Re: [RFC PATCH v1 01/37] KVM: guest_memfd: Introduce per-gmem attributes, use to guard user mappings

[Binbin Wu <binbin.wu@linux.intel.com>]

On 10/18/2025 4:11 AM, Ackerley Tng wrote:
[...]
>
> +static int kvm_gmem_init_inode(struct inode *inode, loff_t size, u64 flags)
> +{
> +	struct gmem_inode *gi = GMEM_I(inode);
> +	MA_STATE(mas, &gi->attributes, 0, (size >> PAGE_SHIFT) - 1);
> +	u64 attrs;
> +	int r;
> +
> +	inode->i_op = &kvm_gmem_iops;
> +	inode->i_mapping->a_ops = &kvm_gmem_aops;
> +	inode->i_mode |= S_IFREG;
> +	inode->i_size = size;
> +	mapping_set_gfp_mask(inode->i_mapping, GFP_HIGHUSER);
> +	mapping_set_inaccessible(inode->i_mapping);
> +	/* Unmovable mappings are supposed to be marked unevictable as well. */
AS_UNMOVABLE has been removed and got merged into AS_INACCESSIBLE, not sure if
it's better to use "Inaccessible" instead of "Unmovable"

> +	WARN_ON_ONCE(!mapping_unevictable(inode->i_mapping));
> +
> +	gi->flags = flags;
> +
> +	mt_set_external_lock(&gi->attributes,
> +			     &inode->i_mapping->invalidate_lock);
> +
> +	/*
> +	 * Store default attributes for the entire gmem instance. Ensuring every
> +	 * index is represented in the maple tree at all times simplifies the
> +	 * conversion and merging logic.
> +	 */
> +	attrs = gi->flags & GUEST_MEMFD_FLAG_INIT_SHARED ? 0 : KVM_MEMORY_ATTRIBUTE_PRIVATE;
> +
> +	/*
> +	 * Acquire the invalidation lock purely to make lockdep happy. There
> +	 * should be no races at this time since the inode hasn't yet been fully
> +	 * created.
> +	 */
> +	filemap_invalidate_lock(inode->i_mapping);
> +	r = mas_store_gfp(&mas, xa_mk_value(attrs), GFP_KERNEL);
> +	filemap_invalidate_unlock(inode->i_mapping);
> +
> +	return r;
> +}
> +
[...]
> @@ -925,13 +986,39 @@ static struct inode *kvm_gmem_alloc_inode(struct super_block *sb)
>
>   	mpol_shared_policy_init(&gi->policy, NULL);
>
> +	/*
> +	 * Memory attributes are protected the filemap invalidation lock, but
                                      ^
                                 protected by
> +	 * the lock structure isn't available at this time.  Immediately mark
> +	 * maple tree as using external locking so that accessing the tree
> +	 * before its fully initialized results in NULL pointer dereferences
> +	 * and not more subtle bugs.
> +	 */
> +	mt_init_flags(&gi->attributes, MT_FLAGS_LOCK_EXTERN);
> +
>   	gi->flags = 0;
>   	return &gi->vfs_inode;
>   }
>
>   static void kvm_gmem_destroy_inode(struct inode *inode)
>   {
> -	mpol_free_shared_policy(&GMEM_I(inode)->policy);
> +	struct gmem_inode *gi = GMEM_I(inode);
> +
> +	mpol_free_shared_policy(&gi->policy);
> +
> +	/*
> +	 * Note!  Checking for an empty tree is functionally necessary to avoid
> +	 * explosions if the tree hasn't been initialized, i.e. if the inode is

It makes sense to skip __mt_destroy() when mtree is empty.
But what explosions it could trigger if mtree is empty?
It seems __mt_destroy() can handle the case if the external lock is not set.


> +	 * being destroyed before guest_memfd can set the external lock.
> +	 */
> +	if (!mtree_empty(&gi->attributes)) {
> +		/*
> +		 * Acquire the invalidation lock purely to make lockdep happy,
> +		 * the inode is unreachable at this point.
> +		 */
> +		filemap_invalidate_lock(inode->i_mapping);
> +		__mt_destroy(&gi->attributes);
> +		filemap_invalidate_unlock(inode->i_mapping);
> +	}
>   }
>
>   static void kvm_gmem_free_inode(struct inode *inode)
> --
> 2.51.0.858.gf9c4a03a3a-goog