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

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

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.


BR,
Jani.


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

-- 
Jani Nikula, Intel Open Source Graphics Center