Re: [PATCH v2 1/2] mm: Make alloc_contig_range handle free hugetlb pages

[Oscar Salvador]

On 2021-02-19 03:10, Mike Kravetz wrote:
> Those counts will be wrong as there are no huge pages on the node.
>
> I'll think about this more tomorrow.
> Pretty sure this is an issue, but I could be wrong.  Just wanted to 
> give
> a heads up.

Yes, this is a problem, although the fixup would be to check whether we 
have any hugepages.

Nevertheless, I think we should not be touching surplus at all but 
rather make the page temporary.
I am exploring making migrate_pages() handle free hugepages as Michal 
suggested, so the approach is cleaner and we do not need extra 
functions. I yet have to see if that is feasible, as some issues come to 
my mind like the page needs to be in a list to go to migrate_pages, but 
if it is in that list, it is not in hugepages_freelist, and that could 
disrupt userspace as it could not dequeue hugepages if it demands it.
I have to check. Shoult not be possible, we can always make the page 
temporary here.

> --
> Mike Kravetz
>
> > +		}
> > +	}
> > +
> > +	return ret;
> > +}
> > +
> > +bool isolate_or_dissolve_huge_page(struct page *page)
> > +{
> > +	struct hstate *h = NULL;
> > +	struct page *head;
> > +	bool ret = false;
> > +
> > +	spin_lock(&hugetlb_lock);
> > +	if (PageHuge(page)) {
> > +		head = compound_head(page);
> > +		h = page_hstate(head);
> > +	}
> > +	spin_unlock(&hugetlb_lock);
> > +
> > +	/*
> > +	 * The page might have been dissolved from under our feet.
> > +	 * If that is the case, return success as if we dissolved it 
> > ourselves.
> > +	 */
> > +	if (!h)
> > +		return true;
> > +
> > +	/*
> > +	 * Fence off gigantic pages as there is a cyclic dependency
> > +	 * between alloc_contig_range and them.
> > +	 */
> > +	if (hstate_is_gigantic(h))
> > +		return ret;
> > +
> > +	if(!page_count(head) && alloc_and_dissolve_huge_page(h, head))
> > +		ret = true;
> > +
> > +	return ret;
> > +}
> > +
> >  struct page *alloc_huge_page(struct vm_area_struct *vma,
> >  				    unsigned long addr, int avoid_reserve)
> >  {

--
Oscar Salvador
SUSE L3