linux-kernel - Re: [PATCH v2 2/6] bitmap: replace _reg_op(REG_OP_ALLOC) with bitmap

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Date:   Fri, 11 Aug 2023 15:31:01 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Yury Norov <yury.norov@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [PATCH v2 2/6] bitmap: replace _reg_op(REG_OP_ALLOC) with
 bitmap_set()

On 11/08/2023 14.56, Yury Norov wrote:
> On Fri, Aug 11, 2023 at 08:21:33AM +0200, Rasmus Villemoes wrote:
>> On 11/08/2023 02.57, Yury Norov wrote:
>>> _reg_op(REG_OP_ALLOC) duplicates bitmap_set(). Fix it.
>>>
>>> Signed-off-by: Yury Norov <yury.norov@...il.com>
>>> ---
>>>  lib/bitmap.c | 5 ++++-
>>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/lib/bitmap.c b/lib/bitmap.c
>>> index 3a589016f5e0..c9afe704fe4b 100644
>>> --- a/lib/bitmap.c
>>> +++ b/lib/bitmap.c
>>> @@ -1352,9 +1352,12 @@ EXPORT_SYMBOL(bitmap_release_region);
>>>   */
>>>  int bitmap_allocate_region(unsigned long *bitmap, unsigned int pos, int order)
>>>  {
>>> +	unsigned int nbits = pos + BIT(order);
>>> +
>>
>> That really doesn't sound right. Have you added self-tests for these
>> functions first and then used those to catch regressions?
> 
> When bitmap_allocate_region() is broken, almost every arch build fails
> to boot. Can you explain what exactly looks wrong to you?

The number of bits we are about to set should not be [position in bitmap
to start from] + [2^order]. The second half of that patch was

-	return __reg_op(bitmap, pos, order, REG_OP_ALLOC);
+	bitmap_set(bitmap, pos, nbits);
+	return 0;

so instead of setting 1<<nbits starting at pos, you're now setting
pos+(1<<nbits) starting at pos. How is that correct?

Rasmus