Re: [Intel-gfx] [PATCH] drm: Fix a infinite loop condition when order becomes 0

[Paul Menzel]

Dear Arunpravin,

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.paneersel...@amd.com>

Please use your full name.
okay

You might also configure that in your email program.
yes

Not done yet though. ;-)

---
    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&amp;data=04%7C01%7CArunpravin.PaneerSelvam%40amd.com%7Cc456573102c04191cf9708da0662f798%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637829319396954551%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&amp;sdata=5Bspe5QGjQ0KHfVI8%2F%2BXqxR45q6tOL4FE2fVD3uwL%2FM%3D&amp;reserved=0
drm-misc-next - 
https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fcgit.freedesktop.org%2Fdrm%2Fdrm-misc%2Ftree%2F&amp;data=04%7C01%7CArunpravin.PaneerSelvam%40amd.com%7Cc456573102c04191cf9708da0662f798%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637829319396954551%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&amp;sdata=g2S14TfsHF5ORo9jTZ3uA0l1BH8mnAxk2OWYJeF5i8k%3D&amp;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.


Kind regards,

Paul