linux-kernel - Re: [PATCH v9 1/8] mm: Introduce memfd_restricted system call to create restricted user memory

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Message-ID: <20221129135844.GA902164@chaop.bj.intel.com>
Date:   Tue, 29 Nov 2022 21:58:44 +0800
From:   Chao Peng <chao.p.peng@...ux.intel.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Michael Roth <michael.roth@....com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-api@...r.kernel.org, linux-doc@...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>,
        Shuah Khan <shuah@...nel.org>, Mike Rapoport <rppt@...nel.org>,
        Steven Price <steven.price@....com>,
        "Maciej S . Szmigiero" <mail@...iej.szmigiero.name>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        luto@...nel.org, jun.nakajima@...el.com, dave.hansen@...el.com,
        ak@...ux.intel.com, david@...hat.com, aarcange@...hat.com,
        ddutile@...hat.com, dhildenb@...hat.com,
        Quentin Perret <qperret@...gle.com>, tabba@...gle.com,
        mhocko@...e.com, Muchun Song <songmuchun@...edance.com>,
        wei.w.wang@...el.com
Subject: Re: [PATCH v9 1/8] mm: Introduce memfd_restricted system call to
 create restricted user memory

On Tue, Nov 29, 2022 at 02:21:39PM +0300, Kirill A. Shutemov wrote:
> On Mon, Nov 28, 2022 at 06:06:32PM -0600, Michael Roth wrote:
> > On Tue, Oct 25, 2022 at 11:13:37PM +0800, Chao Peng wrote:
> > > From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
> > > 
> > 
> > <snip>
> > 
> > > +static struct file *restrictedmem_file_create(struct file *memfd)
> > > +{
> > > +	struct restrictedmem_data *data;
> > > +	struct address_space *mapping;
> > > +	struct inode *inode;
> > > +	struct file *file;
> > > +
> > > +	data = kzalloc(sizeof(*data), GFP_KERNEL);
> > > +	if (!data)
> > > +		return ERR_PTR(-ENOMEM);
> > > +
> > > +	data->memfd = memfd;
> > > +	mutex_init(&data->lock);
> > > +	INIT_LIST_HEAD(&data->notifiers);
> > > +
> > > +	inode = alloc_anon_inode(restrictedmem_mnt->mnt_sb);
> > > +	if (IS_ERR(inode)) {
> > > +		kfree(data);
> > > +		return ERR_CAST(inode);
> > > +	}
> > > +
> > > +	inode->i_mode |= S_IFREG;
> > > +	inode->i_op = &restrictedmem_iops;
> > > +	inode->i_mapping->private_data = data;
> > > +
> > > +	file = alloc_file_pseudo(inode, restrictedmem_mnt,
> > > +				 "restrictedmem", O_RDWR,
> > > +				 &restrictedmem_fops);
> > > +	if (IS_ERR(file)) {
> > > +		iput(inode);
> > > +		kfree(data);
> > > +		return ERR_CAST(file);
> > > +	}
> > > +
> > > +	file->f_flags |= O_LARGEFILE;
> > > +
> > > +	mapping = memfd->f_mapping;
> > > +	mapping_set_unevictable(mapping);
> > > +	mapping_set_gfp_mask(mapping,
> > > +			     mapping_gfp_mask(mapping) & ~__GFP_MOVABLE);
> > 
> > Is this supposed to prevent migration of pages being used for
> > restrictedmem/shmem backend?
> 
> Yes, my bad. I expected it to prevent migration, but it is not true.
> 
> Looks like we need to bump refcount in restrictedmem_get_page() and reduce
> it back when KVM is no longer use it.

The restrictedmem_get_page() has taken a reference, but later KVM
put_page() after populating the secondary page table entry through
kvm_release_pfn_clean(). One option would let the user feature(e.g.
TDX/SEV) to get_page/put_page() during populating the secondary page
table entry, AFAICS, this requirement also comes from these features.

Chao
> 
> Chao, could you adjust it?
> 
> -- 
>   Kiryl Shutsemau / Kirill A. Shutemov