Commit c56d1b62 authored by Peter Xu's avatar Peter Xu Committed by Andrew Morton
Browse files

mm/shmem: handle uffd-wp during fork() 

Normally we skip copy page when fork() for VM_SHARED shmem, but we can't
skip it anymore if uffd-wp is enabled on dst vma.  This should only happen
when the src uffd has UFFD_FEATURE_EVENT_FORK enabled on uffd-wp shmem
vma, so that VM_UFFD_WP will be propagated onto dst vma too, then we
should copy the pgtables with uffd-wp bit and pte markers, because these
information will be lost otherwise.

Since the condition checks will become even more complicated for deciding
"whether a vma needs to copy the pgtable during fork()", introduce a
helper vma_needs_copy() for it, so everything will be clearer.

Link: https://lkml.kernel.org/r/20220405014855.14468-1-peterx@redhat.com


Signed-off-by: default avatarPeter Xu <peterx@redhat.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: "Kirill A . Shutemov" <kirill@shutemov.name>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 019c2d8b

mm/memory.c

+41 −8
Original line number Diff line number Diff line
@@ -867,6 +867,14 @@ copy_nonpresent_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm, 
		if (try_restore_exclusive_pte(src_pte, src_vma, addr))

			return -EBUSY;

		return -ENOENT;

	} else if (is_pte_marker_entry(entry)) {

		/*

		 * We're copying the pgtable should only because dst_vma has

		 * uffd-wp enabled, do sanity check.

		 */

		WARN_ON_ONCE(!userfaultfd_wp(dst_vma));

		set_pte_at(dst_mm, addr, dst_pte, pte);

		return 0;

	}

	if (!userfaultfd_wp(dst_vma))

		pte = pte_swp_clear_uffd_wp(pte);
@@ -1221,6 +1229,38 @@ copy_p4d_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma, 
	return 0;

}



/*

 * Return true if the vma needs to copy the pgtable during this fork().  Return

 * false when we can speed up fork() by allowing lazy page faults later until

 * when the child accesses the memory range.

 */

bool

vma_needs_copy(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma)

{

	/*

	 * Always copy pgtables when dst_vma has uffd-wp enabled even if it's

	 * file-backed (e.g. shmem). Because when uffd-wp is enabled, pgtable

	 * contains uffd-wp protection information, that's something we can't

	 * retrieve from page cache, and skip copying will lose those info.

	 */

	if (userfaultfd_wp(dst_vma))

		return true;



	if (src_vma->vm_flags & (VM_HUGETLB | VM_PFNMAP | VM_MIXEDMAP))

		return true;



	if (src_vma->anon_vma)

		return true;



	/*

	 * Don't copy ptes where a page fault will fill them correctly.  Fork

	 * becomes much lighter when there are big shared or private readonly

	 * mappings. The tradeoff is that copy_page_range is more efficient

	 * than faulting.

	 */

	return false;

}



int

copy_page_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma)

{
@@ -1234,14 +1274,7 @@ copy_page_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma) 
	bool is_cow;

	int ret;



	/*

	 * Don't copy ptes where a page fault will fill them correctly.

	 * Fork becomes much lighter when there are big shared or private

	 * readonly mappings. The tradeoff is that copy_page_range is more

	 * efficient than faulting.

	 */

	if (!(src_vma->vm_flags & (VM_HUGETLB | VM_PFNMAP | VM_MIXEDMAP)) &&

	    !src_vma->anon_vma)

	if (!vma_needs_copy(dst_vma, src_vma))

		return 0;



	if (is_vm_hugetlb_page(src_vma))