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References: <20221012112323.23283-1-fmdefrancesco@gmail.com>
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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Wed, 12 Oct 2022 15:55:44 +0200
Message-ID: <CAJfpegut1su1PR1gBBPS3Gs8NJD=V7_pH2FGOJ6V1deWa1Z=Ew@mail.gmail.com>
Subject: Re: [PATCH] fs/fuse: Replace kmap() with kmap_local_page()
To:     "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
Cc:     Vivek Goyal <vgoyal@redhat.com>,
        Stefan Hajnoczi <stefanha@redhat.com>,
        Christian Brauner <brauner@kernel.org>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org, bpf@vger.kernel.org,
        "Venkataramanan, Anirudh" <anirudh.venkataramanan@intel.com>,
        Ira Weiny <ira.weiny@intel.com>
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On Wed, 12 Oct 2022 at 13:38, Fabio M. De Francesco
<fmdefrancesco@gmail.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=E2=80=99s pool wraps and it might block when the mapping space is fu=
lly
> 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@intel.com>
> Suggested-by: Ira Weiny <ira.weiny@intel.com>
> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>

Applied, thanks.

Miklos