Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable
From:   David Hildenbrand <david@redhat.com>
Mime-Version: 1.0 (1.0)
Subject: Re: [PATCH v17 07/10] mm: introduce memfd_secret system call to create "secret" memory areas
Date:   Sun, 14 Feb 2021 10:58:44 +0100
Message-Id: <052DACE9-986B-424C-AF8E-D6A4277DE635@redhat.com>
References: <20210214091954.GM242749@kernel.org>
Cc:     David Hildenbrand <david@redhat.com>,
        Michal Hocko <mhocko@suse.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andy Lutomirski <luto@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>, Borislav Petkov <bp@alien8.de>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Christopher Lameter <cl@linux.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Elena Reshetova <elena.reshetova@intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        James Bottomley <jejb@linux.ibm.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Matthew Wilcox <willy@infradead.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Rick Edgecombe <rick.p.edgecombe@intel.com>,
        Roman Gushchin <guro@fb.com>,
        Shakeel Butt <shakeelb@google.com>,
        Shuah Khan <shuah@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Tycho Andersen <tycho@tycho.ws>, Will Deacon <will@kernel.org>,
        linux-api@vger.kernel.org, linux-arch@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org,
        linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
        linux-nvdimm@lists.01.org, linux-riscv@lists.infradead.org,
        x86@kernel.org, Hagen Paul Pfeifer <hagen@jauu.net>,
        Palmer Dabbelt <palmerdabbelt@google.com>
In-Reply-To: <20210214091954.GM242749@kernel.org>
To:     Mike Rapoport <rppt@kernel.org>
Precedence: bulk


> Am 14.02.2021 um 10:20 schrieb Mike Rapoport <rppt@kernel.org>:
>=20
> =EF=BB=BFOn Fri, Feb 12, 2021 at 10:18:19AM +0100, David Hildenbrand wrote=
:
>>> On 12.02.21 00:09, Mike Rapoport wrote:
>>> On Thu, Feb 11, 2021 at 01:07:10PM +0100, David Hildenbrand wrote:
>>>> On 11.02.21 12:27, Mike Rapoport wrote:
>>>>> On Thu, Feb 11, 2021 at 10:01:32AM +0100, David Hildenbrand wrote:
>>>>=20
>>>> So let's talk about the main user-visible differences to other memfd fi=
les
>>>> (especially, other purely virtual files like hugetlbfs). With secretmem=
:
>>>>=20
>>>> - File content can only be read/written via memory mappings.
>>>> - File content cannot be swapped out.
>>>>=20
>>>> I think there are still valid ways to modify file content using syscall=
s:
>>>> e.g., fallocate(PUNCH_HOLE). Things like truncate also seems to work ju=
st
>>>> fine.
>>> These work perfectly with any file, so maybe we should have added
>>> memfd_create as a flag to open(2) back then and now the secretmem file
>>> descriptors?
>>=20
>> I think open() vs memfd_create() makes sense: for open, the path specifie=
s
>> main properties (tmpfs, hugetlbfs, filesystem). On memfd, there is no suc=
h
>> path and the "type" has to be specified differently.
>>=20
>> Also, open() might open existing files - memfd always creates new files.
>=20
> Yes, but still open() returns a handle to a file and memfd_create() return=
s
> a handle to a file. The differences may be well hidden by e.g. O_MEMORY an=
d
> than features unique to memfd files will have their set of O_SOMETHING
> flags.
>=20

Let=E2=80=98s agree to disagree.

> It's the same logic that says "we already have an interface that's close
> enough and it's fine to add a bunch of new flags there".

No, not quite. But let=E2=80=98s agree to disagree.

>=20
> And here we come to the question "what are the differences that justify a
> new system call?" and the answer to this is very subjective. And as such w=
e
> can continue bikeshedding forever.

I think this fits into the existing memfd_create() syscall just fine, and I h=
eard no compelling argument why it shouldn=E2=80=98t. That=E2=80=98s all I c=
an say.=