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

[Christian König]

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