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From:   "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
To:     virtualization@lists.linux-foundation.org,
        Stefano Garzarella <sgarzare@redhat.com>
Cc:     stefanha@redhat.com, "Michael S. Tsirkin" <mst@redhat.com>,
        Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>,
        eperezma@redhat.com, netdev@vger.kernel.org,
        Jason Wang <jasowang@redhat.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Stefano Garzarella <sgarzare@redhat.com>, ira.weiny@intel.com
Subject: Re: [PATCH v3 3/8] vringh: replace kmap_atomic() with kmap_local_page()
Date:   Wed, 22 Mar 2023 11:37:27 +0100
Message-ID: <4499457.LvFx2qVVIh@suse>
In-Reply-To: <20230321154228.182769-4-sgarzare@redhat.com>
References: <20230321154228.182769-1-sgarzare@redhat.com>
 <20230321154228.182769-4-sgarzare@redhat.com>
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On marted=EC 21 marzo 2023 16:42:23 CET Stefano Garzarella wrote:
> kmap_atomic() is deprecated in favor of kmap_local_page() since commit
> f3ba3c710ac5 ("mm/highmem: Provide kmap_local*").
>=20
> 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.
>=20
> 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).
>=20
> 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(), 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()).
>=20
> This commit reuses a "boiler plate" commit message from Fabio, who has
> already did this change in several places.
>=20

=46WIW, I can confirm that the conversions here are safe...

Reviewed-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>

Thanks,

=46abio

P.S.: I had to send this message again because my former contained HTML par=
ts=20
and so it was rejected by the mailing lists. I don't yet know how HTML crep=
t=20
into my text.

> Cc: "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> ---
>=20
> Notes:
>     v3:
>     - credited Fabio for the commit message
>     - added reference to the commit that deprecated kmap_atomic() [Jason]
>     v2:
>     - added this patch since checkpatch.pl complained about deprecation
>       of kmap_atomic() touched by next patch
>=20
>  drivers/vhost/vringh.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>=20
> 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;
>=20
> -	kaddr =3D kmap_atomic(iov.bv_page);
> +	kaddr =3D kmap_local_page(iov.bv_page);
>  	from =3D kaddr + iov.bv_offset;
>  	*val =3D vringh16_to_cpu(vrh, READ_ONCE(*(__virtio16 *)from));
> -	kunmap_atomic(kaddr);
> +	kunmap_local(kaddr);
>=20
>  	return 0;
>  }
> @@ -1241,10 +1241,10 @@ static inline int putu16_iotlb(const struct vringh
> *vrh, if (ret < 0)
>  		return ret;
>=20
> -	kaddr =3D kmap_atomic(iov.bv_page);
> +	kaddr =3D kmap_local_page(iov.bv_page);
>  	to =3D kaddr + iov.bv_offset;
>  	WRITE_ONCE(*(__virtio16 *)to, cpu_to_vringh16(vrh, val));
> -	kunmap_atomic(kaddr);
> +	kunmap_local(kaddr);
>=20
>  	return 0;
>  }
> --
> 2.39.2