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Date:   Mon, 30 Mar 2020 14:07:12 +0000
From:   Wei Yang <richard.weiyang@gmail.com>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Wei Yang <richard.weiyang@gmail.com>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/9] XArray: simplify the calculation of shift
Message-ID: <20200330140712.sloduuoqc5jid64m@master>
Reply-To: Wei Yang <richard.weiyang@gmail.com>
References: <20200330123643.17120-1-richard.weiyang@gmail.com>
 <20200330123643.17120-3-richard.weiyang@gmail.com>
 <20200330132028.GA22483@bombadil.infradead.org>
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On Mon, Mar 30, 2020 at 06:20:28AM -0700, Matthew Wilcox wrote:
>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

fls_long(64) = 7

>roundup(6, 6) is 6.
>
>Please be more careful.

-- 
Wei Yang
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