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From:   "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
To:     Bob Peterson <rpeterso@redhat.com>,
        Andreas Gruenbacher <agruenba@redhat.com>,
        cluster-devel@redhat.com, linux-kernel@vger.kernel.org,
        Deepak R Varma <drv@mailo.com>
Cc:     Ira Weiny <ira.weiny@intel.com>,
        Sumitra Sharma <sumitraartsy@gmail.com>,
        Deepak R Varma <drv@mailo.com>
Subject: Re: [PATCH v3 2/6] gfs2: Replace kmap_atomic()+memcpy by memcpy_from_page()
Date:   Sat, 01 Jul 2023 13:17:20 +0200
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In-Reply-To: <4d132d7aa9cc71f88e6e02c8f6fcb0fec56542aa.1688073459.git.drv@mailo.com>
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 <4d132d7aa9cc71f88e6e02c8f6fcb0fec56542aa.1688073459.git.drv@mailo.com>
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On gioved=EC 29 giugno 2023 23:50:07 CEST Deepak R Varma wrote:
> kmap_atomic() is deprecated in favor of kmap_local_{folio,page}().
>=20
> kmap_atomic() disables page-faults and preemption (the latter only for
> !PREEMPT_RT kernels), However, the code within the mapping/un-mapping in
> gfs2_internal_read() does not depend on the above-mentioned side effects.
>=20
> Further, memcpy_{from,to}_page() wrappers combine the
> {kmap, unmap}_local_page() blocks when they are intended exclusively to
> copy contents from/to the temporary mapped page. So, replace the
> kmap_atomic()/kunmap_automic()

s/kunmap_automic/kunmap_atomic/

> block by the memcpy_from_page() API call.
> This change allows to tidy-up code and also eliminate unused variable p.
>=20
> Suggested-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>

I don't like to read the list of the functions replaced by the=20
memcpy_from_page() helper in the subject of the patch. These details are=20
better suited in commit messages.

=46urthermore, you have not been consistent: "kmap_atomic()+memcpy" contain=
s=20
parentheses only in one of the two functions. Both or none.

The thing that really matters here is that You are replacing the deprecated=
=20
kmap_atomic() with kmap_local_page(). This is what the subject should show.

Please cite the use of the suited helper _only_ within the commit message a=
nd=20
explain why it is preferred to open code kmap_local_page() + memcpy() +=20
kunmap_local().=20

> Signed-off-by: Deepak R Varma <drv@mailo.com>
> ---
> Changes in v3:
>    - Split as a separate patch for conversion in gfs2_internal_read()


>    - Use memcpy_from_page() as suggested by Fabio and  Andreas G
>    - Included split version in patch set
>=20
> Changes in v2:
>    - Update patch description to correct the replacement function name
>      from kmap_local_folio to kmap_local _page
>=20
>=20
>=20
>  fs/gfs2/aops.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
>=20
> diff --git a/fs/gfs2/aops.c b/fs/gfs2/aops.c
> index 3eac4f2f5c27..f47fed657763 100644
> --- a/fs/gfs2/aops.c
> +++ b/fs/gfs2/aops.c
> @@ -489,7 +489,6 @@ int gfs2_internal_read(struct gfs2_inode *ip, char *b=
uf,
> loff_t *pos, unsigned copied =3D 0;
>  	unsigned amt;
>  	struct page *page;
> -	void *p;
>=20
>  	do {
>  		page =3D read_cache_page(mapping, index, gfs2_read_folio,=20
NULL);
> @@ -498,12 +497,12 @@ int gfs2_internal_read(struct gfs2_inode *ip, char=
=20
*buf,
> loff_t *pos, continue;
>  			return PTR_ERR(page);
>  		}
> -		p =3D kmap_atomic(page);
> -		amt =3D size - copied;
>  		if (offset + size > PAGE_SIZE)
>  			amt =3D PAGE_SIZE - offset;
> -		memcpy(buf + copied, p + offset, amt);
> -		kunmap_atomic(p);
> +		else
> +			amt =3D size - copied;

This is a different logical change. Please put it in a different patch with=
=20
proper subject and commit message.

Thanks,

=46abio=20

> +
> +		memcpy_from_page(buf, page, offset, amt);
>  		put_page(page);
>  		copied +=3D amt;
>  		index++;
> --
> 2.34.1