Re: [PATCH v12 12/31] mm: protect SPF handler against anon_vma changes

[Jerome Glisse]

On Tue, Apr 16, 2019 at 03:45:03PM +0200, Laurent Dufour wrote:
> The speculative page fault handler must be protected against anon_vma
> changes. This is because page_add_new_anon_rmap() is called during the
> speculative path.
> 
> In addition, don't try speculative page fault if the VMA don't have an
> anon_vma structure allocated because its allocation should be
> protected by the mmap_sem.
> 
> In __vma_adjust() when importer->anon_vma is set, there is no need to
> protect against speculative page faults since speculative page fault
> is aborted if the vma->anon_vma is not set.
> 
> When calling page_add_new_anon_rmap() vma->anon_vma is necessarily
> valid since we checked for it when locking the pte and the anon_vma is
> removed once the pte is unlocked. So even if the speculative page
> fault handler is running concurrently with do_unmap(), as the pte is
> locked in unmap_region() - through unmap_vmas() - and the anon_vma
> unlinked later, because we check for the vma sequence counter which is
> updated in unmap_page_range() before locking the pte, and then in
> free_pgtables() so when locking the pte the change will be detected.
> 
> Signed-off-by: Laurent Dufour <ldufour@xxxxxxxxxxxxx>

Reviewed-by: Jérôme Glisse <jglisse@xxxxxxxxxx>

> ---
>  mm/memory.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 423fa8ea0569..2cf7b6185daa 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -377,7 +377,9 @@ void free_pgtables(struct mmu_gather *tlb, struct vm_area_struct *vma,
>  		 * Hide vma from rmap and truncate_pagecache before freeing
>  		 * pgtables
>  		 */
> +		vm_write_begin(vma);
>  		unlink_anon_vmas(vma);
> +		vm_write_end(vma);
>  		unlink_file_vma(vma);
>  
>  		if (is_vm_hugetlb_page(vma)) {
> @@ -391,7 +393,9 @@ void free_pgtables(struct mmu_gather *tlb, struct vm_area_struct *vma,
>  			       && !is_vm_hugetlb_page(next)) {
>  				vma = next;
>  				next = vma->vm_next;
> +				vm_write_begin(vma);
>  				unlink_anon_vmas(vma);
> +				vm_write_end(vma);
>  				unlink_file_vma(vma);
>  			}
>  			free_pgd_range(tlb, addr, vma->vm_end,
> -- 
> 2.21.0
>