On Mon, 30 Oct 2023, Vlastimil Babka wrote:
On 10/30/23 12:22, Mikulas Patocka wrote:
On Mon, 30 Oct 2023, Vlastimil Babka wrote:
Ah, missed that. And the traces don't show that we would be waiting for that. I'm starting to think the allocation itself is really not the issue here. Also I don't think it deprives something else of large order pages, as per the sysrq listing they still existed.
What I rather suspect is what happens next to the allocated bio such that it works well with order-0 or up to costly_order pages, but there's some problem causing a deadlock if the bio contains larger pages than that?
Yes. There are many "if (order > PAGE_ALLOC_COSTLY_ORDER)" branches in the memory allocation code and I suppose that one of them does something bad and triggers this bug. But I don't know which one.
It's not what I meant. All the interesting branches for costly order in page allocator/compaction only apply with __GFP_DIRECT_RECLAIM, so we can't be hitting those here. The traces I've seen suggest the allocation of the bio suceeded, and problems arised only after it was submitted.
I wouldn't even be surprised if the threshold for hitting the bug was not exactly order > PAGE_ALLOC_COSTLY_ORDER but order > PAGE_ALLOC_COSTLY_ORDER
- 1 or + 2 (has that been tested?) or rather that there's no exact
threshold, but probability increases with order.
It has been tested - Marek tested it with intial order == 4 and he hit the bug. With 3 and less he didn't.
Mikulas