Re: [PATCH] drm/buddy: Fix alloc_range() error handling code

[Matthew Auld <matthew.auld@intel.com>]

On 08/02/2024 13:47, Arunpravin Paneer Selvam wrote:
> Hi Matthew,
> 
> On 2/8/2024 7:00 PM, Matthew Auld wrote:
>> On 07/02/2024 17:44, Arunpravin Paneer Selvam wrote:
>>> Few users have observed display corruption when they boot
>>> the machine to KDE Plasma or playing games. We have root
>>> caused the problem that whenever alloc_range() couldn't
>>> find the required memory blocks the function was returning
>>> SUCCESS in some of the corner cases.
>>
>> Can you please give an example here?
>>
> In the try hard contiguous allocation, for example the requested memory 
> is 1024 pages,
> it might go and pick the highest and last block (of size 512 pages) in 
> the freelist where
> there are no more space exist in the total address range. In this kind 
> of corner case,
> alloc_range was returning success though the allocated size is less than 
> the requested size.
> Hence in try_hard_contiguous_allocation, we will not proceed to the LHS 
> allocation and
> we return only with the RHS allocation having only the 512 pages of 
> allocation. This
> leads to display corruption in many use cases (I think mainly when 
> requested for contiguous huge buffer)
> mainly on APU platforms.

Ok, I guess other thing is doing:

lhs_offset = drm_buddy_block_offset(block) - lhs_size;

I presume it's possible for block_offset < lhs_size here, which might be 
funny?

> 
> Thanks,
> Arun.
>>>
>>> The right approach would be if the total allocated size
>>> is less than the required size, the function should
>>> return -ENOSPC.
>>>
>>> Gitlab ticket link - 
>>> https://gitlab.freedesktop.org/drm/amd/-/issues/3097
>>> Fixes: 0a1844bf0b53 ("drm/buddy: Improve contiguous memory allocation")
>>> Signed-off-by: Arunpravin Paneer Selvam 
>>> <Arunpravin.PaneerSelvam@amd.com>
>>> Tested-by: Mario Limonciello <mario.limonciello@amd.com>
>>> ---
>>>   drivers/gpu/drm/drm_buddy.c | 6 ++++++
>>>   1 file changed, 6 insertions(+)
>>>
>>> diff --git a/drivers/gpu/drm/drm_buddy.c b/drivers/gpu/drm/drm_buddy.c
>>> index f57e6d74fb0e..c1a99bf4dffd 100644
>>> --- a/drivers/gpu/drm/drm_buddy.c
>>> +++ b/drivers/gpu/drm/drm_buddy.c
>>> @@ -539,6 +539,12 @@ static int __alloc_range(struct drm_buddy *mm,
>>>       } while (1);
>>>         list_splice_tail(&allocated, blocks);
>>> +
>>> +    if (total_allocated < size) {
>>> +        err = -ENOSPC;
>>> +        goto err_free;
>>> +    }
>>> +
>>>       return 0;
>>>     err_undo:
>