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Re: [PATCH v1 3/3] migration: multifd: Enable zerocopy
From: |
Peter Xu |
Subject: |
Re: [PATCH v1 3/3] migration: multifd: Enable zerocopy |
Date: |
Tue, 7 Sep 2021 14:32:23 -0400 |
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
- Re: [PATCH v1 3/3] migration: multifd: Enable zerocopy, (continued)
- Re: [PATCH v1 3/3] migration: multifd: Enable zerocopy, Leonardo Bras Soares Passos, 2021/09/02
- Re: [PATCH v1 3/3] migration: multifd: Enable zerocopy, Daniel P . Berrangé, 2021/09/02
- Re: [PATCH v1 3/3] migration: multifd: Enable zerocopy, Leonardo Bras Soares Passos, 2021/09/02
- Re: [PATCH v1 3/3] migration: multifd: Enable zerocopy, Daniel P . Berrangé, 2021/09/02
- Re: [PATCH v1 3/3] migration: multifd: Enable zerocopy, Leonardo Bras Soares Passos, 2021/09/02
- Re: [PATCH v1 3/3] migration: multifd: Enable zerocopy, Daniel P . Berrangé, 2021/09/02
- Re: [PATCH v1 3/3] migration: multifd: Enable zerocopy, Leonardo Bras Soares Passos, 2021/09/02
- Re: [PATCH v1 3/3] migration: multifd: Enable zerocopy, Dr. David Alan Gilbert, 2021/09/07
Re: [PATCH v1 3/3] migration: multifd: Enable zerocopy,
Peter Xu <=
Re: [PATCH v1 3/3] migration: multifd: Enable zerocopy, Dr. David Alan Gilbert, 2021/09/08
Re: [PATCH v1 3/3] migration: multifd: Enable zerocopy, Peter Xu, 2021/09/08
Re: [PATCH v1 3/3] migration: multifd: Enable zerocopy, Jason Wang, 2021/09/08
Re: [PATCH v1 3/3] migration: multifd: Enable zerocopy, Leonardo Bras Soares Passos, 2021/09/02