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

[Oscar Salvador <osalvador@suse.de>]

On Fri, Jan 29, 2021 at 01:46:33PM +0100, David Hildenbrand wrote:
> >   static void __meminit free_pagetable(struct page *page, int order)
> >   {
> > @@ -1008,10 +1073,10 @@ remove_pte_table(pte_t *pte_start, unsigned long addr, unsigned long end,
> >   			 * with 0xFD, and remove the page when it is wholly
> >   			 * filled with 0xFD.
> >   			 */
> > -			memset((void *)addr, PAGE_INUSE, next - addr);
> > +			memset((void *)addr, PAGE_UNUSED, next - addr);
> >   			page_addr = page_address(pte_page(*pte));
> > -			if (!memchr_inv(page_addr, PAGE_INUSE, PAGE_SIZE)) {
> > +			if (!memchr_inv(page_addr, PAGE_UNUSED, PAGE_SIZE)) {
> >   				free_pagetable(pte_page(*pte), 0);
> 
> I remember already raising this, in the context of other cleanups, but let's
> start anew:
> 
> How could we ever even end up in "!PAGE_ALIGNED(addr) &&
> PAGE_ALIGNED(next)"? As the comment correctly indicates, it would only make
> sense for "freeing vmemmap pages".
> 
> This would mean we are removing parts of a vmemmap page (4k), calling
> vmemmap_free()->remove_pagetable() on sub-page granularity.
> 
> Even sub-sections (2MB - 512 pages) have a memmap size with base pages:
> - 56 bytes: 7 pages
> - 64 bytes: 8 pages
> - 72 bytes: 9 pages
> 
> sizeof(struct page) is always multiples of 8 bytes, so that will hold.
> 
> E.g., in __populate_section_memmap(), we already enforce proper subsection
> alignment.
> 
> 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.

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).

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?

-- 
Oscar Salvador
SUSE L3