Re: [PATCH 2/9] XArray: simplify the calculation of shift

Wei Yang
2020-03-30T14:07:16Z

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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