linux-kernel - Re: [PATCH] dax: use switch statement over chained ifs

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Date:   Tue, 17 Jan 2023 02:41:37 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Amy Parker <apark0006@...dent.cerritos.edu>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dax: use switch statement over chained ifs

On Mon, Jan 16, 2023 at 06:11:00PM -0800, Amy Parker wrote:
> This patch uses a switch statement for pe_order, which improves
> readability and on some platforms may minorly improve performance. It
> also, to improve readability, recognizes that `PAGE_SHIFT - PAGE_SHIFT' is
> a constant, and uses 0 in its place instead.
> 
> Signed-off-by: Amy Parker <apark0006@...dent.cerritos.edu>

Hi Amy,

Thanks for the patch!  Two problems.  First, your mailer seems to have
mangled the patch; in my tree these are tab indents, and the patch has
arrived with four-space indents, so it can't be applied.

The second problem is that this function should simply not exist.
I forget how we ended up with enum page_entry_size, but elsewhere
we simply pass 'order' around.  So what I'd really like to see is
a patch series that eliminates page_entry_size everywhere.

I can outline a way to do that in individual patches if that would be
helpful.

>  fs/dax.c | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/dax.c b/fs/dax.c
> index c48a3a93ab29..e8beed601384 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -32,13 +32,16 @@
> 
>  static inline unsigned int pe_order(enum page_entry_size pe_size)
>  {
> -    if (pe_size == PE_SIZE_PTE)
> -        return PAGE_SHIFT - PAGE_SHIFT;
> -    if (pe_size == PE_SIZE_PMD)
> +    switch (pe_size) {
> +    case PE_SIZE_PTE:
> +        return 0;
> +    case PE_SIZE_PMD:
>          return PMD_SHIFT - PAGE_SHIFT;
> -    if (pe_size == PE_SIZE_PUD)
> +    case PE_SIZE_PUD:
>          return PUD_SHIFT - PAGE_SHIFT;
> -    return ~0;
> +    default:
> +        return ~0;
> +    }
>  }
> 
>  /* We choose 4096 entries - same as per-zone page wait tables */
> -- 
> 2.39.0