Re: KASLR causes intermittent boot failures on some systems

[Baoquan He <bhe@redhat.com>]

On 04/24/17 at 04:18pm, Dan Williams wrote:
> On Mon, Apr 24, 2017 at 4:07 PM, Baoquan He <bhe@redhat.com> wrote:
> > On 04/24/17 at 01:52pm, Dan Williams wrote:
> [..]
> >> When using the memmap= parameter we're using this call by default:
> >>
> >>         } else if (pmem_should_map_pages(dev)) {
> >>                 addr = devm_memremap_pages(dev, &nsio->res,
> >>                                 &q->q_usage_counter, NULL);
> >>                 pmem->pfn_flags |= PFN_MAP;
> >>         } else
> >>
> >> ...where we are assuming that the memmap= parameter does not specify a
> >> range-size that will exhaust all of system-memory just to hold the
> >> struct page array.
> >
> > Yeah, according to my debugging tracking, it goes as Dan said. And the
> > is_ram is REGION_DISJOINT. And till arch_add_memory, the parameters
> > passed to arch_add_memory are "arch_add_memory, align_start:0x10000000000, align_size:0x3000000000",
> > seems it's going well.
> >
> > Hi Dan,
> >
> > I am always confused that in devm_memremap_pages, the passed in
> > parameter altmap is NULL, while it used devres_alloc_node to allocate a
> > page_map and that page_map contained a altmap instance, not pointer.
> > Then the addr range were inserted into pgmap_radix with value of
> > page_map. Why later in __add_pages, to_vmem_altmap() return NULL
> > according to my debugging code?
> 
> We expect altmap to always be NULL in this case. The only time it is
> not NULL is when the namespace is configured to allocate the struct
> page array from capacity on the namespace itself. I.e. instead of
> allocating struct page from page allocator pages the driver creates an
> altmap and vmemmap_populate_hugepages() uses that to allocate the
> array from "alternate" capacity.
> 
> You can force this by running:
> 
>     ndctl create-namespace -f -e namespace0.0 -m memory -M dev
> 
> ...which says "put the struct page map on the device".

Ah, got. It only assigned page_map->altmap to pgmap->altmap when the
passed in altmap is not NULL. 

pgmap->altmap = &page_map->altmap;

I didn't notice this. Thanks for telling. So in this case without
alternate capacity, it will allocate from normal ram memory struct page
array.

Thanks
Baoquan