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
