@ -579,7 +580,7 @@ gist_indexsortbuild_levelstate_flush(GISTBuildState *state,
 
 		/* Create page and copy data */
 		data = (char *) (dist->list);
-		target = palloc0(BLCKSZ);
+		target = palloc_aligned(BLCKSZ, PG_IO_ALIGN_SIZE, MCXT_ALLOC_ZERO);
 		gistinitpage(target, isleaf ? F_LEAF : 0);
 		for (int i = 0; i < dist->block.num; i++)
 		{
@@ -630,7 +631,7 @@ gist_indexsortbuild_levelstate_flush(GISTBuildState *state,
 		if (parent == NULL)
 		{
 			parent = palloc0(sizeof(GistSortedBuildLevelState));
-			parent->pages[0] = (Page) palloc(BLCKSZ);
+			parent->pages[0] = (Page) palloc_aligned(BLCKSZ, PG_IO_ALIGN_SIZE, 0);
 			parent->parent = NULL;
 			gistinitpage(parent->pages[0], 0);
 

src/backend/access/hash/hashpage.c

@@ -992,7 +992,7 @@ static bool
 _hash_alloc_buckets(Relation rel, BlockNumber firstblock, uint32 nblocks)
 {
 	BlockNumber lastblock;
-	PGAlignedBlock zerobuf;
+	PGIOAlignedBlock zerobuf;
 	Page		page;
 	HashPageOpaque ovflopaque;
 

src/backend/access/heap/rewriteheap.c

@@ -255,7 +255,7 @@ begin_heap_rewrite(Relation old_heap, Relation new_heap, TransactionId oldest_xm
 
 	state->rs_old_rel = old_heap;
 	state->rs_new_rel = new_heap;
-	state->rs_buffer = (Page) palloc(BLCKSZ);
+	state->rs_buffer = (Page) palloc_aligned(BLCKSZ, PG_IO_ALIGN_SIZE, 0);
 	/* new_heap needn't be empty, just locked */
 	state->rs_blockno = RelationGetNumberOfBlocks(new_heap);
 	state->rs_buffer_valid = false;

src/backend/access/nbtree/nbtree.c

@@ -154,7 +154,7 @@ btbuildempty(Relation index)
 	Page		metapage;
 
 	/* Construct metapage. */
-	metapage = (Page) palloc(BLCKSZ);
+	metapage = (Page) palloc_aligned(BLCKSZ, PG_IO_ALIGN_SIZE, 0);
 	_bt_initmetapage(metapage, P_NONE, 0, _bt_allequalimage(index, false));
 
 	/*

src/backend/access/nbtree/nbtsort.c

@@ -619,7 +619,7 @@ _bt_blnewpage(uint32 level)
 	Page		page;
 	BTPageOpaque opaque;
 
-	page = (Page) palloc(BLCKSZ);
+	page = (Page) palloc_aligned(BLCKSZ, PG_IO_ALIGN_SIZE, 0);
 
 	/* Zero the page and set up standard page header info */
 	_bt_pageinit(page, BLCKSZ);
@@ -660,7 +660,9 @@ _bt_blwritepage(BTWriteState *wstate, Page page, BlockNumber blkno)
 	while (blkno > wstate->btws_pages_written)
 	{
 		if (!wstate->btws_zeropage)
-			wstate->btws_zeropage = (Page) palloc0(BLCKSZ);
+			wstate->btws_zeropage = (Page) palloc_aligned(BLCKSZ,
+														  PG_IO_ALIGN_SIZE,
+														  MCXT_ALLOC_ZERO);
 		/* don't set checksum for all-zero page */
 		smgrextend(RelationGetSmgr(wstate->index), MAIN_FORKNUM,
 				   wstate->btws_pages_written++,
@@ -1170,7 +1172,7 @@ _bt_uppershutdown(BTWriteState *wstate, BTPageState *state)
 	 * set to point to "P_NONE").  This changes the index to the "valid" state
 	 * by filling in a valid magic number in the metapage.
 	 */
-	metapage = (Page) palloc(BLCKSZ);
+	metapage = (Page) palloc_aligned(BLCKSZ, PG_IO_ALIGN_SIZE, 0);
 	_bt_initmetapage(metapage, rootblkno, rootlevel,
 					 wstate->inskey->allequalimage);
 	_bt_blwritepage(wstate, metapage, BTREE_METAPAGE);

src/backend/access/spgist/spginsert.c

@@ -158,7 +158,7 @@ spgbuildempty(Relation index)
 	Page		page;
 
 	/* Construct metapage. */
-	page = (Page) palloc(BLCKSZ);
+	page = (Page) palloc_aligned(BLCKSZ, PG_IO_ALIGN_SIZE, 0);
 	SpGistInitMetapage(page);
 
 	/*

src/backend/access/transam/generic_xlog.c

@@ -58,14 +58,17 @@ typedef struct
 	char		delta[MAX_DELTA_SIZE];	/* delta between page images */
 } PageData;
 
-/* State of generic xlog record construction */
+/*
+ * State of generic xlog record construction.  Must be allocated at an I/O
+ * aligned address.
+ */
 struct GenericXLogState
 {
+	/* Page images (properly aligned, must be first) */
+	PGIOAlignedBlock images[MAX_GENERIC_XLOG_PAGES];
 	/* Info about each page, see above */
 	PageData	pages[MAX_GENERIC_XLOG_PAGES];
 	bool		isLogged;
-	/* Page images (properly aligned) */
-	PGAlignedBlock images[MAX_GENERIC_XLOG_PAGES];
 };
 
 static void writeFragment(PageData *pageData, OffsetNumber offset,
@@ -269,7 +272,9 @@ GenericXLogStart(Relation relation)
 	GenericXLogState *state;
 	int			i;
 
-	state = (GenericXLogState *) palloc(sizeof(GenericXLogState));
+	state = (GenericXLogState *) palloc_aligned(sizeof(GenericXLogState),
+												PG_IO_ALIGN_SIZE,
+												0);
 	state->isLogged = RelationNeedsWAL(relation);
 
 	for (i = 0; i < MAX_GENERIC_XLOG_PAGES; i++)

src/backend/access/transam/xlog.c

@@ -4506,7 +4506,7 @@ XLOGShmemSize(void)
 	/* xlblocks array */
 	size = add_size(size, mul_size(sizeof(XLogRecPtr), XLOGbuffers));
 	/* extra alignment padding for XLOG I/O buffers */
-	size = add_size(size, XLOG_BLCKSZ);
+	size = add_size(size, Max(XLOG_BLCKSZ, PG_IO_ALIGN_SIZE));
 	/* and the buffers themselves */
 	size = add_size(size, mul_size(XLOG_BLCKSZ, XLOGbuffers));