Re: [RFC v2 PATCH 01/13] mm/shmem: Introduce F_SEAL_GUEST

[Paolo Bonzini]

On 11/19/21 14:47, Chao Peng wrote:
> +static void guest_invalidate_page(struct inode *inode,
> +                                 struct page *page, pgoff_t start, pgoff_t end)
> +{
> +       struct shmem_inode_info *info = SHMEM_I(inode);
> +
> +       if (!info->guest_ops || !info->guest_ops->invalidate_page_range)
> +               return;
> +
> +       start = max(start, page->index);
> +       end = min(end, page->index + thp_nr_pages(page)) - 1;
> +
> +       info->guest_ops->invalidate_page_range(inode, info->guest_owner,
> +                                              start, end);
> +}

The lack of protection makes the API quite awkward to use;
the usual way to do this is with refcount_inc_not_zero (aka 
kvm_get_kvm_safe).

Can you use the shmem_inode_info spinlock to protect against this?  If 
register/unregister take the spinlock, the invalidate and fallocate can 
take a reference under the same spinlock, like this:

        if (!info->guest_ops)
                return;

        spin_lock(&info->lock);
        ops = info->guest_ops;
        if (!ops) {
                spin_unlock(&info->lock);
                return;
        }

        /* Calls kvm_get_kvm_safe.  */
        r = ops->get_guest_owner(info->guest_owner);
        spin_unlock(&info->lock);
        if (r < 0)
                return;

        start = max(start, page->index);
        end = min(end, page->index + thp_nr_pages(page)) - 1;

        ops->invalidate_page_range(inode, info->guest_owner,
                                               start, end);
        ops->put_guest_owner(info->guest_owner);

Considering that you have to take a mutex anyway in patch 13, and that 
the critical section here is very small, the extra indirect calls are 
cheaper than walking the vm_list; and it makes the API clearer.

Paolo