linux-kernel - Re: [PATCH] mm/highmem: Add notes about conversions from kmap{,

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Message-ID: <Y472ipY908pHip+B@linutronix.de>
Date:   Tue, 6 Dec 2022 09:00:10 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Cc:     Jonathan Corbet <corbet@....net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Mike Rapoport <rppt@...nel.org>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] mm/highmem: Add notes about conversions from
 kmap{,_atomic}()

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