Re: [PATCH V2 1/2] arm64/mm: Fix pfn_valid() for ZONE_DEVICE based memory

[David Hildenbrand]

On 11.02.21 13:10, Anshuman Khandual wrote:
> On 2/11/21 5:23 PM, Will Deacon wrote:
> > On Fri, Feb 05, 2021 at 06:55:53PM +0000, Will Deacon wrote:
> > > On Wed, Feb 03, 2021 at 09:20:39AM +0530, Anshuman Khandual wrote:
> > > > On 2/2/21 6:26 PM, David Hildenbrand wrote:
> > > > > On 02.02.21 13:51, Will Deacon wrote:
> > > > > > On Tue, Feb 02, 2021 at 01:39:29PM +0100, David Hildenbrand wrote:
> > > > > > > As I expressed already, long term we should really get rid of the arm64
> > > > > > > variant and rather special-case the generic one. Then we won't go out of
> > > > > > > sync - just as it happened with ZONE_DEVICE handling here.
> > > > > >
> > > > > > Why does this have to be long term? This ZONE_DEVICE stuff could be the
> > > > > > carrot on the stick :)
> > > > >
> > > > > Yes, I suggested to do it now, but Anshuman convinced me that doing a
> > > > > simple fix upfront might be cleaner --- for example when it comes to
> > > > > backporting :)
> > > >
> > > > Right. The current pfn_valid() breaks for ZONE_DEVICE memory and this fixes
> > > > the problem in the present context which can be easily backported if required.
> > > >
> > > > Changing or rather overhauling the generic code with new configs as proposed
> > > > earlier (which I am planning to work on subsequently) would definitely be an
> > > > improvement for the current pfn_valid() situation in terms of maintainability
> > > > but then it should not stop us from fixing the problem now.
> > >
> > > Alright, I've mulled this over a bit. I don't agree that this patch helps
> > > with maintainability (quite the opposite, in fact), but perfection is the
> > > enemy of the good so I'll queue the series for 5.12. However, I'll revert
> > > the changes at the first sign of a problem, so please do work towards a
> > > generic solution which can replace this in the medium term.
> >
> > ... and dropped. These patches appear to be responsible for a boot
> > regression reported by CKI:
>
> Ahh, boot regression ? These patches only change the behaviour
> for non boot memory only.
>
> > https://lore.kernel.org/r/cki.8D1CB60FEC.K6NJMEFQPV@xxxxxxxxxx
>
> Will look into the logs and see if there is something pointing to
> the problem.

It's strange. One thing I can imagine is a mis-detection of early 
sections. However, I don't see that happening:

In sparse_init_nid(), we:
1. Initialize the memmap
2. Set SECTION_IS_EARLY | SECTION_HAS_MEM_MAP via
   sparse_init_one_section()

Only hotplugged sections (DIMMs, dax/kmem) set SECTION_HAS_MEM_MAP 
without SECTION_IS_EARLY - which is correct, because these are not early.

So once we know that we have valid_section() -- SECTION_HAS_MEM_MAP is 
set -- early_section() should be correct.

Even if someone would be doing a pfn_valid() after 
memblocks_present()->memory_present() but before
sparse_init_nid(), we should be fine (!valid_section() -> return 0).


As it happens early during boot, I doubt that some NVDIMMs that get 
detected and added early during boot as system RAM (via dax/kmem). Are 
the problem.

--
Thanks,

David / dhildenb