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From:   "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
To:     dsterba@suse.cz
Cc:     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 01:42:06 +0200
Message-ID: <11997068.O9o76ZdvQC@opensuse>
In-Reply-To: <1936552.usQuhbGJ8B@opensuse>
References: <20220611135203.27992-1-fmdefrancesco@gmail.com> <20220613183913.GD20633@twin.jikos.cz> <1936552.usQuhbGJ8B@opensuse>
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On marted=C3=AC 14 giugno 2022 01:22:50 CEST Fabio M. De Francesco wrote:
> 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=20
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?
>=20
> 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=20
like=20
> the following:
>=20
> /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)
>       |                                   ~~~~~~^~~~~
>=20
> 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?
>=20
> (+Cc Andrew Morton).
>=20
> 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=20
const=20
> void to kunmap_local(). Therefore, I was not sure if changing the API=20
were=20
> better suited (however I have already discussed this with Ira).
>=20
> > 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
>=20
> Yes, correct, kunmap_local() does _not_ touch the buffer.
>=20
> > and 'const
> > void*' would work too, then it should be fixed.
>=20
> I'll send an RFC patch for changing __kunmap_local() and the other
> functions of the calls chain down to kunmap_local_indexed().
> Furthermore, changes to kunmap_local_indexed() prototype require also=20
> changes to __kunmap_atomic() (if I recall correctly...).
>=20
> Thanks for your review,
>=20
> Fabio
>=20
Sorry, I forgot to paste a link:
[1] https://lore.kernel.org/lkml/20220611020451.28170-1-fmdefrancesco@gmail=
=2Ecom/

=46abio