Re: [PATCH v1 3/3] migration: multifd: Enable zerocopy

[Jason Wang]

On Wed, Sep 8, 2021 at 2:32 AM Peter Xu <peterx@redhat.com> wrote:
>
> On Thu, Sep 02, 2021 at 04:22:55AM -0300, Leonardo Bras Soares Passos wrote:
> > > I don't think it is valid to unconditionally enable this feature due to 
> > > the
> > > resource usage implications
> > >
> > > https://www.kernel.org/doc/html/v5.4/networking/msg_zerocopy.html
> > >
> > >   "A zerocopy failure will return -1 with errno ENOBUFS. This happens
> > >    if the socket option was not set, the socket exceeds its optmem
> > >    limit or the user exceeds its ulimit on locked pages."
> >
> > You are correct, I unfortunately missed this part in the docs :(
> >
> > > The limit on locked pages is something that looks very likely to be
> > > exceeded unless you happen to be running a QEMU config that already
> > > implies locked memory (eg PCI assignment)
> >
> > Do you mean the limit an user has on locking memory?
> >
> > If so, that makes sense. I remember I needed to set the upper limit of 
> > locked
> > memory for the user before using it, or adding a capability to qemu before.
>
> So I'm a bit confused on why MSG_ZEROCOPY requires checking RLIMIT_MEMLOCK.
>
> The thing is IIUC that's accounting for pinned pages only with either mlock()
> (FOLL_MLOCK) or vfio (FOLL_PIN).
>
> I don't really think MSG_ZEROCOPY is doing that at all...  I'm looking at
> __zerocopy_sg_from_iter() -> iov_iter_get_pages().

It happens probably here:

E.g

__ip_append_data()
    msg_zerocopy_realloc()
        mm_account_pinned_pages()

Thanks

>
> Say, I think there're plenty of iov_iter_get_pages() callers and normal GUPs, 
> I
> think most of them do no need such accountings.
>
> --
> Peter Xu
>