linux-kernel - Re: [PATCH v8 1/8] mm/memfd: Introduce userspace inaccessible memfd

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Date:   Thu, 3 Nov 2022 21:57:11 +0530
From:   Vishal Annapurve <vannapurve@...gle.com>
To:     "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        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,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        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>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>, luto@...nel.org,
        jun.nakajima@...el.com, dave.hansen@...el.com, ak@...ux.intel.com,
        david@...hat.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>, wei.w.wang@...el.com
Subject: Re: [PATCH v8 1/8] mm/memfd: Introduce userspace inaccessible memfd

On Mon, Oct 24, 2022 at 8:30 PM Kirill A . Shutemov
<kirill.shutemov@...ux.intel.com> wrote:
>
> On Fri, Oct 21, 2022 at 04:18:14PM +0000, Sean Christopherson wrote:
> > On Fri, Oct 21, 2022, Chao Peng wrote:
> > > >
> > > > In the context of userspace inaccessible memfd, what would be a
> > > > suggested way to enforce NUMA memory policy for physical memory
> > > > allocation? mbind[1] won't work here in absence of virtual address
> > > > range.
> > >
> > > How about set_mempolicy():
> > > https://www.man7.org/linux/man-pages/man2/set_mempolicy.2.html
> >
> > Andy Lutomirski brought this up in an off-list discussion way back when the whole
> > private-fd thing was first being proposed.
> >
> >   : The current Linux NUMA APIs (mbind, move_pages) work on virtual addresses.  If
> >   : we want to support them for TDX private memory, we either need TDX private
> >   : memory to have an HVA or we need file-based equivalents. Arguably we should add
> >   : fmove_pages and fbind syscalls anyway, since the current API is quite awkward
> >   : even for tools like numactl.
>
> Yeah, we definitely have gaps in API wrt NUMA, but I don't think it be
> addressed in the initial submission.
>
> BTW, it is not regression comparing to old KVM slots, if the memory is
> backed by memfd or other file:
>
> MBIND(2)
>        The  specified policy will be ignored for any MAP_SHARED mappings in the
>        specified memory range.  Rather the pages will be allocated according to
>        the  memory  policy  of the thread that caused the page to be allocated.
>        Again, this may not be the thread that called mbind().
>
> It is not clear how to define fbind(2) semantics, considering that multiple
> processes may compete for the same region of page cache.
>
> Should it be per-inode or per-fd? Or maybe per-range in inode/fd?
>

David's analysis on mempolicy with shmem seems to be right. set_policy
on virtual address range does seem to change the shared policy for the
inode irrespective of the mapping type.

Maybe having a way to set numa policy per-range in the inode would be
at par with what we can do today via mbind on virtual address ranges.



> fmove_pages(2) should be relatively straight forward, since it is
> best-effort and does not guarantee that the page will note be moved
> somewhare else just after return from the syscall.
>
> --
>   Kiryl Shutsemau / Kirill A. Shutemov