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From:   "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
To:     dhowells@redhat.com, marc.dionne@auristor.com,
        zhaoxiao <zhaoxiao@uniontech.com>
Cc:     linux-afs@lists.infradead.org, linux-kernel@vger.kernel.org,
        zhaoxiao <zhaoxiao@uniontech.com>, ira.weiny@intel.com
Subject: Re: [PATCH] afs: convert kmap() to kmap_local_page() in mntpt.c
Date:   Mon, 15 Aug 2022 16:42:19 +0200
Message-ID: <1752846.TLkxdtWsSY@opensuse>
In-Reply-To: <20220812080432.14909-1-zhaoxiao@uniontech.com>
References: <20220812080432.14909-1-zhaoxiao@uniontech.com>
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On venerd=C3=AC 12 agosto 2022 10:04:32 CEST zhaoxiao wrote:
> kmap() is being deprecated in favor of kmap_local_page().
>=20
> There are two main problems with kmap(): (1) It comes with an overhead as
> 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.
>=20
> 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 are still valid.
>=20
> Since its use in mntpt.c is safe everywhere, it should be preferred.
>=20
> Therefore, replace kmap() with kmap_local_page() in mntpt.c.
>=20
> Tested in a QEMU/KVM x86_32 VM, 6GB RAM, booting a kernel with
> HIGHMEM64GB enabled.
>=20
> Signed-off-by: zhaoxiao <zhaoxiao@uniontech.com>
> ---
>  fs/afs/mntpt.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>=20
> diff --git a/fs/afs/mntpt.c b/fs/afs/mntpt.c
> index 97f50e9fd9eb..7e3cacb11780 100644
> --- a/fs/afs/mntpt.c
> +++ b/fs/afs/mntpt.c
> @@ -132,11 +132,11 @@ static int afs_mntpt_set_params(struct fs_context *=
fc,
> struct dentry *mntpt) if (IS_ERR(page))
>  			return PTR_ERR(page);
>=20
> -		buf =3D kmap(page);
> +		buf =3D kmap_local_page(page);
>  		ret =3D -EINVAL;
>  		if (buf[size - 1] =3D=3D '.')
>  			ret =3D vfs_parse_fs_string(fc, "source", buf,=20
size - 1);
> -		kunmap(page);
> +		kunmap_local(buf);
>  		put_page(page);
>  		if (ret < 0)
>  			return ret;
> --
> 2.20.1

I doubt that zhaoxiao is your legal name.
If I'm not wrong, can you please submit a v2 with the name that you use to=
=20
sign documents with legal value? Otherwise, please discard this warning.

Aside from the above, this conversion looks good to me, so feel free to=20
forward my tag to next version...

Reviewed-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>

Thanks,

=46abio

P.S.: I'm adding Ira to the list of recipients. Ira and I have been
working through converting kmap() calls to kmap_local_page(). Please let Ir=
a=20
or me know if you plan on tacking more conversions so we can mark it
off the list and avoid duplicate works.