[PATCH] drm: Fix a infinite loop condition when order becomes 0

[Paul Menzel]

Dear Arunprivin,


Am 16.03.22 um 07:49 schrieb Arunpravin Paneer Selvam:

> On 15/03/22 9:14 pm, Paul Menzel wrote:

>> Am 15.03.22 um 16:42 schrieb Arunpravin:
>>
>>> On 15/03/22 2:35 pm, Paul Menzel wrote:
>>
>>>> Am 15.03.22 um 10:01 schrieb Arunpravin:
>>>>
>>>>> On 15/03/22 1:49 pm, Paul Menzel wrote:
>>>>
>>>>>> Am 14.03.22 um 20:40 schrieb Arunpravin:
>>>>>>> handle a situation in the condition order-- == min_order,
>>>>>>> when order = 0, leading to order = -1, it now won't exit
>>>>>>> the loop. To avoid this problem, added a order check in
>>>>>>> the same condition, (i.e) when order is 0, we return
>>>>>>> -ENOSPC
>>>>>>>
>>>>>>> Signed-off-by: Arunpravin <Arunpravin.PaneerSelvam at amd.com>
>>>>>>
>>>>>> Please use your full name.
>>>>> okay
>>>>
>>>> You might also configure that in your email program.
>>> yes
>>
>> Not done yet though. ;-)
>>
> done in v2 :)
>>>>>>> ---
>>>>>>>      drivers/gpu/drm/drm_buddy.c | 2 +-
>>>>>>>      1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>>>
>>>>>>> diff --git a/drivers/gpu/drm/drm_buddy.c b/drivers/gpu/drm/drm_buddy.c
>>>>>>> index 72f52f293249..5ab66aaf2bbd 100644
>>>>>>> --- a/drivers/gpu/drm/drm_buddy.c
>>>>>>> +++ b/drivers/gpu/drm/drm_buddy.c
>>>>>>
>>>>>> In what tree is that file?
>>>>>>
>>>>> drm-tip - https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fcgit.freedesktop.org%2Fdrm-tip%2Ftree%2F&data=04%7C01%7CArunpravin.PaneerSelvam%40amd.com%7C3610aafe216d421c715c08da069ac1d7%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637829559006306914%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=GM3iXiDQCx%2BM4pD1nmivRFRvkehwTNd2Jtd713cF51g%3D&reserved=0
>>>>> drm-misc-next - https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fcgit.freedesktop.org%2Fdrm%2Fdrm-misc%2Ftree%2F&data=04%7C01%7CArunpravin.PaneerSelvam%40amd.com%7C3610aafe216d421c715c08da069ac1d7%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637829559006306914%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=i7pvmDJu310XRX7h3cQ344j5RYHq7fBZ520l%2F%2Br1%2BQU%3D&reserved=0
>>
>> Thank Outlook. Now everybody feels safe.
>>
>>>>>>> @@ -685,7 +685,7 @@ int drm_buddy_alloc_blocks(struct drm_buddy *mm,
>>>>>>>      			if (!IS_ERR(block))
>>>>>>>      				break;
>>>>>>>      
>>>>>>> -			if (order-- == min_order) {
>>>>>>> +			if (!order || order-- == min_order) {
>>>>>>>      				err = -ENOSPC;
>>>>>>>      				goto err_free;
>>>>>>>      			}
>>>>
>>>> Thank you for the hint. So the whole function is:
>>>>
>>>> 	do {
>>>> 		order = min(order, (unsigned int)fls(pages) - 1);
>>>> 		BUG_ON(order > mm->max_order);
>>>> 		BUG_ON(order < min_order);
>>>>
>>>> 		do {
>>>> 			if (flags & DRM_BUDDY_RANGE_ALLOCATION)
>>>> 				/* Allocate traversing within the range */
>>>> 				block = alloc_range_bias(mm, start, end, order);
>>>> 			else
>>>> 				/* Allocate from freelist */
>>>> 				block = alloc_from_freelist(mm, order, flags);
>>>>
>>>> 			if (!IS_ERR(block))
>>>> 				break;
>>>>
>>>> 			if (order-- == min_order) {
>>>> 				err = -ENOSPC;
>>>> 				goto err_free;
>>>> 			}
>>>> 		} while (1);
>>>>
>>>> 		mark_allocated(block);
>>>> 		mm->avail -= drm_buddy_block_size(mm, block);
>>>> 		kmemleak_update_trace(block);
>>>> 		list_add_tail(&block->link, &allocated);
>>>>
>>>> 		pages -= BIT(order);
>>>>
>>>> 		if (!pages)
>>>> 			break;
>>>> 	} while (1);
>>>>
>>>> Was the BUG_ON triggered for your case?
>>>>
>>>> 	BUG_ON(order < min_order);
>>> no, this BUG_ON is not triggered for this bug
>>>>
>>>> Please give more details.
>>>
>>> there is a chance when there is no space to allocate, order value
>>> decrements and reaches to 0 at one point, here we should exit the loop,
>>> otherwise, further order value decrements to -1 and do..while loop
>>> doesn't exit. Hence added a check to exit the loop if order value becomes 0.
>>
>> Sorry, I do not see it. How can that be with order ≥ min_order and the
>> check `order-- == min_order`? Is min_order 0? Please explain that in the
>> next commit message.
>>
> please check v2, yes when min_order is 0, the above said situation may
> occur.And, since the order is unsigned int, I think it will not trigger
> the BUG_ON(order < min_order) when order becomes -1. Hence I think we
> needed a check !order to exit the loop.

Thank you for clarifying this. I still do not understand it though. With

	order = fls(pages) - 1;
	min_order = ilog2(min_page_size) - ilog2(mm->chunk_size);

is zorder` always non-negative? Let’s assume it is. Also, can min_order 
get “negative” (wraps around)?

I would add BUG_ON statements for these cases?

     BUG_ON(fls(pages) - 1 < 1);
     BUG_ON(ilog2(min_page_size) - ilog2(mm->chunk_size) < 1);

Assuming “negative” is not possible, your case can only happen if 
`order` and `min_order` are 0, right? If `order` is greater than 0, and 
`min_order` is 0, the first BUG_ON in the while loop would be hit. If 
`order` is 0 and `min_order` is greater than 0, everything should work 
as the condition in `if (order-- == min_order)` is going to be true 
eventually.

Could you please analyze this more. The current patch looks more like 
papering over something, or I am missing something.


Kind regards,

Paul


PS: The commit message summary of your v2 should also be updated.