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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Thu, 19 Oct 2023 14:45:21 -0700
Message-ID: <CAJuCfpF2hM9MmUdv4K8a1meKdwsdWb5Q0fFihUtsngidehTPnA@mail.gmail.com>
Subject: Re: [PATCH v3 2/3] userfaultfd: UFFDIO_MOVE uABI
To:     Peter Xu <peterx@redhat.com>
Cc:     David Hildenbrand <david@redhat.com>,
        Lokesh Gidra <lokeshgidra@google.com>,
        akpm@linux-foundation.org, viro@zeniv.linux.org.uk,
        brauner@kernel.org, shuah@kernel.org, aarcange@redhat.com,
        hughd@google.com, mhocko@suse.com, axelrasmussen@google.com,
        rppt@kernel.org, willy@infradead.org, Liam.Howlett@oracle.com,
        jannh@google.com, zhangpeng362@huawei.com, bgeffon@google.com,
        kaleshsingh@google.com, ngeoffray@google.com, jdduke@google.com,
        linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
        kernel-team@android.com
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On Fri, Oct 13, 2023 at 9:08=E2=80=AFAM Peter Xu <peterx@redhat.com> wrote:
>
> On Fri, Oct 13, 2023 at 11:56:31AM +0200, David Hildenbrand wrote:
> > Hi Peter,
>
> Hi, David,
>
> >
> > > I used to have the same thought with David on whether we can simplify=
 the
> > > design to e.g. limit it to single mm.  Then I found that the trickies=
t is
> > > actually patch 1 together with the anon_vma manipulations, and the pr=
oblem
> > > is that's not avoidable even if we restrict the api to apply on singl=
e mm.
> > >
> > > What else we can benefit from single mm?  One less mmap read lock, bu=
t
> > > probably that's all we can get; IIUC we need to keep most of the rest=
 of
> > > the code, e.g. pgtable walks, double pgtable lockings, etc.
> >
> > No existing mechanisms move anon pages between unrelated processes, tha=
t
> > naturally makes me nervous if we're doing it "just because we can".
>
> IMHO that's also the potential, when guarded with userfaultfd descriptor
> being shared between two processes.
>
> See below with more comment on the raised concerns.
>
> >
> > >
> > > Actually, even though I have no solid clue, but I had a feeling that =
there
> > > can be some interesting way to leverage this across-mm movement, whil=
e
> > > keeping things all safe (by e.g. elaborately requiring other proc to =
create
> > > uffd and deliver to this proc).
> >
> > Okay, but no real use cases yet.
>
> I can provide a "not solid" example.  I didn't mention it because it's
> really something that just popped into my mind when thinking cross-mm, so=
 I
> never discussed with anyone yet nor shared it anywhere.
>
> Consider VM live upgrade in a generic form (e.g., no VFIO), we can do tha=
t
> very efficiently with shmem or hugetlbfs, but not yet anonymous.  We can =
do
> extremely efficient postcopy live upgrade now with anonymous if with REMA=
P.
>
> Basically I see it a potential way of moving memory efficiently especiall=
y
> with thp.
>
> >
> > >
> > > Considering Andrea's original version already contains those bits and=
 all
> > > above, I'd vote that we go ahead with supporting two MMs.
> >
> > You can do nasty things with that, as it stands, on the upstream codeba=
se.
> >
> > If you pin the page in src_mm and move it to dst_mm, you successfully b=
roke
> > an invariant that "exclusive" means "no other references from other
> > processes". That page is marked exclusive but it is, in fact, not exclu=
sive.
>
> It is still exclusive to the dst mm?  I see your point, but I think you'r=
e
> taking exclusiveness altogether with pinning, and IMHO that may not be
> always necessary?
>
> >
> > Once you achieved that, you can easily have src_mm not have MMF_HAS_PIN=
NED,
>
> (I suppose you meant dst_mm here)
>
> > so you can just COW-share that page. Now you successfully broke the
> > invariant that COW-shared pages must not be pinned. And you can even tr=
igger
> > VM_BUG_ONs, like in sanity_check_pinned_pages().
>
> Yeah, that's really unfortunate.  But frankly, I don't think it's the fau=
lt
> of this new feature, but the rest.
>
> Let's imagine if the MMF_HAS_PINNED wasn't proposed as a per-mm flag, but
> per-vma, which I don't see why we can't because it's simply a hint so far=
.
> Then if we apply the same rule here, UFFDIO_REMAP won't even work for
> single-mm as long as cross-vma. Then UFFDIO_REMAP as a whole feature will
> be NACKed simply because of this..
>
> And I don't think anyone can guarantee a per-vma MMF_HAS_PINNED can never
> happen, or any further change to pinning solution that may affect this.  =
So
> far it just looks unsafe to remap a pin page to me.
>
> I don't have a good suggestion here if this is a risk.. I'd think it risk=
y
> then to do REMAP over pinned pages no matter cross-mm or single-mm.  It
> means probably we just rule them out: folio_maybe_dma_pinned() may not ev=
en
> be enough to be safe with fast-gup.  We may need page_needs_cow_for_dma()
> with proper write_protect_seq no matter cross-mm or single-mm?
>
> >
> > Can it all be fixed? Sure, with more complexity. For something without =
clear
> > motivation, I'll have to pass.
>
> I think what you raised is a valid concern, but IMHO it's better fixed no
> matter cross-mm or single-mm.  What do you think?
>
> In general, pinning lose its whole point here to me for an userspace eith=
er
> if it DONTNEEDs it or REMAP it.  What would be great to do here is we unp=
in
> it upon DONTNEED/REMAP/whatever drops the page, because it loses its
> coherency anyway, IMHO.
>
> >
> > Once there is real demand, we can revisit it and explore what else we w=
ould
> > have to take care of (I don't know how memcg behaves when moving betwee=
n
> > completely unrelated processes, maybe that works as expected, I don't k=
now
> > and I have no time to spare on reviewing features with no real use case=
s)
> > and announce it as a new feature.
>
> Good point.  memcg is probably needed..
>
> So you reminded me to do a more thorough review against zap/fault paths, =
I
> think what's missing are (besides page pinning):
>
>   - mem_cgroup_charge()/mem_cgroup_uncharge():

Good point. Will add in the next version.

>
>     (side note: I think folio_throttle_swaprate() is only for when
>      allocating new pages, so not needed here)
>
>   - check_stable_address_space() (under pgtable lock)

Ack.

>
>   - tlb flush
>
>     Hmm???????????????? I can't see anywhere we did tlb flush, batched or
>     not, either single-mm or cross-mm should need it.  Is this missing?

As Lokesh pointed out we do that but we don't batch them. I'll try to
add batching in the next version.

>
> >
> >
> > Note: that (with only reading the documentation) it also kept me wonder=
ing
> > how the MMs are even implied from
> >
> >        struct uffdio_move {
> >            __u64 dst;    /* Destination of move */
> >            __u64 src;    /* Source of move */
> >            __u64 len;    /* Number of bytes to move */
> >            __u64 mode;   /* Flags controlling behavior of move */
> >            __s64 move;   /* Number of bytes moved, or negated error */
> >        };
> >
> > That probably has to be documented as well, in which address space dst =
and
> > src reside.
>
> Agreed, some better documentation will never hurt.  Dst should be in the =
mm
> address space that was bound to the userfault descriptor.  Src should be =
in
> the current mm address space.

Ack. Will add. Thanks!

>
> Thanks,
>
> --
> Peter Xu
>
> --
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