linux-kernel - Re: [PATCH v3 1/4] mm/highmem: Fix kernel-doc warnings in highmem*.h

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Date:   Thu, 28 Apr 2022 13:05:21 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
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>,
        "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,
        linux-doc@...r.kernel.org, outreachy@...ts.linux.dev,
        Mike Rapoport <rppt@...ux.ibm.com>
Subject: Re: [PATCH v3 1/4] mm/highmem: Fix kernel-doc warnings in highmem*.h

On 2022-04-28 12:54:14 [+0200], Fabio M. De Francesco wrote:
> No, it's not sufficient because Matthew Wilcox said that something like "It 
> is the counterpart of kmap_atomic() for unmapping" (or anything similar) is 
> _not_ what he wants to see. 
> 
> Furthermore, a large part of this text has been written by him (I'm talking 
> of a couple of weeks ago, when this patch was not part of this series - it 
> was on its own until Ira Weiny asked me to gather 4 patches in one only 
> series).

Sure.

> > This indicates the "migration" is disabled for
> > !PREEMPT_RT which is not the case.
> 
> I read again how kmap_atomic() is defined. There are lots of 'if' 
> statements. Only if the code gets to __kmap_local_pfn_prot(), users can be 
> assured that it unconditionally calls both migrate_disable() and 
> preempt_disable(). 

Right, that part. Then keep it.

> > So maybe something like
> > 
> >  * Unmaps an address previously mapped by kmap_atomic() and re-enables
> >  * pagefaults, CPU migration (CONFIG_PREEMPT_RT) or preemption
> >  * (!CONFIG_PREEMPT_RT). Mappings should be unmapped in the reverse
> > 
> > will make it clear.
> 
> I'm starting to think that this level of detail is too much for users who 
> just need to understand how to use this function as well as  
> kmap_local_page().
> 
> I prefer something like the following:
> 
> + * Unmaps an address previously mapped by kmap_atomic() and re-enables
> + * pagefaults and possibly also CPU migration and/or preemption. However, 
> + * users should not count on disable of migration and/or preemption as a 
> + * side effect of calling kmap_atomic(). Mappings must be unmapped in the 
> + * reverse [...]
> 
> I'd also like to write the same paragraph for kmap_local_page().
> 
> What do you think of being less detailed and instead using the text I wrote 
> above? 

Sounds perfect.

> Thanks,
> 
> Fabio

Sebastian