Re: [PATCH] mm,memory_hotplug: Explicitly pass the head to isolate_huge_page

[Oscar Salvador <osalvador@suse.de>]

On Tue, Feb 12, 2019 at 09:33:29AM +0100, Michal Hocko wrote:
> >  
> >  		if (PageHuge(page)) {
> >  			struct page *head = compound_head(page);
> > -			pfn = page_to_pfn(head) + (1<<compound_order(head)) - 1;
> >  			if (compound_order(head) > PFN_SECTION_SHIFT) {
> >  				ret = -EBUSY;
> >  				break;
> >  			}
> 
> Why are we doing this, btw? 

I assume you are referring to:

> >                     if (compound_order(head) > PFN_SECTION_SHIFT) {
> >                             ret = -EBUSY;
> >                             break;
> >                     }

I thought it was in case we stumble upon a gigantic page, and commit
(c8721bbbdd36 mm: memory-hotplug: enable memory hotplug to handle hugepage)
confirms it.

But I am not really sure if the above condition would still hold on powerpc,
I wanted to check it but it is a bit more tricky than it is in x86_64 because
of the different hugetlb sizes.
Could it be that the above condition is not true, but still the order of that
hugetlb page goes beyond MAX_ORDER? It is something I have to check.

Anyway, I think that a safer way to check this would be using hstate_is_gigantic(),
which checks whether the order of the hstate goes beyond MAX_ORDER.
In the end, I think that all we care about is if we can get the pages to migrate
to via the buddy allocator, since gigantic pages need to use another method.

Actually, alloc_migrate_huge_page() checks for it:

<---
static struct page *alloc_migrate_huge_page(struct hstate *h, gfp_t gfp_mask,
		int nid, nodemask_t *nmask)
{

	if (hstate_is_gigantic(h))
		return NULL;
--->

Another thing is that AFAICS, as long as the memblock we try to offline contains
a gigantic page, it will not be able to be offlined.
Moreover, the -EBUSY we return in that case is not checked anywhere, although that
is not really an issue because scan_movable_pages will skip it in the next loop.

Now, this is more rambling than anything:
Maybe I am missing half of the picture, but I have been thinking for a while whether
we could do better when it comes to gigantic pages vs hotplug.
I think that we could try to migrate those in case any of the other nodes
have a spare pre-allocated gigantic page.

-- 
Oscar Salvador
SUSE L3