Re: [PATCH] mm/highmem: Add notes about conversions from kmap{,_atomic}()

Sebastian Andrzej Siewior
2022-12-06T08:00:22Z

On 2022-12-06 08:00:29 [+0100], Fabio M. De Francesco wrote:
> diff --git a/Documentation/mm/highmem.rst b/Documentation/mm/highmem.rst
> index 0f731d9196b0..9523e92299f6 100644
> --- a/Documentation/mm/highmem.rst
> +++ b/Documentation/mm/highmem.rst
> @@ -100,10 +101,21 @@ list shows them in order of preference of use.
> (included in the "Functions" section) for details on how to manage nested
> mappings.
>
> -* kmap_atomic(). This permits a very short duration mapping of a single
> - page. Since the mapping is restricted to the CPU that issued it, it
> - performs well, but the issuing task is therefore required to stay on that
> - CPU until it has finished, lest some other task displace its mappings.
> +* kmap_atomic(). This function has been deprecated; use kmap_local_page().
> +
> + NOTE: Conversions to kmap_local_page() must take care to follow the mapping
> + restrictions imposed on kmap_local_page(). Furthermore, code between the
> + map/unmap operations may implicitly depended on the side effects of
> + kmap_atomic(), such as disabling pagefaults, migration, and/or preemption.
> + Such conversions should be changed to make explicit calls for those
> + requirements.

Furthermore, code between the kmap_atomic() and kunmap_atomic()
functions may implicitly depended on the side effects of kmap_atomic()
namely disabling pagefaults or preemption or both.

> + [Legacy documentation]
> +
> + This permits a very short duration mapping of a single page. Since the
> + mapping is restricted to the CPU that issued it, it performs well, but
> + the issuing task is therefore required to stay on that CPU until it has
> + finished, lest some other task displace its mappings.
>
> kmap_atomic() may also be used by interrupt contexts, since it does not
> sleep and the callers too may not sleep until after kunmap_atomic() is

Sebastian