linux-kernel - Re: [PATCH] fs/fuse: Replace kmap() with kmap_local

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Message-ID: <CAJfpegut1su1PR1gBBPS3Gs8NJD=V7_pH2FGOJ6V1deWa1Z=Ew@mail.gmail.com>
Date:   Wed, 12 Oct 2022 15:55:44 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Cc:     Vivek Goyal <vgoyal@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        Christian Brauner <brauner@...nel.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, bpf@...r.kernel.org,
        "Venkataramanan, Anirudh" <anirudh.venkataramanan@...el.com>,
        Ira Weiny <ira.weiny@...el.com>
Subject: Re: [PATCH] fs/fuse: Replace kmap() with kmap_local_page()

On Wed, 12 Oct 2022 at 13:38, Fabio M. De Francesco
<fmdefrancesco@...il.com> 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.
>
> Therefore, replace kmap() with kmap_local_page() in fuse_readdir_cached(),
> it being the only call site of kmap() currently left in fs/fuse.
>
> Cc: "Venkataramanan, Anirudh" <anirudh.venkataramanan@...el.com>
> Suggested-by: Ira Weiny <ira.weiny@...el.com>
> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>

Applied, thanks.

Miklos