linux-kernel - Re: [PATCH v4 04/12] mm/shmem: Support memfile

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Message-ID: <314affa4-fbcb-2cb9-deb7-f61a2ac99260@kernel.org>
Date:   Fri, 11 Feb 2022 15:40:09 -0800
From:   Andy Lutomirski <luto@...nel.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:     Paolo Bonzini <pbonzini@...hat.com>,
        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>,
        jun.nakajima@...el.com, dave.hansen@...el.com, ak@...ux.intel.com,
        david@...hat.com
Subject: Re: [PATCH v4 04/12] mm/shmem: Support memfile_notifier

On 1/18/22 05:21, Chao Peng wrote:
> It maintains a memfile_notifier list in shmem_inode_info structure and
> implements memfile_pfn_ops callbacks defined by memfile_notifier. It
> then exposes them to memfile_notifier via
> shmem_get_memfile_notifier_info.
> 
> We use SGP_NOALLOC in shmem_get_lock_pfn since the pages should be
> allocated by userspace for private memory. If there is no pages
> allocated at the offset then error should be returned so KVM knows that
> the memory is not private memory.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Signed-off-by: Chao Peng <chao.p.peng@...ux.intel.com>

>   static int memfile_get_notifier_info(struct inode *inode,
>   				     struct memfile_notifier_list **list,
>   				     struct memfile_pfn_ops **ops)
>   {
> -	return -EOPNOTSUPP;
> +	int ret = -EOPNOTSUPP;
> +#ifdef CONFIG_SHMEM
> +	ret = shmem_get_memfile_notifier_info(inode, list, ops);
> +#endif
> +	return ret;
>   }

> +int shmem_get_memfile_notifier_info(struct inode *inode,
> +				    struct memfile_notifier_list **list,
> +				    struct memfile_pfn_ops **ops)
> +{
> +	struct shmem_inode_info *info;
> +
> +	if (!shmem_mapping(inode->i_mapping))
> +		return -EINVAL;
> +
> +	info = SHMEM_I(inode);
> +	*list = &info->memfile_notifiers;
> +	if (ops)
> +		*ops = &shmem_pfn_ops;
> +
> +	return 0;

I can't wrap my head around exactly who is supposed to call these 
functions and when, but there appears to be a missing check that the 
inode is actually a shmem inode.

What is this code trying to do?  It's very abstract.