+ mm-khugepaged-free-unused-page-table-when-setting-a-huge-pmd-fails.patch added to mm-new branch

+ mm-khugepaged-free-unused-page-table-when-setting-a-huge-pmd-fails.patch added to mm-new branch

[Andrew Morton]

The patch titled
     Subject: mm: free unused page table allocated by do_set_pmd()
has been added to the -mm mm-new branch.  Its filename is
     mm-khugepaged-free-unused-page-table-when-setting-a-huge-pmd-fails.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-khugepaged-free-unused-page-table-when-setting-a-huge-pmd-fails.patch

This patch will later appear in the mm-new branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Note, mm-new is a provisional staging ground for work-in-progress
patches, and acceptance into mm-new is a notification for others take
notice and to finish up reviews.  Please do not hesitate to respond to
review feedback and post updated versions to replace or incrementally
fixup patches in mm-new.

The mm-new branch of mm.git is not included in linux-next

If a few days of testing in mm-new is successful, the patch will me moved
into mm.git's mm-unstable branch, which is included in linux-next

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and is updated there most days

------------------------------------------------------
From: "David Hildenbrand (Arm)" <david@kernel.org>
Subject: mm: free unused page table allocated by do_set_pmd()
Date: Wed, 4 Nov 2026 20:56:37 +0100

do_set_pmd() can allocate vmf->prealloc_pte on architectures that need a
deposited page table, such as PowerPC with the hash MMU.  If the PMD is no
longer empty when it takes the PMD lock, it returns VM_FAULT_FALLBACK
without consuming that table.

Since collapse_pte_mapped_thp() uses mmap_read_lock(), a concurrent
userfaultfd operation can install a PTE table before taking the folio
lock.  If that races with set_huge_pmd(), the unused preallocated table is
lost when its local vm_fault goes out of scope.

Track whether do_set_pmd() allocated the table and free it after dropping
the PMD lock if it was not consumed.  Preserve any preallocated table
supplied by the caller for PTE fallback.  Successful PMD installation
consumes the table and clears the pointer.

Returning without a preallocated table matches the existing behavior on
architectures that do not need a deposited table.  All callers already
handle that case.

Link: https://lore.kernel.org/c13cdd25-7d7b-4059-a22d-af0a48fabaac@kernel.org
Fixes: 1043173eb5eb ("mm/khugepaged: collapse_pte_mapped_thp() with mmap_read_lock()")
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Nico Pache <nico.pache@linux.dev>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Usama Arif <usama.arif@linux.dev>
Cc: Qi Zheng <qi.zheng@linux.dev>
Cc: Hugh Dickins <hughd@google.com>
Cc: <stable@vger.kernel.org>
---

 mm/memory.c |    6 ++++++
 1 file changed, 6 insertions(+)

--- a/mm/memory.c~mm-khugepaged-free-unused-page-table-when-setting-a-huge-pmd-fails
+++ a/mm/memory.c
@@ -5694,6 +5694,7 @@ vm_fault_t do_set_pmd(struct vm_fault *v
 	struct vm_area_struct *vma = vmf->vma;
 	bool write = vmf->flags & FAULT_FLAG_WRITE;
 	unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
+	bool pgtable_allocated = false;
 	pmd_t entry;
 	vm_fault_t ret = VM_FAULT_FALLBACK;
 
@@ -5732,6 +5733,7 @@ vm_fault_t do_set_pmd(struct vm_fault *v
 		vmf->prealloc_pte = pte_alloc_one(vma->vm_mm);
 		if (!vmf->prealloc_pte)
 			return VM_FAULT_OOM;
+		pgtable_allocated = true;
 	}
 
 	vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
@@ -5762,6 +5764,10 @@ vm_fault_t do_set_pmd(struct vm_fault *v
 	count_vm_event(THP_FILE_MAPPED);
 out:
 	spin_unlock(vmf->ptl);
+	if (pgtable_allocated && vmf->prealloc_pte) {
+		pte_free(vma->vm_mm, vmf->prealloc_pte);
+		vmf->prealloc_pte = NULL;
+	}
 	return ret;
 }
 #else
_

Patches currently in -mm which might be from david@kernel.org are

mm-memory-failure-fix-folio-refcount-leak-and-min_order_for_split-locking.patch
mm-mprotect-dont-skip-uffd-marker-install-when-racing-with-pte-table-reclaim.patch
mm-khugepaged-free-unused-page-table-when-setting-a-huge-pmd-fails.patch