Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
From:   "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Evgeniy Dushistov <dushistov@mail.ru>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Ira Weiny <ira.weiny@intel.com>, linux-kernel@vger.kernel.org,
        bpf@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v5 0/4] fs/ufs: Replace kmap() with kmap_local_page
Date:   Mon, 03 Apr 2023 07:27:40 +0200
Message-ID: <10129918.0AQdONaE2F@suse>
In-Reply-To: <ZCGY3c5avRefahms@casper.infradead.org>
References: <20221229225100.22141-1-fmdefrancesco@gmail.com> <11383508.F0gNSz5aLb@suse>
 <ZCGY3c5avRefahms@casper.infradead.org>
MIME-Version: 1.0
Content-Transfer-Encoding: quoted-printable
Content-Type: text/plain; charset="utf-8"
Precedence: bulk

On luned=C3=AC 27 marzo 2023 15:23:41 CEST Matthew Wilcox wrote:
> On Mon, Mar 27, 2023 at 12:13:08PM +0200, Fabio M. De Francesco wrote:
> > On gioved=C3=AC 29 dicembre 2022 23:50:56 CEST Fabio M. De Francesco wr=
ote:
> > > kmap() is being deprecated in favor of kmap_local_page().
> > >=20
> > > There are two main problems with kmap(): (1) It comes with an overhea=
d=20
as
> > > the mapping space is restricted and protected by a global lock for
> > > synchronization and (2) it also requires global TLB invalidation when=
=20
the
> > > kmap=E2=80=99s pool wraps and it might block when the mapping space i=
s fully
> > > utilized until a slot becomes available.
> > >=20
> > > With kmap_local_page() the mappings are per thread, CPU local, can ta=
ke
> > > 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,=
=20
the
> > > kernel virtual addresses are restored and still valid.
> > >=20
> > > Since its use in fs/ufs is safe everywhere, it should be preferred.
> > >=20
> > > Therefore, replace kmap() with kmap_local_page() in fs/ufs.=20
kunmap_local()
> > > requires the mapping address, so return that address from ufs_get_pag=
e()
> > > to be used in ufs_put_page().
> >=20
> > Hi Al,
> >=20
> > I see that this series is here since Dec 29, 2022.
> > Is there anything that prevents its merging?
> > Can you please its four patches in your tree?
>=20
> I'm pretty sure UFS directories should simply be allocated from lowmem.
> There's really no reason to put them in highmem these days.

Matthew,

It's been a few days since you wrote but I still haven't done anything or=20
replied. For now, I just want you to know that it's not my intention to ign=
ore=20
your comment.

The only reasons I hesitate to follow your directions are that I built this=
=20
series based on specific suggestions from Al.=20

While I understand and, for sure, heed what you said, I also think I should=
=20
wait for Al to chime in with what he thinks about all this before taking a=
=20
different route for removing kmap() from fs/ufs.

Thanks for looking at this series.

=46abio=20