Re: [PATCH v3] arm64: mm: Fix NOMAP page initialization

[Hanjun Guo]

On 2017/1/4 21:56, Ard Biesheuvel wrote:
> On 16 December 2016 at 16:54, Robert Richter <rrichter@xxxxxxxxxx> wrote:
> > On ThunderX systems with certain memory configurations we see the
> > following BUG_ON():
> >
> >  kernel BUG at mm/page_alloc.c:1848!
> >
> > This happens for some configs with 64k page size enabled. The BUG_ON()
> > checks if start and end page of a memmap range belongs to the same
> > zone.
> >
> > The BUG_ON() check fails if a memory zone contains NOMAP regions. In
> > this case the node information of those pages is not initialized. This
> > causes an inconsistency of the page links with wrong zone and node
> > information for that pages. NOMAP pages from node 1 still point to the
> > mem zone from node 0 and have the wrong nid assigned.
> >
> > The reason for the mis-configuration is a change in pfn_valid() which
> > reports pages marked NOMAP as invalid:
> >
> >  68709f45385a arm64: only consider memblocks with NOMAP cleared for linear mapping
> >
> > This causes pages marked as nomap being no longer reassigned to the
> > new zone in memmap_init_zone() by calling __init_single_pfn().
> >
> > Fixing this by implementing an arm64 specific early_pfn_valid(). This
> > causes all pages of sections with memory including NOMAP ranges to be
> > initialized by __init_single_page() and ensures consistency of page
> > links to zone, node and section.
>
> I like this solution a lot better than the first one, but I am still
> somewhat uneasy about having the kernel reason about attributes of
> pages it should not touch in the first place. But the fact that
> early_pfn_valid() is only used a single time in the whole kernel does
> give some confidence that we are not simply moving the problem
> elsewhere.
>
> Given that you are touching arch/arm/ as well as arch/arm64, could you
> explain why only arm64 needs this treatment? Is it simply because we
> don't have NUMA support there?
>
> Considering that Hisilicon D05 suffered from the same issue, I would
> like to get some coverage there as well. Hanjun, is this something you
> can arrange? Thanks

Sure, we will test this patch with LTP MM stress test (which triggers
the bug on D05), and give the feedback.

Thanks
Hanjun