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

From: "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Ira Weiny <ira.weiny@intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	"Matthew Wilcox (Oracle)" <willy@infradead.org>,
	Will Deacon <will@kernel.org>,
	Peter Collingbourne <pcc@google.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	linux-kernel@vger.kernel.org, Jonathan Corbet <corbet@lwn.net>,
	linux-doc@vger.kernel.org, outreachy@lists.linux.dev,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v2 4/4] Documentation/vm: Rework "Temporary Virtual Mappings" section
Date: Tue, 26 Apr 2022 20:31:12 +0200	[thread overview]
Message-ID: <3429395.iIbC2pHGDl@leap> (raw)
In-Reply-To: <Ymfb1saaHVuq4IUl@linutronix.de>

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