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

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Message-ID: <13394075-fca0-6f2b-92a2-f1291fcec9a3@redhat.com>
Date:   Fri, 5 Aug 2022 15:22:58 +0200
From:   David Hildenbrand <david@...hat.com>
To:     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-api@...r.kernel.org,
        linux-doc@...r.kernel.org, qemu-devel@...gnu.org,
        linux-kselftest@...r.kernel.org
Cc:     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 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?

> 
> 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?

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.

-- 
Thanks,

David / dhildenb