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Date:   Mon, 9 Oct 2023 03:16:44 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        seanjc@google.com
Subject: Re: [PATCH gmem FIXUP] kvm: guestmem: do not use a file system
Message-ID: <20231009021644.GC800259@ZenIV>
References: <20230928180651.1525674-1-pbonzini@redhat.com>
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On Thu, Sep 28, 2023 at 02:06:51PM -0400, Paolo Bonzini wrote:
> Use a run-of-the-mill anonymous inode, there is nothing useful
> being provided by kvm_gmem_fs.

> -	inode = alloc_anon_inode(mnt->mnt_sb);
> -	if (IS_ERR(inode))
> -		return PTR_ERR(inode);
> +	fd = get_unused_fd_flags(0);
> +	if (fd < 0)
> +		return fd;
>  
> -	err = security_inode_init_security_anon(inode, &qname, NULL);
> -	if (err)
> -		goto err_inode;
> +	gmem = kzalloc(sizeof(*gmem), GFP_KERNEL);
> +	if (!gmem) {
> +		err = -ENOMEM;
> +		goto err_fd;
> +	}
> +
> +	file = anon_inode_getfile(anon_name, &kvm_gmem_fops, gmem,
> +				  O_RDWR);

> +	inode = file->f_inode;
> +	WARN_ON(file->f_mapping != inode->i_mapping);
>  
>  	inode->i_private = (void *)(unsigned long)flags;
>  	inode->i_op = &kvm_gmem_iops;

That's very badly broken.  The whole point of anon_inode_getfile() is
that *ALL* resulting files share the same inode.  You are not allowed
to modify the damn thing.