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

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Message-ID: <6d7f7775-5703-c27a-e57b-03aafb4de712@redhat.com>
Date:   Tue, 29 Nov 2022 12:39:06 +0100
From:   David Hildenbrand <david@...hat.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Michael Roth <michael.roth@....com>
Cc:     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, 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, 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 29.11.22 12:21, 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.

Maybe add a comment that these pages are not movable and we don't want 
to place them into movable pageblocks (including CMA and ZONE_MOVABLE). 
That's the primary purpose of the GFP mask here.

-- 
Thanks,

David / dhildenb