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

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Date:   Thu, 30 Mar 2023 12:56:27 +0200
From:   Christian König <christian.koenig@....com>
To:     Jani Nikula <jani.nikula@...ux.intel.com>,
        David Gow <davidgow@...gle.com>,
        Luís Mendes <luis.p.mendes@...il.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Maíra Canal <mairacanal@...eup.net>,
        Arthur Grillo <arthurgrillo@...eup.net>
Cc:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] drm: buddy_allocator: Fix buddy allocator init on
 32-bit systems

Am 30.03.23 um 12:53 schrieb Jani Nikula:
> On Wed, 29 Mar 2023, David Gow <davidgow@...gle.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@...il.com>
>> Link: https://lore.kernel.org/lkml/CAEzXK1oghXAB_KpKpm=-CviDQbNaH0qfgYTSSjZgvvyj4U78AA@mail.gmail.com/T/
>> Signed-off-by: David Gow <davidgow@...gle.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.

Regards,
Christian.

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