linux-kernel - Re: [PATCH v5 01/13] mm/memfd: Introduce MFD

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Message-ID: <20220412131128.GE8013@chaop.bj.intel.com>
Date:   Tue, 12 Apr 2022 21:11:28 +0800
From:   Chao Peng <chao.p.peng@...ux.intel.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-api@...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>,
        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
Subject: Re: [PATCH v5 01/13] mm/memfd: Introduce MFD_INACCESSIBLE flag

On Mon, Apr 11, 2022 at 06:10:23PM +0300, Kirill A. Shutemov wrote:
> On Thu, Mar 10, 2022 at 10:08:59PM +0800, Chao Peng wrote:
> > From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
> > 
> > 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.
> > 
> > Since page migration/swapping is not yet supported for such usages
> > so these pages are currently marked as UNMOVABLE and UNEVICTABLE
> > which makes them behave like long-term pinned pages.
> > 
> > The flag can not coexist with MFD_ALLOW_SEALING, future sealing is
> > also impossible for a memfd created with this flag.
> > 
> > At this time only shmem implements this flag.
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> > Signed-off-by: Chao Peng <chao.p.peng@...ux.intel.com>
> > ---
> >  include/linux/shmem_fs.h   |  7 +++++
> >  include/uapi/linux/memfd.h |  1 +
> >  mm/memfd.c                 | 26 +++++++++++++++--
> >  mm/shmem.c                 | 57 ++++++++++++++++++++++++++++++++++++++
> >  4 files changed, 88 insertions(+), 3 deletions(-)
> > 
> > diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
> > index e65b80ed09e7..2dde843f28ef 100644
> > --- a/include/linux/shmem_fs.h
> > +++ b/include/linux/shmem_fs.h
> > @@ -12,6 +12,9 @@
> >  
> >  /* inode in-kernel data */
> >  
> > +/* shmem extended flags */
> > +#define SHM_F_INACCESSIBLE	0x0001  /* prevent ordinary MMU access (e.g. read/write/mmap) to file content */
> > +
> >  struct shmem_inode_info {
> >  	spinlock_t		lock;
> >  	unsigned int		seals;		/* shmem seals */
> > @@ -24,6 +27,7 @@ struct shmem_inode_info {
> >  	struct shared_policy	policy;		/* NUMA memory alloc policy */
> >  	struct simple_xattrs	xattrs;		/* list of xattrs */
> >  	atomic_t		stop_eviction;	/* hold when working on inode */
> > +	unsigned int		xflags;		/* shmem extended flags */
> >  	struct inode		vfs_inode;
> >  };
> >  
> 
> AFAICS, only two bits of 'flags' are used. And that's very strange that
> VM_ flags are used for the purpose. My guess that someone was lazy to
> introduce new constants for this.
> 
> I think we should fix this: introduce SHM_F_LOCKED and SHM_F_NORESERVE
> alongside with SHM_F_INACCESSIBLE and stuff them all into info->flags.
> It also makes shmem_file_setup_xflags() go away.

Did a quick search and sounds we only use SHM_F_LOCKED/SHM_F_NORESERVE and
that definitely don't have to be VM_ flags.

Chao
> 
> -- 
>  Kirill A. Shutemov