[PATCH] arm64: mm: Fix memmap to be initialized for the entire section

[Robert Richter]

Mark, Will, any opinion here?

Thanks,

-Robert

On 06.10.16 18:11:14, Robert Richter wrote:
> Ard,
> 
> thank you for your answer and you explanation.
> 
> On 06.10.16 11:00:33, Ard Biesheuvel wrote:
> > On 6 October 2016 at 10:52, Robert Richter <rrichter at cavium.com> wrote:
> > > There is a memory setup problem on ThunderX systems with certain
> > > memory configurations. The symptom is
> > >
> > >  kernel BUG at mm/page_alloc.c:1848!
> > >
> > > This happens for some configs with 64k page size enabled. The bug
> > > triggers for page zones with some pages in the zone not assigned to
> > > this particular zone. In my case some pages that are marked as nomap
> > > were not reassigned to the new zone of node 1, so those are still
> > > assigned to node 0.
> > >
> > > 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
> > >
> > 
> > These pages are owned by the firmware, which may map it with
> > attributes that conflict with the attributes we use for the linear
> > mapping. This means they should not be covered by the linear mapping.
> > 
> > > This causes pages marked as nomap being no long reassigned to the new
> > > zone in memmap_init_zone() by calling __init_single_pfn().
> > >
> > 
> > This sounds like the root cause of your issue. Could we not fix that instead?
> 
> Yes, this is proposal b) from my last mail that would work too: I
> implemented an arm64 private early_pfn_valid() function that uses
> memblock_is_memory() to setup all pages of a zone. Though, I think
> this is the wrong way and thus I prefer this patch instead. I see
> serveral reasons for this:
> 
> Inconsistent use of struct *page, it is initialized but never used
> again.
> 
> Other archs only do a basic range check in pfn_valid(), the default
> implementation just returns if the whole section is valid. As I
> understand the code, if the mem range is not aligned to the section,
> then there will be pfn's in the section that don't have physical mem
> attached. The page is then just initialized, it's not marked reserved
> nor the refcount is non-zero. It is then simply not used. This is how
> no-map pages should be handled too.
> 
> I think pfn_valid() is just a quick check if the pfn's struct *page
> can be used. There is a good description for this in include/linux/
> mmzone.h. So there can be memory holes that have a valid pfn.
> 
> If the no-map memory needs special handling, then additional checks
> need to be added to the particular code (as in ioremap.c). It's imo
> wrong to (mis-)use pfn_valid for that.
> 
> Variant b) involves generic mm code to fix it for arm64, this patch is
> an arm64 change only. This makes it harder to get a fix for it.
> (Though maybe only a problem of patch logistics.)
> 
> > 
> > > Fixing this by restoring the old behavior of pfn_valid() to use
> > > memblock_is_memory().
> > 
> > This is incorrect imo. In general, pfn_valid() means ordinary memory
> > covered by the linear mapping and the struct page array. Returning
> > reserved ranges that the kernel should not even touch only to please
> > the NUMA code seems like an inappropriate way to deal with this issue.
> 
> As said above, it is not marked as reserved, it is treated like
> non-existing memory.
> 
> This has been observed for non-numa kernels too and can happen for
> each zone that is only partly initialized.
> 
> I think the patch addresses your concerns. I can't see there the
> kernel uses memory marked as nomap in a wrong way.
> 
> Thanks,
> 
> -Robert
> 
> > 
> > > Also changing users of pfn_valid() in arm64 code
> > > to use memblock_is_map_memory() where necessary. This only affects
> > > code in ioremap.c. The code in mmu.c still can use the new version of
> > > pfn_valid().
> > >
> > > Should be marked stable v4.5..
> > >
> > > Signed-off-by: Robert Richter <rrichter at cavium.com>
> > > ---
> > >  arch/arm64/mm/init.c    | 2 +-
> > >  arch/arm64/mm/ioremap.c | 5 +++--
> > >  2 files changed, 4 insertions(+), 3 deletions(-)
> --
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