linux-kernel - Re: [PATCH v7 05/14] mm/memfd: Introduce MFD

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Message-ID: <64ab9678-c72d-b6d9-8532-346cc9c06814@redhat.com>
Date:   Wed, 10 Aug 2022 11:55:19 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Chao Peng <chao.p.peng@...ux.intel.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-api@...r.kernel.org, linux-doc@...r.kernel.org,
        qemu-devel@...gnu.org, linux-kselftest@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
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        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>,
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        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>,
        Michael Roth <michael.roth@....com>, mhocko@...e.com,
        Muchun Song <songmuchun@...edance.com>
Subject: Re: [PATCH v7 05/14] mm/memfd: Introduce MFD_INACCESSIBLE flag

On 10.08.22 11:37, Chao Peng wrote:
> On Fri, Aug 05, 2022 at 03:28:50PM +0200, David Hildenbrand wrote:
>> On 06.07.22 10:20, Chao Peng wrote:
>>> Introduce a new memfd_create() flag indicating the content of the
>>> created memfd is inaccessible from userspace through ordinary MMU
>>> access (e.g., read/write/mmap). However, the file content can be
>>> accessed via a different mechanism (e.g. KVM MMU) indirectly.
>>>
>>> It provides semantics required for KVM guest private memory support
>>> that a file descriptor with this flag set is going to be used as the
>>> source of guest memory in confidential computing environments such
>>> as Intel TDX/AMD SEV but may not be accessible from host userspace.
>>>
>>> The flag can not coexist with MFD_ALLOW_SEALING, future sealing is
>>> also impossible for a memfd created with this flag.
>>
>> It's kind of weird to have it that way. Why should the user have to
>> care? It's the notifier requirement to have that, no?
>>
>> Why can't we handle that when register a notifier? If anything is
>> already mapped, fail registering the notifier if the notifier has these
>> demands. If registering succeeds, block it internally.
>>
>> Or what am I missing? We might not need the memfile set flag semantics
>> eventually and would not have to expose such a flag to user space.
> 
> This makes sense if doable. The major concern was: is there a reliable
> way to detect this (already mapped) at the time of memslot registering.

If too complicated, we could simplify to "was this ever mapped" and fail
for now. Hooking into shmem_mmap() might be sufficient for that to get
notified about the first mmap.

As an alternative, mapping_mapped() or similar *might* do what we want.



-- 
Thanks,

David / dhildenb