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

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Date:   Thu, 17 Feb 2022 21:23:25 +0800
From:   Chao Peng <chao.p.peng@...ux.intel.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        qemu-devel@...gnu.org, 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 Fri, Feb 11, 2022 at 03:40:09PM -0800, Andy Lutomirski wrote:
> 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.

This is to be called by memfile_(un)register_notifier in patch-03 to
allow shmem to be connected to memfile_notifer. But as Mike pointed out,
probably introducing a memfile_notifier_register_backing_store() sounds
better so backing store (e.g. shmem) can register itself to
memfile_notifier.

Chao