Re: [PATCH v2 1/2] mm: free unused page table allocated by do_set_pmd()

[Lance Yang <lance.yang@linux.dev>]

On Fri, Nov 06, 2026 at 01:36:12PM +0100, David Hildenbrand (Arm) wrote:
>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.
>
>Fixes: 1043173eb5eb ("mm/khugepaged: collapse_pte_mapped_thp() with mmap_read_lock()")
>Cc: stable@vger.kernel.org
>Reviewed-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
>Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
>---
> mm/memory.c | 6 ++++++
> 1 file changed, 6 insertions(+)
>
>diff --git a/mm/memory.c b/mm/memory.c
>index becf53a2a0944..421064aef3cf0 100644
>--- a/mm/memory.c
>+++ b/mm/memory.c
>@@ -5687,6 +5687,7 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct folio *folio, struct page *pa
> 	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;
> 
>@@ -5725,6 +5726,7 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct folio *folio, struct page *pa
> 		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);
>@@ -5755,6 +5757,10 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct folio *folio, struct page *pa
> 	count_vm_event(THP_FILE_MAPPED);
> out:
> 	spin_unlock(vmf->ptl);
>+	if (pgtable_allocated && vmf->prealloc_pte) {

Took me a sec ... to see why we can't just check vmf->prealloc_pte here
...

Emm ... as it turns out, the caller might have supplied it for fallback :D

LGTM1

Reviewed-by: Lance Yang <lance.yang@linux.dev>

>+		pte_free(vma->vm_mm, vmf->prealloc_pte);
>+		vmf->prealloc_pte = NULL;
>+	}
> 	return ret;
> }
> #else
>
>-- 
>2.43.0
>
>