Re: [PATCH v2 05/10] mm/hugetlb: Make userfaultfd_huge_must_wait() safe to pmd unshare

[John Hubbard]

On 12/7/22 12:30, Peter Xu wrote:
> We can take the hugetlb walker lock, here taking vma lock directly.
>
> Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
> Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> ---
>   fs/userfaultfd.c | 18 ++++++++++++++----
>   1 file changed, 14 insertions(+), 4 deletions(-)
>
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 07c81ab3fd4d..a602f008dde5 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -376,7 +376,8 @@ static inline unsigned int userfaultfd_get_blocking_state(unsigned int flags)
>    */
>   vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
>   {
> -	struct mm_struct *mm = vmf->vma->vm_mm;
> +	struct vm_area_struct *vma = vmf->vma;
> +	struct mm_struct *mm = vma->vm_mm;
>   	struct userfaultfd_ctx *ctx;
>   	struct userfaultfd_wait_queue uwq;
>   	vm_fault_t ret = VM_FAULT_SIGBUS;
> @@ -403,7 +404,7 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
>   	 */
>   	mmap_assert_locked(mm);
>   
> -	ctx = vmf->vma->vm_userfaultfd_ctx.ctx;
> +	ctx = vma->vm_userfaultfd_ctx.ctx;
>   	if (!ctx)
>   		goto out;
>   
> @@ -493,6 +494,13 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
>   
>   	blocking_state = userfaultfd_get_blocking_state(vmf->flags);
>   
> +	/*
> +	 * This stablizes pgtable for hugetlb on e.g. pmd unsharing.  Need
> +	 * to be before setting current state.
> +	 */

Looking at this code, I am not able to come up with a reason for why the
vma lock/unlock placement is exactly where it is. It looks quite arbitrary.

Why not, for example, take and drop the vma lock within
userfaultfd_huge_must_wait()? That makes more sense to me, but I'm not familiar
with userfaultfd so of course I'm missing something.

But the comment above certainly doesn't supply that something.


thanks,
--
John Hubbard
NVIDIA

> +	if (is_vm_hugetlb_page(vma))
> +		hugetlb_vma_lock_read(vma);
> +
>   	spin_lock_irq(&ctx->fault_pending_wqh.lock);
>   	/*
>   	 * After the __add_wait_queue the uwq is visible to userland
> @@ -507,13 +515,15 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
>   	set_current_state(blocking_state);
>   	spin_unlock_irq(&ctx->fault_pending_wqh.lock);
>   
> -	if (!is_vm_hugetlb_page(vmf->vma))
> +	if (!is_vm_hugetlb_page(vma))
>   		must_wait = userfaultfd_must_wait(ctx, vmf->address, vmf->flags,
>   						  reason);
>   	else
> -		must_wait = userfaultfd_huge_must_wait(ctx, vmf->vma,
> +		must_wait = userfaultfd_huge_must_wait(ctx, vma,
>   						       vmf->address,
>   						       vmf->flags, reason);
> +	if (is_vm_hugetlb_page(vma))
> +		hugetlb_vma_unlock_read(vma);
>   	mmap_read_unlock(mm);
>   
>   	if (likely(must_wait && !READ_ONCE(ctx->released))) {