* Jakub Matěna <matenajakub@xxxxxxxxx> [220218 07:21]:
> Adjust page offset of a VMA when it's moved to a new location by mremap.
> This is made possible for all VMAs that do not share their anonymous
> pages with other processes. Previously this was possible only for not
> yet faulted VMAs.
> When the page offset does not correspond to the virtual address
> of the anonymous VMA any merge attempt with another VMA will fail.
I don't know enough to fully answer this but I think this may cause
issues with rmap? I would think the anon vma lock is necessary but I
may be missing something.
>
> Signed-off-by: Jakub Matěna <matenajakub@xxxxxxxxx>
> ---
> mm/mmap.c | 101 ++++++++++++++++++++++++++++++++++++++++++++++++++----
> 1 file changed, 95 insertions(+), 6 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index b55e11f20571..8d253b46b349 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -3224,6 +3224,91 @@ int insert_vm_struct(struct mm_struct *mm, struct vm_area_struct *vma)
> return 0;
> }
>
> +bool rbst_no_children(struct anon_vma *av, struct rb_node *node)
> +{
> + struct anon_vma_chain *model;
> + struct anon_vma_chain *avc;
> +
> + if (node == NULL) /* leaf node */
> + return true;
> + avc = container_of(node, typeof(*(model)), rb);
> + if (avc->vma->anon_vma != av)
> + /*
> + * Inequality implies avc belongs
> + * to a VMA of a child process
> + */
> + return false;
> + return (rbst_no_children(av, node->rb_left) &&
> + rbst_no_children(av, node->rb_right));
> +}
> +
> +/*
> + * Check if none of the VMAs connected to the given
> + * anon_vma via anon_vma_chain are in child relationship
> + */
> +bool rbt_no_children(struct anon_vma *av)
> +{
> + struct rb_node *root_node;
> +
> + if (av == NULL || av->degree <= 1) /* Higher degree might not necessarily imply children */
> + return true;
> + root_node = av->rb_root.rb_root.rb_node;
> + return rbst_no_children(av, root_node);
> +}
> +
> +/**
> + * update_faulted_pgoff() - Update faulted pages of a vma
> + * @vma: VMA being moved
> + * @addr: new virtual address
> + * @pgoff: pointer to pgoff which is updated
> + * If the vma and its pages are not shared with another process, update
> + * the new pgoff and also update index parameter (copy of the pgoff) in
> + * all faulted pages.
> + */
> +bool update_faulted_pgoff(struct vm_area_struct *vma, unsigned long addr, pgoff_t *pgoff)
> +{
> + unsigned long pg_iter = 0;
> + unsigned long pg_iters = (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;
> +
> + /* 1.] Check vma is not shared with other processes */
> + if (vma->anon_vma->root != vma->anon_vma || !rbt_no_children(vma->anon_vma))
> + return false;
> +
> + /* 2.] Check all pages are not shared */
> + for (; pg_iter < pg_iters; ++pg_iter) {
> + bool pages_not_shared = true;
> + unsigned long shift = pg_iter << PAGE_SHIFT;
> + struct page *phys_page = follow_page(vma, vma->vm_start + shift, FOLL_GET);
> +
> + if (phys_page == NULL)
> + continue;
> +
> + /* Check page is not shared with other processes */
> + if (page_mapcount(phys_page) > 1)
> + pages_not_shared = false;
> + put_page(phys_page);
> + if (!pages_not_shared)
> + return false;
> + }
> +
> + /* 3.] Update index in all pages to this new pgoff */
> + pg_iter = 0;
> + *pgoff = addr >> PAGE_SHIFT;
> +
> + for (; pg_iter < pg_iters; ++pg_iter) {
> + unsigned long shift = pg_iter << PAGE_SHIFT;
> + struct page *phys_page = follow_page(vma, vma->vm_start + shift, FOLL_GET);
> +
> + if (phys_page == NULL)
> + continue;
> + lock_page(phys_page);
> + phys_page->index = *pgoff + pg_iter;
> + unlock_page(phys_page);
> + put_page(phys_page);
> + }
> + return true;
> +}
> +
> /*
> * Copy the vma structure to a new location in the same mm,
> * prior to moving page table entries, to effect an mremap move.
> @@ -3237,15 +3322,19 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
> struct mm_struct *mm = vma->vm_mm;
> struct vm_area_struct *new_vma, *prev;
> struct rb_node **rb_link, *rb_parent;
> - bool faulted_in_anon_vma = true;
> + bool anon_pgoff_updated = false;
>
> /*
> - * If anonymous vma has not yet been faulted, update new pgoff
> + * Try to update new pgoff for anonymous vma
> * to match new location, to increase its chance of merging.
> */
> - if (unlikely(vma_is_anonymous(vma) && !vma->anon_vma)) {
> - pgoff = addr >> PAGE_SHIFT;
> - faulted_in_anon_vma = false;
> + if (unlikely(vma_is_anonymous(vma))) {
> + if (!vma->anon_vma) {
> + pgoff = addr >> PAGE_SHIFT;
> + anon_pgoff_updated = true;
> + } else {
> + anon_pgoff_updated = update_faulted_pgoff(vma, addr, &pgoff);
> + }
> }
>
> if (find_vma_links(mm, addr, addr + len, &prev, &rb_link, &rb_parent))
> @@ -3271,7 +3360,7 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
> * safe. It is only safe to keep the vm_pgoff
> * linear if there are no pages mapped yet.
> */
> - VM_BUG_ON_VMA(faulted_in_anon_vma, new_vma);
> + VM_BUG_ON_VMA(!anon_pgoff_updated, new_vma);
> *vmap = vma = new_vma;
> }
> *need_rmap_locks = (new_vma->vm_pgoff <= vma->vm_pgoff);
> --
> 2.34.1
>
