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

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Date:   Tue, 19 Apr 2022 13:44:14 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Cc:     Ira Weiny <ira.weiny@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Peter Collingbourne <pcc@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        linux-kernel@...r.kernel.org, outreachy@...ts.linux.dev
Subject: Re: [PATCH v2] mm/highmem: Fix kernel-doc warnings in highmem*.h

On Mon, Apr 18, 2022 at 07:56:38PM +0200, Fabio M. De Francesco wrote:
> +/**
> + * kunmap_atomic - Unmap the virtual address mapped by kmap_atomic()
> + * @__addr:       Virtual address to be unmapped
> + *
> + * Counterpart to kmap_atomic().

I don't think this is a terribly useful paragraph?

> + * Effectively a wrapper around kunmap_local() which additionally undoes
> + * the side effects of kmap_atomic(), i.e. reenabling pagefaults and
> + * preemption. Prevent people trying to call kunmap_atomic() as if it
> + * were kunmap() because kunmap_atomic() should get the return value of
> + * kmap_atomic(), not its argument which is a pointer to struct page.

I'd rather this were useful advice to the caller than documentation of
how it works.  How about:

 * Unmap an address previously mapped by kmap_atomic().  Mappings
 * should be unmapped in the reverse order that they were mapped.
 * See kmap_local_page() for details.  @__addr can be any address within
 * the mapped page, so there is no need to subtract any offset that has
 * been added.  In contrast to kunmap(), this function takes the address
 * returned from kmap_atomic(), not the page passed to kmap_atomic().
 * The compiler will warn you if you pass the page.