linux-kernel - Re: [RFC v2 PATCH 01/13] mm/shmem: Introduce F_SEAL

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Date:   Tue, 23 Nov 2021 09:54:52 +0100
From:   Paolo Bonzini <bonzini@....org>
To:     Chao Peng <chao.p.peng@...ux.intel.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, qemu-devel@...gnu.org
Cc:     Jonathan Corbet <corbet@....net>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jeff Layton <jlayton@...nel.org>,
        "J . Bruce Fields" <bfields@...ldses.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        luto@...nel.org, john.ji@...el.com, susie.li@...el.com,
        jun.nakajima@...el.com, dave.hansen@...el.com, ak@...ux.intel.com,
        david@...hat.com
Subject: Re: [RFC v2 PATCH 01/13] mm/shmem: Introduce F_SEAL_GUEST

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