linux-kernel - Re: [PATCH v2 1/4] mm/highmem: Fix kernel-doc warnings in highmem*.h

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Date:   Wed, 25 May 2022 11:34:16 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Will Deacon <will@...nel.org>,
        Peter Collingbourne <pcc@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
        outreachy@...ts.linux.dev,
        "Acked-by : Mike Rapoport" <rppt@...ux.ibm.com>
Subject: Re: [PATCH v2 1/4] mm/highmem: Fix kernel-doc warnings in highmem*.h

On 2022-04-29 08:59:26 [-0700], Ira Weiny wrote:
> I think some discussion needs to happen around this API.
> 
> Highmem has little use.  I don't think anyone disagrees with Linus there.
> (Although I think there are still a few users out there.)

arm32 is still built and they have sometimes 1 - 2 GiB of memory.

> kmap may be a poor name for an API without the highmem functionality.  But
> perhaps not.  One could interpret it to mean simply getting the kernel mapping
> of the page rather than creating one.  After all that is what 64bit has done
> all along.
> 
> This interpretation helps when you consider features which attempt to layer the
> direct map with additional protections like PKS.[1]  Those protections mean
> that a simple page_address() is insufficient to access the direct map.
> 
> As far as calling kmap() and kmap_atomic() deprecated I'm ok with that if the
> community is.
> 
> The current kmap() call sites need work and Fabio's work on auditing them is
> extremely helpful.  That said, if we officially deprecate kmap_atomic() then
> those sites could be added to the list for rework.

Maybe I oversee something obvious but there is no problem with removing
kmap_atomic*() and keeping only kmap_local*() around, is there?
I never intended to deprecated kmap(), only kmap_atomic*() in favour of
kmap_local*().

> Ira

Sebastian