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From:   "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
To:     Wang Yugui <wangyugui@e16-tech.com>
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>
Subject: Re: [PATCH] btrfs: Replace kmap() with kmap_local_page() in zstd.c
Date:   Thu, 14 Jul 2022 09:46:00 +0200
Message-ID: <8091114.T7Z3S40VBb@opensuse>
In-Reply-To: <20220714082519.A7C9.409509F4@e16-tech.com>
References: <20220611135203.27992-1-fmdefrancesco@gmail.com> <20220714082519.A7C9.409509F4@e16-tech.com>
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On gioved=C3=AC 14 luglio 2022 02:25:20 CEST Wang Yugui wrote:
> Hi,
>=20
> Compiler warning:
>=20
> fs/btrfs/zstd.c:478:55: warning: passing argument 1 of =E2=80=98__kunmap_=
local=E2=80=99=20
discards =E2=80=98const=E2=80=99 qualifier from pointer target type [-Wdisc=
arded-
qualifiers]
>   478 |                         kunmap_local(workspace->in_buf.src);
> ./include/linux/highmem-internal.h:284:24: note: in definition of macro=20
=E2=80=98kunmap_local=E2=80=99
>   284 |         __kunmap_local(__addr);                                 \
>       |                        ^~~~~~
> ./include/linux/highmem-internal.h:200:41: note: expected =E2=80=98void *=
=E2=80=99 but=20
argument is of type =E2=80=98const void *=E2=80=99
>   200 | static inline void __kunmap_local(void *addr)
>       |                                   ~~~~~~^~~~
>=20
> Best Regards
> Wang Yugui (wangyugui@e16-tech.com)
> 2022/07/14

Thanks for the build test, but you're a little late :)

This patch is superseded by a series of two: please see
[PATCH v6 0/2] btrfs: Replace kmap() with kmap_local_page() in zstd.c
at https://lore.kernel.org/lkml/20220706111520.12858-1-fmdefrancesco@gmail.=
com/

In that series, patch 1/2 changes __kunmap_{local,atomic}() prototypes to=20
take pointers to const void.

Regards,

=46abio

> > 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