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

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Date:   Sat, 31 Dec 2022 15:55:17 -0500 (EST)
From:   Nicolas Pitre <nico@...xnic.net>
To:     Ira Weiny <ira.weiny@...el.com>
cc:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        Christian Brauner <brauner@...nel.org>,
        linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
        "Venkataramanan, Anirudh" <anirudh.venkataramanan@...el.com>
Subject: Re: [PATCH] fs/cramfs: Convert kmap() to kmap_local_data()

On Fri, 30 Dec 2022, Ira Weiny wrote:

> On Wed, Oct 12, 2022 at 11:13:56AM +0200, 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().
> > 
> > Cc: "Venkataramanan, Anirudh" <anirudh.venkataramanan@...el.com>
> > Suggested-by: Ira Weiny <ira.weiny@...el.com>
> 
> Nicolas, I was auditing the kmap conversion status and I see you reviewed this
> but did not merge it.  Are you still the maintainer for fs/cramfs?  Should
> Fabio send this to someone else?

Please send it to Al Viro with my reviewed-by.


Nicolas