linux-kernel - [PATCH v2 10/11] mm: thp: always enable mTHP support

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Message-ID: <29e8dfc2772af4b6e0db24134ca3563ec422b91a.1770675272.git.luizcap@redhat.com>
Date: Mon,  9 Feb 2026 17:14:32 -0500
From: Luiz Capitulino <luizcap@...hat.com>
To: linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	david@...nel.org,
	baolin.wang@...ux.alibaba.com
Cc: ryan.roberts@....com,
	akpm@...ux-foundation.org,
	lorenzo.stoakes@...cle.com
Subject: [PATCH v2 10/11] mm: thp: always enable mTHP support

If PMD-sized pages are not supported on an architecture (ie. the
arch implements arch_has_pmd_leaves() and it returns false) then the
current code disables all THP, including mTHP.

This commit fixes this by allowing mTHP to be always enabled for all
archs. When PMD-sized pages are not supported, its sysfs entry won't be
created and their mapping will be disallowed at page-fault time.

Similarly, this commit implements the following changes for shmem:

 - In shmem_allowable_huge_orders(): drop the pgtable_has_pmd_leaves()
   check so that mTHP sizes are considered
 - In shmem_alloc_and_add_folio(): don't consider PMD and PUD orders
   when PMD-sized pages are not supported by the CPU

Signed-off-by: Luiz Capitulino <luizcap@...hat.com>
---
 mm/huge_memory.c | 11 +++++++----
 mm/shmem.c       |  4 +++-
 2 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 1e5ea2e47f79..882331592928 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -115,6 +115,9 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
 	else
 		supported_orders = THP_ORDERS_ALL_FILE_DEFAULT;
 
+	if (!pgtable_has_pmd_leaves())
+		supported_orders &= ~(BIT(PMD_ORDER) | BIT(PUD_ORDER));
+
 	orders &= supported_orders;
 	if (!orders)
 		return 0;
@@ -122,7 +125,7 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
 	if (!vma->vm_mm)		/* vdso */
 		return 0;
 
-	if (!pgtable_has_pmd_leaves() || vma_thp_disabled(vma, vm_flags, forced_collapse))
+	if (vma_thp_disabled(vma, vm_flags, forced_collapse))
 		return 0;
 
 	/* khugepaged doesn't collapse DAX vma, but page fault is fine. */
@@ -806,6 +809,9 @@ static int __init hugepage_init_sysfs(struct kobject **hugepage_kobj)
 	}
 
 	orders = THP_ORDERS_ALL_ANON | THP_ORDERS_ALL_FILE_DEFAULT;
+	if (!pgtable_has_pmd_leaves())
+		orders &= ~(BIT(PMD_ORDER) | BIT(PUD_ORDER));
+
 	order = highest_order(orders);
 	while (orders) {
 		thpsize = thpsize_create(order, *hugepage_kobj);
@@ -905,9 +911,6 @@ static int __init hugepage_init(void)
 	int err;
 	struct kobject *hugepage_kobj;
 
-	if (!pgtable_has_pmd_leaves())
-		return -EINVAL;
-
 	/*
 	 * hugepages can't be allocated by the buddy allocator
 	 */
diff --git a/mm/shmem.c b/mm/shmem.c
index 1c98e84667a4..cb325d1e2d1e 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1827,7 +1827,7 @@ unsigned long shmem_allowable_huge_orders(struct inode *inode,
 	vm_flags_t vm_flags = vma ? vma->vm_flags : 0;
 	unsigned int global_orders;
 
-	if (!pgtable_has_pmd_leaves() || (vma && vma_thp_disabled(vma, vm_flags, shmem_huge_force)))
+	if (vma && vma_thp_disabled(vma, vm_flags, shmem_huge_force))
 		return 0;
 
 	global_orders = shmem_huge_global_enabled(inode, index, write_end,
@@ -1935,6 +1935,8 @@ static struct folio *shmem_alloc_and_add_folio(struct vm_fault *vmf,
 
 	if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
 		orders = 0;
+	else if (!pgtable_has_pmd_leaves())
+		orders &= ~(BIT(PMD_ORDER) | BIT(PUD_ORDER));
 
 	if (orders > 0) {
 		suitable_orders = shmem_suitable_orders(inode, vmf,
-- 
2.53.0