Re: [PATCH v3 3/8] vringh: replace kmap_atomic() with kmap_local_page()

Fabio M. De Francesco
2023-03-22T10:37:39Z

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@gmail.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@gmail.com>
> Signed-off-by: Stefano Garzarella <sgarzare@redhat.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