Re: [PATCH 1/2] drm: buddy_allocator: Fix buddy allocator init on 32-bit systems

[Jani Nikula <jani.nikula@linux.intel.com>]

On Thu, 30 Mar 2023, Christian König <christian.koenig@amd.com> wrote:
> Am 30.03.23 um 12:53 schrieb Jani Nikula:
>> On Wed, 29 Mar 2023, David Gow <davidgow@google.com> wrote:
>>> The drm buddy allocator tests were broken on 32-bit systems, as
>>> rounddown_pow_of_two() takes a long, and the buddy allocator handles
>>> 64-bit sizes even on 32-bit systems.
>>>
>>> This can be reproduced with the drm_buddy_allocator KUnit tests on i386:
>>> 	./tools/testing/kunit/kunit.py run --arch i386 \
>>> 	--kunitconfig ./drivers/gpu/drm/tests drm_buddy
>>>
>>> (It results in kernel BUG_ON() when too many blocks are created, due to
>>> the block size being too small.)
>>>
>>> This was independently uncovered (and fixed) by Luís Mendes, whose patch
>>> added a new u64 variant of rounddown_pow_of_two(). This version instead
>>> recalculates the size based on the order.
>>>
>>> Reported-by: Luís Mendes <luis.p.mendes@gmail.com>
>>> Link: https://lore.kernel.org/lkml/CAEzXK1oghXAB_KpKpm=-CviDQbNaH0qfgYTSSjZgvvyj4U78AA@mail.gmail.com/T/
>>> Signed-off-by: David Gow <davidgow@google.com>
>>> ---
>>>   drivers/gpu/drm/drm_buddy.c | 4 ++--
>>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/drm_buddy.c b/drivers/gpu/drm/drm_buddy.c
>>> index 3d1f50f481cf..7098f125b54a 100644
>>> --- a/drivers/gpu/drm/drm_buddy.c
>>> +++ b/drivers/gpu/drm/drm_buddy.c
>>> @@ -146,8 +146,8 @@ int drm_buddy_init(struct drm_buddy *mm, u64 size, u64 chunk_size)
>>>   		unsigned int order;
>>>   		u64 root_size;
>>>   
>>> -		root_size = rounddown_pow_of_two(size);
>>> -		order = ilog2(root_size) - ilog2(chunk_size);
>>> +		order = ilog2(size) - ilog2(chunk_size);
>>> +		root_size = chunk_size << order;
>> Just noticed near the beginning of the function there's also:
>>
>> 	if (!is_power_of_2(chunk_size))
>> 		return -EINVAL;
>>
>> which is also wrong for 32-bit.
>
> Yeah, but that isn't vital. We just use u64 for the chunk_size for 
> consistency.
>
> In reality I wouldn't except more than 256K here.

Right. It's just not pedantically correct either. ;)

is_power_of_2() is pretty scary as it is, since it just truncates.

BR,
Jani.


>
> Regards,
> Christian.
>
>>
>>
>> BR,
>> Jani.
>>
>>
>>>   
>>>   		root = drm_block_alloc(mm, NULL, order, offset);
>>>   		if (!root)
>

-- 
Jani Nikula, Intel Open Source Graphics Center