linux-kernel - Re: [PATCH v7 03/14] mm: Introduce memfile

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Date:   Wed, 10 Aug 2022 17:22:32 +0800
From:   Chao Peng <chao.p.peng@...ux.intel.com>
To:     David Hildenbrand <david@...hat.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>,
        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>,
        Michael Roth <michael.roth@....com>, mhocko@...e.com,
        Muchun Song <songmuchun@...edance.com>
Subject: Re: [PATCH v7 03/14] mm: Introduce memfile_notifier

On Fri, Aug 05, 2022 at 03:22:58PM +0200, David Hildenbrand wrote:
> On 06.07.22 10:20, Chao Peng wrote:
> > This patch introduces memfile_notifier facility so existing memory file
> > subsystems (e.g. tmpfs/hugetlbfs) can provide memory pages to allow a
> > third kernel component to make use of memory bookmarked in the memory
> > file and gets notified when the pages in the memory file become
> > invalidated.
> 
> Stupid question, but why is this called "memfile_notifier" and not
> "memfd_notifier". We're only dealing with memfd's after all ... which
> are anonymous files essentially. Or what am I missing? Are there any
> other plans for fs than plain memfd support that I am not aware of?

There were some discussions on this in v3.
  https://lkml.org/lkml/2021/12/28/484
Sean commented it's OK to abstract it from memfd but he also wants the
kAPI (name) should not bind to memfd to make room for future non-memfd
usages.

> 
> > 
> > It will be used for KVM to use a file descriptor as the guest memory
> > backing store and KVM will use this memfile_notifier interface to
> > interact with memory file subsystems. In the future there might be other
> > consumers (e.g. VFIO with encrypted device memory).
> > 
> > It consists below components:
> >  - memfile_backing_store: Each supported memory file subsystem can be
> >    implemented as a memory backing store which bookmarks memory and
> >    provides callbacks for other kernel systems (memfile_notifier
> >    consumers) to interact with.
> >  - memfile_notifier: memfile_notifier consumers defines callbacks and
> >    associate them to a file using memfile_register_notifier().
> >  - memfile_node: A memfile_node is associated with the file (inode) from
> >    the backing store and includes feature flags and a list of registered
> >    memfile_notifier for notifying.
> > 
> > In KVM usages, userspace is in charge of guest memory lifecycle: it first
> > allocates pages in memory backing store and then passes the fd to KVM and
> > lets KVM register memory slot to memory backing store via
> > memfile_register_notifier.
> 
> Can we add documentation/description in any form how the different
> functions exposed in linux/memfile_notifier.h are supposed to be used?

Yeah, code comments can be added.

> 
> Staring at memfile_node_set_flags() and memfile_notifier_invalidate()
> it's not immediately clear to me who's supposed to call that and under
> which conditions.

I will also amend the commit message.

Chao
> 
> -- 
> Thanks,
> 
> David / dhildenb