Commit c0620487 authored Apr 02, 2026 by Mike Rapoport (Microsoft) Committed by Andrew Morton Apr 18, 2026

userfaultfd: introduce mfill_copy_folio_locked() helper

Patch series "mm, kvm: allow uffd support in guest_memfd", v4.

These patches enable support for userfaultfd in guest_memfd.

As the groundwork I refactored userfaultfd handling of PTE-based memory
types (anonymous and shmem) and converted them to use vm_uffd_ops for
allocating a folio or getting an existing folio from the page cache. 
shmem also implements callbacks that add a folio to the page cache after
the data passed in UFFDIO_COPY was copied and remove the folio from the
page cache if page table update fails.

In order for guest_memfd to notify userspace about page faults, there are
new VM_FAULT_UFFD_MINOR and VM_FAULT_UFFD_MISSING that a ->fault() handler
can return to inform the page fault handler that it needs to call
handle_userfault() to complete the fault.

Nikita helped to plumb these new goodies into guest_memfd and provided
basic tests to verify that guest_memfd works with userfaultfd.  The
handling of UFFDIO_MISSING in guest_memfd requires ability to remove a
folio from page cache, the best way I could find was exporting
filemap_remove_folio() to KVM.

I deliberately left hugetlb out, at least for the most part.  hugetlb
handles acquisition of VMA and more importantly establishing of parent
page table entry differently than PTE-based memory types.  This is a
different abstraction level than what vm_uffd_ops provides and people
objected to exposing such low level APIs as a part of VMA operations.

Also, to enable uffd in guest_memfd refactoring of hugetlb is not needed
and I prefer to delay it until the dust settles after the changes in this
set.


This patch (of 4):

Split copying of data when locks held from mfill_atomic_pte_copy() into a
helper function mfill_copy_folio_locked().

This makes improves code readability and makes complex
mfill_atomic_pte_copy() function easier to comprehend.

No functional change.

Link: https://lore.kernel.org/20260402041156.1377214-1-rppt@kernel.org
Link: https://lore.kernel.org/20260402041156.1377214-2-rppt@kernel.org


Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Acked-by: Peter Xu <peterx@redhat.com>
Reviewed-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Harry Yoo (Oracle) <harry@kernel.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andrei Vagin <avagin@google.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: James Houghton <jthoughton@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Harry Yoo <harry.yoo@oracle.com>
Cc: Nikita Kalyazin <kalyazin@amazon.com>
Cc: David Carlier <devnexen@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

parent dc44f32f

mm/userfaultfd.c

+35 −24

Original line number	Diff line number	Diff line
		@@ -238,23 +238,10 @@ int mfill_atomic_install_pte(pmd_t *dst_pmd,
		return ret;
		}

		static int mfill_atomic_pte_copy(pmd_t *dst_pmd,
		struct vm_area_struct *dst_vma,
		unsigned long dst_addr,
		unsigned long src_addr,
		uffd_flags_t flags,
		struct folio **foliop)
		static int mfill_copy_folio_locked(struct folio *folio, unsigned long src_addr)
		{
		void *kaddr;
		int ret;
		struct folio *folio;

		if (!*foliop) {
		ret = -ENOMEM;
		folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0, dst_vma,
		dst_addr);
		if (!folio)
		goto out;

		kaddr = kmap_local_folio(folio, 0);
		/*
		@@ -278,6 +265,32 @@ static int mfill_atomic_pte_copy(pmd_t *dst_pmd,
		pagefault_enable();
		kunmap_local(kaddr);

		if (ret)
		return -EFAULT;

		flush_dcache_folio(folio);
		return ret;
		}

		static int mfill_atomic_pte_copy(pmd_t *dst_pmd,
		struct vm_area_struct *dst_vma,
		unsigned long dst_addr,
		unsigned long src_addr,
		uffd_flags_t flags,
		struct folio **foliop)
		{
		int ret;
		struct folio *folio;

		if (!*foliop) {
		ret = -ENOMEM;
		folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0, dst_vma,
		dst_addr);
		if (!folio)
		goto out;

		ret = mfill_copy_folio_locked(folio, src_addr);

		/* fallback to copy_from_user outside mmap_lock */
		if (unlikely(ret)) {
		ret = -ENOENT;
		@@ -285,8 +298,6 @@ static int mfill_atomic_pte_copy(pmd_t *dst_pmd,
		/* don't free the page */
		goto out;
		}

		flush_dcache_folio(folio);
		} else {
		folio = *foliop;
		*foliop = NULL;