linux-kernel - Re: [PATCH v3 4/6] gfs2: Replace kmap_atomic() by kmap_local

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Message-ID: <5948996.alqRGMn8q6@suse>
Date:   Sat, 01 Jul 2023 15:16:48 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Bob Peterson <rpeterso@...hat.com>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        cluster-devel@...hat.com, linux-kernel@...r.kernel.org,
        Deepak R Varma <drv@...lo.com>
Cc:     Ira Weiny <ira.weiny@...el.com>,
        Sumitra Sharma <sumitraartsy@...il.com>,
        Deepak R Varma <drv@...lo.com>
Subject: Re: [PATCH v3 4/6] gfs2: Replace kmap_atomic() by kmap_local_page() in lops.c

On giovedì 29 giugno 2023 23:51:17 CEST Deepak R Varma wrote:
> kmap_atomic() is deprecated in favor of kmap_local_{folio,page}().

Deepak,

Can you please add a reference to the highmem documentation and to the patch 
from Ira that added a deprecation check for kmap() and kmap_atomic() in his 
commit regarding checkpatch.pl?

There may be maintainers / reviewers who are still unaware of this 
information. It would surely help them with reviewing. Furthermore it might 
suggest maintainers to convert their subsystem / driver to the new API or 
remove and use plain page_address() (if it is possible to prove that pages 
can't come from ZONE_HIGHMEM).

> 
> Therefore, replace kmap_atomic() with kmap_local_page() in following
> functions of lops.c:
>    - gfs2_jhead_pg_srch()
>    - gfs2_check_magic()
>    - gfs2_before_commit()
> 
> kmap_atomic() disables page-faults and preemption (the latter only for
> !PREEMPT_RT kernels), However, the code within the mapping/un-mapping in
> stuffed_readpage() does not depend on the above-mentioned side effects.
> 
> Therefore, a mere replacement of the old API with the new one is all that
> is required (i.e., there is no need to explicitly add any calls to
> pagefault_disable() and/or preempt_disable()).
> 
> Suggested-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
> Signed-off-by: Deepak R Varma <drv@...lo.com>
> ---
> Changes in v3:
>    - Patch included in patch series
> 
> Changes in v2:
>    - None
> 
> 
>  fs/gfs2/lops.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/gfs2/lops.c b/fs/gfs2/lops.c
> index 1902413d5d12..a7c2296cb3c6 100644
> --- a/fs/gfs2/lops.c
> +++ b/fs/gfs2/lops.c
> @@ -427,7 +427,7 @@ static bool gfs2_jhead_pg_srch(struct gfs2_jdesc *jd,
>  {
>  	struct gfs2_sbd *sdp = GFS2_SB(jd->jd_inode);
>  	struct gfs2_log_header_host lh;
> -	void *kaddr = kmap_atomic(page);
> +	void *kaddr = kmap_local_page(page);
>  	unsigned int offset;
>  	bool ret = false;
>
Deepak,

Are we mixing declarations with functions calls? Is it good practice? If not, 
I'd suggest to move the mapping to a better suited place.
> 
> @@ -441,7 +441,7 @@ static bool gfs2_jhead_pg_srch(struct gfs2_jdesc *jd,
>  			}
>  		}
>  	}
> -	kunmap_atomic(kaddr);
> +	kunmap_local(kaddr);
>  	return ret;
>  }
> 
> @@ -626,11 +626,11 @@ static void gfs2_check_magic(struct buffer_head *bh)
>  	__be32 *ptr;
> 
>  	clear_buffer_escaped(bh);
> -	kaddr = kmap_atomic(bh->b_page);
> +	kaddr = kmap_local_page(bh->b_page);
>  	ptr = kaddr + bh_offset(bh);
>  	if (*ptr == cpu_to_be32(GFS2_MAGIC))
>  		set_buffer_escaped(bh);
> -	kunmap_atomic(kaddr);
> +	kunmap_local(kaddr);
>  }
> 
>  static int blocknr_cmp(void *priv, const struct list_head *a,
> @@ -699,10 +699,10 @@ static void gfs2_before_commit(struct gfs2_sbd *sdp,
> unsigned int limit, void *kaddr;
>  				page = mempool_alloc(gfs2_page_pool, 
GFP_NOIO);
>  				ptr = page_address(page);
> -				kaddr = kmap_atomic(bd2->bd_bh-
>b_page);
> +				kaddr = kmap_local_page(bd2->bd_bh-
>b_page);
>  				memcpy(ptr, kaddr + bh_offset(bd2-
>bd_bh),
>  				       bd2->bd_bh->b_size);
>
Deepak,

How about memcpy_from_page()?

Thanks,

Fabio
>
> -				kunmap_atomic(kaddr);
> +				kunmap_local(kaddr);
>  				*(__be32 *)ptr = 0;
>  				clear_buffer_escaped(bd2->bd_bh);
>  				unlock_buffer(bd2->bd_bh);
> --
> 2.34.1