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

[Peter Xu <peterx@redhat.com>]

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().

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