linux-kernel - Re: [PATCH v3 5/6] gfs2: Replace kmap() by kmap_local_page() in gfs2_read

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Date:   Sat, 01 Jul 2023 15:45:24 +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 5/6] gfs2: Replace kmap() by kmap_local_page() in
 gfs2_read_super

On giovedì 29 giugno 2023 23:51:53 CEST Deepak R Varma wrote:
> The use of kmap() is being deprecated in favor of kmap_local_page().
> 
> There are two main problems with kmap(): (1) It comes with an overhead as
> the mapping space is restricted and protected by a global lock for
> synchronization and (2) it also requires global TLB invalidation when the
> kmap’s pool wraps and it might block when the mapping space is fully
> utilized until a slot becomes available.
> 
> With kmap_local_page() the mappings are per thread, CPU local, can take
> page faults, and can be called from any context (including interrupts).
> It is faster than kmap() in kernels with HIGHMEM enabled. Furthermore,
> the tasks can be preempted and, when they are scheduled to run again, the
> kernel virtual addresses are restored and still valid.
> 
> Therefore, replace kmap() with kmap_local_page() in gfs2_read_super().
> 
> 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 set
> 
> Changes in v2:
>    - None
> 
>  fs/gfs2/ops_fstype.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/gfs2/ops_fstype.c b/fs/gfs2/ops_fstype.c
> index 8a27957dbfee..80fe61662412 100644
> --- a/fs/gfs2/ops_fstype.c
> +++ b/fs/gfs2/ops_fstype.c
> @@ -264,9 +264,9 @@ static int gfs2_read_super(struct gfs2_sbd *sdp, 
sector_t
> sector, int silent) __free_page(page);
>  		return -EIO;
>  	}
> -	p = kmap(page);
> +	p = kmap_local_page(page);
>
Deepak,

Could this page ever come from ZONE_HIGHMEM? Can you please check whether we 
really need to kmap*() it or not? 

Fabio
>
>  	gfs2_sb_in(sdp, p);
> -	kunmap(page);
> +	kunmap_local(p);
>  	__free_page(page);
>  	return gfs2_check_sb(sdp, silent);
>  }
> --
> 2.34.1