linux-kernel - Re: [RFC PATCH v2 32/51] KVM: guest_memfd: Support guestmem

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Message-ID: <aDBSJ+Se1nU8HmnQ@yzhao56-desk.sh.intel.com>
Date: Fri, 23 May 2025 18:47:03 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Ackerley Tng <ackerleytng@...gle.com>
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Subject: Re: [RFC PATCH v2 32/51] KVM: guest_memfd: Support guestmem_hugetlb
 as custom allocator

On Wed, May 14, 2025 at 04:42:11PM -0700, Ackerley Tng wrote:
>  enum shareability {
> @@ -40,6 +47,44 @@ static struct kvm_gmem_inode_private *kvm_gmem_private(struct inode *inode)
>  	return inode->i_mapping->i_private_data;
>  }
>  
> +#ifdef CONFIG_KVM_GMEM_HUGETLB
> +
> +static const struct guestmem_allocator_operations *
> +kvm_gmem_allocator_ops(struct inode *inode)
> +{
> +	return kvm_gmem_private(inode)->allocator_ops;
> +}
> +
> +static void *kvm_gmem_allocator_private(struct inode *inode)
> +{
> +	return kvm_gmem_private(inode)->allocator_private;
> +}
> +
> +static bool kvm_gmem_has_custom_allocator(struct inode *inode)
> +{

+       if (!kvm_gmem_private(inode))
+               return NULL;

> +	return kvm_gmem_allocator_ops(inode) != NULL;
> +}
> +
...

> +static void kvm_gmem_evict_inode(struct inode *inode)
> +{
> +	truncate_inode_pages_final_prepare(inode->i_mapping);
> +
> +	if (kvm_gmem_has_custom_allocator(inode)) {

The i_private_data of the root inode in pseudo fs is NULL.
Without the two lines added above, evicting the root inode during unmount will
cause NULL pointer deference.

> +		size_t nr_pages = inode->i_size >> PAGE_SHIFT;
> +
> +		kvm_gmem_truncate_inode_aligned_pages(inode, 0, nr_pages);
> +	} else {
> +		truncate_inode_pages(inode->i_mapping, 0);
> +	}
> +
> +	clear_inode(inode);
> +}
> +
>  static const struct super_operations kvm_gmem_super_operations = {
>  	.statfs		= simple_statfs,
> +	.evict_inode	= kvm_gmem_evict_inode,
>  	.destroy_inode	= kvm_gmem_destroy_inode,
>  	.free_inode	= kvm_gmem_free_inode,
>  };