linux-kernel - [PATCH for-next 2/2] book3s64/radix : Optimize vmemmap start alignment

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Message-ID: <14f9b6ef72fa689f3a4d44dfa67d2279bdfd28a6.1746255312.git.donettom@linux.ibm.com>
Date: Sat,  3 May 2025 12:31:59 +0530
From: Donet Tom <donettom@...ux.ibm.com>
To: Madhavan Srinivasan <maddy@...ux.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>
Cc: Ritesh Harjani <ritesh.list@...il.com>,
        Hari Bathini <hbathini@...ux.ibm.com>, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org (open list),
        Donet Tom <donettom@...ux.ibm.com>
Subject: [PATCH for-next 2/2] book3s64/radix : Optimize vmemmap start alignment

If we always align the vmemmap start to PAGE_SIZE, there is a
chance that we may end up allocating page-sized vmemmap backing
pages in RAM in the altmap not present case, because a PAGE_SIZE
aligned address is not PMD_SIZE-aligned.

In this patch, we are aligning the vmemmap start address to
PMD_SIZE if altmap is not present. This ensures that a PMD_SIZE
page is always allocated for the vmemmap mapping if altmap is
not present.

If altmap is present, Make sure we align the start vmemmap addr to
PAGE_SIZE so that we calculate the correct start_pfn in altmap
boundary check to decide whether we should use altmap or RAM based
backing memory allocation. Also the address need to be aligned for
set_pte operation. If the start addr is already PMD_SIZE aligned
and with in the altmap boundary then we will try to use a pmd size
altmap mapping else we go for page size  mapping.

So if altmap is present, we try to use the maximum number of
altmap pages; otherwise, we allocate a PMD_SIZE RAM page.

Signed-off-by: Donet Tom <donettom@...ux.ibm.com>
---
 arch/powerpc/mm/book3s64/radix_pgtable.c | 29 +++++++++++++++---------
 1 file changed, 18 insertions(+), 11 deletions(-)

diff --git a/arch/powerpc/mm/book3s64/radix_pgtable.c b/arch/powerpc/mm/book3s64/radix_pgtable.c
index 3d67aee8c8ca..c630cece8ed4 100644
--- a/arch/powerpc/mm/book3s64/radix_pgtable.c
+++ b/arch/powerpc/mm/book3s64/radix_pgtable.c
@@ -1122,18 +1122,25 @@ int __meminit radix__vmemmap_populate(unsigned long start, unsigned long end, in
 	pte_t *pte;
 
 	/*
-	 * Make sure we align the start vmemmap addr so that we calculate
-	 * the correct start_pfn in altmap boundary check to decided whether
-	 * we should use altmap or RAM based backing memory allocation. Also
-	 * the address need to be aligned for set_pte operation.
-
-	 * If the start addr is already PMD_SIZE aligned we will try to use
-	 * a pmd mapping. We don't want to be too aggressive here beacause
-	 * that will cause more allocations in RAM. So only if the namespace
-	 * vmemmap start addr is PMD_SIZE aligned we will use PMD mapping.
+	 * If altmap is present, Make sure we align the start vmemmap addr
+	 * to PAGE_SIZE so that we calculate the correct start_pfn in
+	 * altmap boundary check to decide whether we should use altmap or
+	 * RAM based backing memory allocation. Also the address need to be
+	 * aligned for set_pte operation. If the start addr is already
+	 * PMD_SIZE aligned and with in the altmap boundary then we will
+	 * try to use a pmd size altmap mapping else we go for page size
+	 * mapping.
+	 *
+	 * If altmap is not present, align the vmemmap addr to PMD_SIZE and
+	 * always allocate a PMD size page for vmemmap backing.
+	 *
 	 */
 
-	start = ALIGN_DOWN(start, PAGE_SIZE);
+	if (altmap)
+		start = ALIGN_DOWN(start, PAGE_SIZE);
+	else
+		start = ALIGN_DOWN(start, PMD_SIZE);
+
 	for (addr = start; addr < end; addr = next) {
 		next = pmd_addr_end(addr, end);
 
@@ -1159,7 +1166,7 @@ int __meminit radix__vmemmap_populate(unsigned long start, unsigned long end, in
 			 * in altmap block allocation failures, in which case
 			 * we fallback to RAM for vmemmap allocation.
 			 */
-			if (!IS_ALIGNED(addr, PMD_SIZE) || (altmap &&
+			if (altmap && (!IS_ALIGNED(addr, PMD_SIZE) ||
 			    altmap_cross_boundary(altmap, addr, PMD_SIZE))) {
 				/*
 				 * make sure we don't create altmap mappings
-- 
2.48.1