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Date:   Mon, 30 Mar 2020 06:20:28 -0700
From:   Matthew Wilcox <willy@infradead.org>
To:     Wei Yang <richard.weiyang@gmail.com>
Cc:     linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/9] XArray: simplify the calculation of shift
Message-ID: <20200330132028.GA22483@bombadil.infradead.org>
References: <20200330123643.17120-1-richard.weiyang@gmail.com>
 <20200330123643.17120-3-richard.weiyang@gmail.com>
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On Mon, Mar 30, 2020 at 12:36:36PM +0000, Wei Yang wrote:
> When head is NULL, shift is calculated from max. Currently we use a loop
> to detect how many XA_CHUNK_SHIFT is need to cover max.
> 
> To achieve this, we can get number of bits max expands and round it up
> to XA_CHUNK_SHIFT.
> 
> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
> ---
>  lib/xarray.c | 6 +-----
>  1 file changed, 1 insertion(+), 5 deletions(-)
> 
> diff --git a/lib/xarray.c b/lib/xarray.c
> index 1d9fab7db8da..6454cf3f5b4c 100644
> --- a/lib/xarray.c
> +++ b/lib/xarray.c
> @@ -560,11 +560,7 @@ static int xas_expand(struct xa_state *xas, void *head)
>  	unsigned long max = xas_max(xas);
>  
>  	if (!head) {
> -		if (max == 0)
> -			return 0;
> -		while ((max >> shift) >= XA_CHUNK_SIZE)
> -			shift += XA_CHUNK_SHIFT;
> -		return shift + XA_CHUNK_SHIFT;
> +		return roundup(fls_long(max), XA_CHUNK_SHIFT);

This doesn't give the same number.  Did you test this?

Consider max = 64.  The current code does:

shift = 0;
64 >> 0 >= 64 (true)
shift += 6;
64 >> 6 < 64
return 12

Your replacement does:

fls_long(64) = 6
roundup(6, 6) is 6.

Please be more careful.