Re: [PATCH v2] x86/vmemmap: Handle unpopulated sub-pmd ranges

[David Hildenbrand <david@redhat.com>]

>> IMHO, we should rip out that code here and enforce page alignment in
>> vmemmap_populate()/vmemmap_free().
>>
>> Am I missing something?
> 
> Thanks David for bringing this up, I must say I was not aware that this
> topic was ever discussed.

Yeah, last time I raised it was in

https://lkml.kernel.org/r/20200703013435.GA11340@L-31X9LVDL-1304.local

but I never got to clean it up myself.

> 
> Ok, I've been having a look into this.
> At first I was concerced because of a pure SPARSEMEM configuration, but I
> see that those allocations are done in a very diferent way so it does not
> bother us.
> 
> So we have the following enforcements during hotplug:
> 
> add_memory_resource
>   check_hotplug_memory_range : Checks range aligned to memory_block_size_bytes,
>                              : which means it must be section-size aligned
> 
> populate_section_memmap
>   __populate_section_memmap  : Checks range aligned to sub-section size
> 
> So, IIRC we have two cases during hotplug:
>   1) the ones that want memory blocks
>   2) the ones that do not want them (pmem stuff)
> 
> For #1, we always enforce section alignment in add_memory_resource, and for
> #2 we always make sure the range is at least sub-section aligned.
> 
> And the important stuff is that boot memory is no longer to be hot-removed
> (boot memory had some strange layout sometimes).

The vmemmap of boot mem sections is always fully populated, even with 
strange memory layouts (e.g., see comment in pfn_valid()). In addition, 
we can only offline+remove whole sections, so that should be fine.

> 
> So, given the above, I think it should be safe to drop that check in
> remote_pte_table.
> But do we really need to force page alignment in vmemmap_populate/vmemmap_free?
> vmemmap_populate should already receive a page-aligned chunk because
>   __populate_section_memmap made sure of that, and vmemmap_free() should be ok
> as we already filtered out at hot-adding stage.
> 
> Of course, this will hold as long as struct page size of multiple of 8.
> Should that change we might get trouble, but I do not think that can ever
> happened (tm).
> 
> But anyway, I am fine with placing a couple of checks in vmemmap_{populate,free}
> just to double check.
> 
> What do you think?

I'd just throw in 1 or 2 VM_BUG_ON() to self-document what we expect and 
that we thought about these conditions. It's then easy to identify the 
relevant commit where we explain the rationale.

I don't have a strong opinion, the other archs also don't seem to care 
about documenting/enforcing it.

-- 
Thanks,

David / dhildenb