linux-kernel - Re: [PATCH v2 4/4] Documentation/vm: Rework "Temporary Virtual Mappings" section

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Date:   Tue, 26 Apr 2022 20:31:12 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
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,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
        outreachy@...ts.linux.dev, Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2 4/4] Documentation/vm: Rework "Temporary Virtual Mappings" section

On martedì 26 aprile 2022 13:47:34 CEST Sebastian Andrzej Siewior wrote:
> 
> Hmm. It is thread-local in the end. There are slots 0 … KM_MAX_IDX for
> the mappings. Slot 0 for task A can be different from slot 0 for task B
> while both run on CPU0. So the same address, that is returned from
> kmap_local(), will point to a different page for both tasks. Both tasks
> can't be migrated to another CPU while the mapping is active.
> "CPU local" sounds like something that is same to everyone on the same
> CPU which is what this_cpu_read() for instance does.
> 

OK, I agree with you :) 

I just got three notices from Greg K-H stating that he has applied three of 
my driver / Android patches. The patches are some conversions from kmap() 
and / or kmap_atomic() to kmap_local_page() (or wrappers around it):

https://lore.kernel.org/lkml/20220425175754.8180-4-fmdefrancesco@gmail.com/
https://lore.kernel.org/lkml/20220425175754.8180-3-fmdefrancesco@gmail.com/
https://lore.kernel.org/lkml/20220425175754.8180-2-fmdefrancesco@gmail.com/

I had forgotten that I wrote the following sentence in all three commit 
messages: "[] With kmap_local_page(), the mapping is per thread, CPU local 
and not globally visible. []"

Therefore, I'll add "thread-local" or "per thread". I probably like your 
wording more than mine: "thread-local" is more suitable.

For consistency (again) I like the other change you proposed, which is to 
add "deprecated!" also in kunmap_atomic(), exactly as it is already in 
kmap_atomic() kernel-docs.

However, I will wait one more day before sending v3, in case there are 
other people who want to suggest further changes.

If I remember correctly, I'm overlooking nothing else. Do I overlook 
something?

Thanks for your help,

Fabio