linux-kernel - Re: [PATCH v3 3/8] vringh: replace kmap_atomic() with kmap_local

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Message-ID: <4499457.LvFx2qVVIh@suse>
Date:   Wed, 22 Mar 2023 11:37:27 +0100
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     virtualization@...ts.linux-foundation.org,
        Stefano Garzarella <sgarzare@...hat.com>
Cc:     stefanha@...hat.com, "Michael S. Tsirkin" <mst@...hat.com>,
        Andrey Zhadchenko <andrey.zhadchenko@...tuozzo.com>,
        eperezma@...hat.com, netdev@...r.kernel.org,
        Jason Wang <jasowang@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Stefano Garzarella <sgarzare@...hat.com>, ira.weiny@...el.com
Subject: Re: [PATCH v3 3/8] vringh: replace kmap_atomic() with kmap_local_page()

On martedì 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*").
> 
> 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(), 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()).
> 
> This commit reuses a "boiler plate" commit message from Fabio, who has
> already did this change in several places.
> 

FWIW, I can confirm that the conversions here are safe...

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

Thanks,

Fabio

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

> Cc: "Fabio M. De Francesco" <fmdefrancesco@...il.com>
> Signed-off-by: Stefano Garzarella <sgarzare@...hat.com>
> ---
> 
> 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
> 
>  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