linux-kernel - Re: [PATCH v2 3/8] vringh: replace kmap_atomic() with kmap_local

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Message-ID: <CAGxU2F4k-UHxHxpLcsvKvJdvcXfb3WpV+wU=8ZpnJwMNkx0rdA@mail.gmail.com>
Date:   Thu, 16 Mar 2023 09:09:29 +0100
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Cc:     Jason Wang <jasowang@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        Andrey Zhadchenko <andrey.zhadchenko@...tuozzo.com>,
        eperezma@...hat.com, netdev@...r.kernel.org, stefanha@...hat.com,
        linux-kernel@...r.kernel.org,
        "Michael S. Tsirkin" <mst@...hat.com>, kvm@...r.kernel.org
Subject: Re: [PATCH v2 3/8] vringh: replace kmap_atomic() with kmap_local_page()

On Wed, Mar 15, 2023 at 10:12 PM Fabio M. De Francesco
<fmdefrancesco@...il.com> wrote:
>
> On martedì 14 marzo 2023 04:56:08 CET Jason Wang wrote:
> > On Thu, Mar 2, 2023 at 7:34 PM Stefano Garzarella <sgarzare@...hat.com>
> wrote:
> > > kmap_atomic() is deprecated in favor of kmap_local_page().
> >
> > It's better to mention the commit or code that introduces this.
> >
> > > With kmap_local_page() the mappings are per thread, CPU local, can take
> > > page-faults, and can be called from any context (including interrupts).
> > > Furthermore, the tasks can be preempted and, when they are scheduled to
> > > run again, the kernel virtual addresses are restored and still valid.
> > >
> > > kmap_atomic() is implemented like a kmap_local_page() which also disables
> > > page-faults and preemption (the latter only for !PREEMPT_RT kernels,
> > > otherwise it only disables migration).
> > >
> > > The code within the mappings/un-mappings in getu16_iotlb() and
> > > putu16_iotlb() don't depend on the above-mentioned side effects of
> > > kmap_atomic(),
> >
> > Note we used to use spinlock to protect simulators (at least until
> > patch 7, so we probably need to re-order the patches at least) so I
> > think this is only valid when:
> >
> > The vringh IOTLB helpers are not used in atomic context (e.g spinlock,
> > interrupts).
>
> I'm probably missing some context but it looks that you are saying that
> kmap_local_page() is not suited for any use in atomic context (you are
> mentioning spinlocks).
>
> The commit message (that I know pretty well since it's the exact copy, word by
> word, of my boiler plate commits)

I hope it's not a problem for you, should I mention it somehow?

I searched for the last commits that made a similar change and found
yours that explained it perfectly ;-)

Do I need to rephrase?

> explains that kmap_local_page() is perfectly
> usable in atomic context (including interrupts).
>
> I don't know this code, however I am not able to see why these vringh IOTLB
> helpers cannot work if used under spinlocks. Can you please elaborate a little
> more?
>
> > If yes, should we document this? (Or should we introduce a boolean to
> > say whether an IOTLB variant can be used in an atomic context)?
>
> Again, you'll have no problems from the use of kmap_local_page() and so you
> don't need any boolean to tell whether or not the code is running in atomic
> context.
>
> Please take a look at the Highmem documentation which has been recently
> reworked and extended by me: https://docs.kernel.org/mm/highmem.html
>
> Anyway, I have been ATK 12 or 13 hours in a row. So I'm probably missing the
> whole picture.

Thanks for your useful info!
Stefano