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

[Oscar Salvador <osalvador@suse.de>]

On Tue, Feb 02, 2021 at 09:35:09AM +0100, David Hildenbrand wrote:
> 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.

I see.

> > 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.

You are right.

> 
> > 
> > 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.

Fine by me, also on a second thought it is good to have some sort of clue
when looking at the code.
I will add that cleanup before the actual "fix" of the sub-pmd stuff.

thanks!

-- 
Oscar Salvador
SUSE L3