linux-kernel - Re: [RESEND PATCH] fs/cramfs: Convert kmap() to kmap_local

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Message-ID: <Y74eH0+++0HqHXV7@ZenIV>
Date:   Wed, 11 Jan 2023 02:25:35 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        "Venkataramanan, Anirudh" <anirudh.venkataramanan@...el.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Nicolas Pitre <nico@...xnic.net>,
        Christian Brauner <brauner@...nel.org>
Subject: Re: [RESEND PATCH] fs/cramfs: Convert kmap() to kmap_local_data()

On Mon, Jan 09, 2023 at 07:28:43PM +0100, Fabio M. De Francesco wrote:
> The use of kmap() is being deprecated in favor of kmap_local_page().
> 
> There are two main problems with kmap(): (1) It comes with an overhead as
> the mapping space is restricted and protected by a global lock for
> synchronization and (2) it also requires global TLB invalidation when the
> kmap’s pool wraps and it might block when the mapping space is fully
> utilized until a slot becomes available.
> 
> With kmap_local_page() the mappings are per thread, CPU local, can take
> page faults, and can be called from any context (including interrupts).
> It is faster than kmap() in kernels with HIGHMEM enabled. Furthermore,
> the tasks can be preempted and, when they are scheduled to run again, the
> kernel virtual addresses are restored and still valid.
> 
> Since its use in fs/cramfs is safe everywhere, it should be preferred.
> 
> Therefore, replace kmap() with kmap_local_page() in fs/cramfs. Instead
> of open-coding kmap_local_page() + memcpy(), use memcpy_from_page().

Applied (#work.misc).