Re: kernel BUG at mm/huge_memory.c:2613!

Andrea Arcangeli
2020-06-22T21:57:10Z

Hello,

On Mon, Jun 22, 2020 at 04:30:41PM +0100, Robin Murphy wrote:
> On 2020-06-22 13:46, Joerg Roedel wrote:
> > + Robin
> >
> > Robin, any idea on this?
>
> After a bit of archaeology, this dates back to the original review:
>
> https://lore.kernel.org/linux-arm-kernel/54C285D4.3070802@arm.com/
> https://lore.kernel.org/linux-arm-kernel/54DA2666.9030003@arm.com/
>
> In summary: originally this inherited from other arch code that did
> simply strip __GFP_COMP; that was deemed questionable because of the
> nonsensical comment about CONFIG_HUGETLBFS that was stuck to it; the
> current code is like it is because in 5 and a half years nobody said
> that it's wrong :)
>
> If there actually *are* good reasons for stripping __GFP_COMP, then I've
> certainly no objection to doing so.

The main question is if there's any good reasons for not forbidding
__GFP_COMP to be specified in the callers. The reason given in the
comment isn't convincing.

I don't see how a caller that gets a pointer can care about how the
page structure looks like and in turn why it's asking for __GFP_COMP.

As far as I can tell there are two orthogonal issues in play here:

1) The comment about __GFP_COMP facilitating the sound driver to do
partial mapping doesn't make much sense. It's probably best to
WARN_ON immediately in dma_alloc_coherent if __GFP_COMP is
specified, not only down the call stack in the
__iommu_dma_alloc_pages() path.

Note: the CMA paths would already ignore __GFP_COMP if it's
specified so that __GFP_COMP request can already be ignored. It
sounds preferable to warn the caller it's asking something it can't
get, than to silently ignore __GFP_COMP.

On a side note: hugetlbfs/THP pages can only be allocated with
__GFP_COMP because for example put_page() must work on all tail
pages (you can't call compound_head() unless the tail page is part
of a compound page). But for private driver pages mapped by
remap_pfn_range, any full or partial mapping is done manually and
nobody can call GUP on VM_PFNMAP|VM_IO anyway (there's not even the
requirement of a page struct backing those mappings in fact).

2) __iommu_dma_alloc_pages cannot use __GFP_COMP if it intends to
return an array of small pages, which is the only thing that the
current sg_alloc_table_from_pages() supports in input. split_page
will work as expected to generate small pages from non-compound
order>0 pages, incidentally it's implement on mm/page_alloc.c, not
in huge_memory.c.

split_huge_page as opposed is not intended to be used on newly
allocated compound page. Maybe we should renamed it to
split_trans_huge_page to make it more explicit, since it won't even
work on hugetlbfs (compound) pages.

Thanks,
Andrea