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

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Date:   Wed, 15 Mar 2023 22:12:04 +0100
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Stefano Garzarella <sgarzare@...hat.com>,
        Jason Wang <jasowang@...hat.com>
Cc:     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 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) 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, 

Fabio

> Thanks
> 
> > so that mere replacements of the old API with the new one
> > is all that is required (i.e., there is no need to explicitly add calls
> > to pagefault_disable() and/or preempt_disable()).
> > 
> > Signed-off-by: Stefano Garzarella <sgarzare@...hat.com>
> > ---
> > 
> > Notes:
> >     v2:
> >     - added this patch since checkpatch.pl complained about deprecation
> >     
> >       of kmap_atomic() touched by next patch
> >  
> >  drivers/vhost/vringh.c | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c
> > index a1e27da54481..0ba3ef809e48 100644
> > --- a/drivers/vhost/vringh.c
> > +++ b/drivers/vhost/vringh.c
> > @@ -1220,10 +1220,10 @@ static inline int getu16_iotlb(const struct vringh
> > *vrh,> 
> >         if (ret < 0)
> >         
> >                 return ret;
> > 
> > -       kaddr = kmap_atomic(iov.bv_page);
> > +       kaddr = kmap_local_page(iov.bv_page);
> > 
> >         from = kaddr + iov.bv_offset;
> >         *val = vringh16_to_cpu(vrh, READ_ONCE(*(__virtio16 *)from));
> > 
> > -       kunmap_atomic(kaddr);
> > +       kunmap_local(kaddr);
> > 
> >         return 0;
> >  
> >  }
> > 
> > @@ -1241,10 +1241,10 @@ static inline int putu16_iotlb(const struct vringh
> > *vrh,> 
> >         if (ret < 0)
> >         
> >                 return ret;
> > 
> > -       kaddr = kmap_atomic(iov.bv_page);
> > +       kaddr = kmap_local_page(iov.bv_page);
> > 
> >         to = kaddr + iov.bv_offset;
> >         WRITE_ONCE(*(__virtio16 *)to, cpu_to_vringh16(vrh, val));
> > 
> > -       kunmap_atomic(kaddr);
> > +       kunmap_local(kaddr);
> > 
> >         return 0;
> >  
> >  }
> > 
> > --
> > 2.39.2