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

[Catalin Marinas <catalin.marinas@arm.com>]

On Thu, Feb 11, 2021 at 01:35:56PM +0100, David Hildenbrand wrote:
> On 11.02.21 13:10, Anshuman Khandual wrote:
> > On 2/11/21 5:23 PM, Will Deacon wrote:
> > > ... 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@redhat.com
> > 
> > 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).

I couldn't figure out how this could fail with Anshuman's patches.
Will's suspicion is that some invalid/null pointer gets dereferenced
before being initialised but the only case I see is somewhere in
pfn_section_valid() (ms->usage) if valid_section() && !early_section().

Assuming that we do get a valid_section(ms) && !early_section(ms), is
there a case where ms->usage is not initialised? I guess races with
section_deactivate() are not possible this early.

Another situation could be that pfn_valid() returns true when no memory
is mapped for that pfn.

> 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.

It is indeed very early, we can't even get the early console output.
Debugging this is even harder as it's only misbehaving on a board we
don't have access to.

On the logic in this patch, is the hot-added memory always covering a
full subsection? The arm64 pfn_valid() currently relies on
memblock_is_map_memory() but the patch changes it to
pfn_section_valid(). So if hot-added memory doesn't cover the full
subsection, it may return true even if the pfn is not mapped.

Regarding the robustness of the pfn_valid for ZONE_DEVICE memory, could
we instead have a SECTION_IS_DEVICE flag and only check for that as not
to disturb the hotplugged memory check via memblock_is_map_memory()?

-- 
Catalin