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:     dsterba@suse.cz
Cc:     "Fabio M. De Francesco" <fmdefrancesco@gmail.com>,
        Chris Mason <clm@fb.com>, Josef Bacik <josef@toxicpanda.com>,
        David Sterba <dsterba@suse.com>,
        Nick Terrell <terrelln@fb.com>, linux-btrfs@vger.kernel.org,
        linux-kernel@vger.kernel.org, Filipe Manana <fdmanana@kernel.org>,
        Ira Weiny <ira.weiny@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH] btrfs: Replace kmap() with kmap_local_page() in zstd.c
Date:   Tue, 14 Jun 2022 01:22:50 +0200
Message-ID: <1936552.usQuhbGJ8B@opensuse>
In-Reply-To: <20220613183913.GD20633@twin.jikos.cz>
References: <20220611135203.27992-1-fmdefrancesco@gmail.com> <20220613183913.GD20633@twin.jikos.cz>
MIME-Version: 1.0
Content-Transfer-Encoding: quoted-printable
Content-Type: text/plain; charset="utf-8"
Precedence: bulk

On luned=C3=AC 13 giugno 2022 20:39:13 CEST David Sterba wrote:
> On Sat, Jun 11, 2022 at 03:52:03PM +0200, Fabio M. De Francesco wrote:
> > The use of kmap() is being deprecated in favor of kmap_local_page().=20
With
> > kmap_local_page(), the mapping is per thread, CPU local and not=20
globally
> > visible.
> >=20
> > Therefore, use kmap_local_page() / kunmap_local() in zstd.c because in
> > this file the mappings are per thread and are not visible in other
> > contexts; meanwhile refactor zstd_compress_pages() to comply with=20
nested
> > local mapping / unmapping ordering rules.
> >=20
> > Tested with xfstests on a QEMU + KVM 32 bits VM with 4GB of RAM and
> > HIGHMEM64G enabled.
> >=20
> > Cc: Filipe Manana <fdmanana@kernel.org>
> > Suggested-by: Ira Weiny <ira.weiny@intel.com>
> > Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
> > ---
> >=20
> > @@ -477,15 +479,16 @@ int zstd_compress_pages(struct list_head *ws,=20
struct address_space *mapping,
> >  		/* Check if we need more input */
> >  		if (workspace->in_buf.pos =3D=3D workspace->in_buf.size) {
> >  			tot_in +=3D PAGE_SIZE;
> > -			kunmap(in_page);
> > +			kunmap_local(workspace->out_buf.dst);
> > +			kunmap_local((void *)workspace->in_buf.src);
>=20
> Why is the cast needed?

As I wrote in an email I sent some days ago ("[RFC PATCH] btrfs: Replace=20
kmap() with kmap_local_page() in zstd.c")[1] I get a series of errors like=
=20
the following:

/usr/src/git/kernels/linux/fs/btrfs/zstd.c:547:33: warning: passing=20
argument 1 of '__kunmap_local' discards 'const' qualifier from pointer=20
target type [-Wdiscarded-qualifiers]
  547 |   kunmap_local(workspace->in_buf.src);
      |                ~~~~~~~~~~~~~~~~~^~~~
/usr/src/git/kernels/linux/include/linux/highmem-internal.h:284:17: note:=20
in definition of macro 'kunmap_local'
  284 |  __kunmap_local(__addr);     \
      |                 ^~~~~~
/usr/src/git/kernels/linux/include/linux/highmem-internal.h:92:41: note:=20
expected 'void *' but argument is of type 'const void *'
   92 | static inline void __kunmap_local(void *vaddr)
      |                                   ~~~~~~^~~~~

Therefore, this is a (bad?) hack to make these changes compile.
A better solution is changing the prototype of __kunmap_local(); I
suppose that Andrew won't object, but who knows?

(+Cc Andrew Morton).

I was waiting for your comments. At now I've done about 15 conversions=20
across the kernel but it's the first time I had to pass a pointer to const=
=20
void to kunmap_local(). Therefore, I was not sure if changing the API were=
=20
better suited (however I have already discussed this with Ira).

> I see that it leads to a warning but we pass a
> const buffer and that breaks the API contract as in kunmap it would be
> accessed as non-const and potentially changed without warning or
> compiler error. If kunmap_local does not touch the buffer

Yes, correct, kunmap_local() does _not_ touch the buffer.

> and 'const
> void*' would work too, then it should be fixed.

I'll send an RFC patch for changing __kunmap_local() and the other
functions of the calls chain down to kunmap_local_indexed().
=46urthermore, changes to kunmap_local_indexed() prototype require also=20
changes to __kunmap_atomic() (if I recall correctly...).

Thanks for your review,

=46abio