Re: [PATCH mm-unstable v2 8/8] mm/hugetlb: convert demote_free_huge_page to folios

[Sidhartha Kumar]

On 1/10/23 6:00 PM, Mike Kravetz wrote:
> On 01/10/23 21:40, Matthew Wilcox wrote:
> > On Tue, Jan 10, 2023 at 03:28:21PM -0600, Sidhartha Kumar wrote:
> > > @@ -3505,6 +3505,7 @@ static int demote_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed)
> > >   {
> > >   	int nr_nodes, node;
> > >   	struct page *page;
> > > +	struct folio *folio;
> > >   
> > >   	lockdep_assert_held(&hugetlb_lock);
> > >   
> > > @@ -3518,8 +3519,8 @@ static int demote_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed)
> > >   		list_for_each_entry(page, &h->hugepage_freelists[node], lru) {
> > >   			if (PageHWPoison(page))
> > >   				continue;
> > > -
> > > -			return demote_free_huge_page(h, page);
> > > +			folio = page_folio(page);
> > > +			return demote_free_hugetlb_folio(h, folio);
> > >   		}
> > >   	}
> >
> > Can't this be
> > 		list_for_each_entry(folio, &h->hugepage_freelists[node], lru)
> >
> > which avoids the call to page_folio() here.
> >
> > I think the call to PageHWPoison is actually wrong here.  That would
> > only check the hwpoison bit on the first page, whereas we want to know
> > about the hwpoison bit on any page (don't we?)  So this should be
> > folio_test_has_hwpoisoned()?
> >
> > Or is that a THP-thing that is different for hugetlb pages?
>
> I believe it is different for hugetlb pages.  See hugetlb_set_page_hwpoison()
> where it sets PageHWPoison on head page as well as allocating a raw_hwp_page

I agree, this line in hugetlb_set_page_hwpoison (which is now 
folio_set_hugetlb_hwpoison) sets the HWPoison flag on the head page

int ret = folio_test_set_hwpoison(folio) ? -EHWPOISON : 0;

so the correct code in demote_pool_huge_page() would be:

	list_for_each_entry(folio, &h->hugepage_freelists[node], lru) {
			if (folio_test_hwpoison(folio))
				continue;
			return demote_free_hugetlb_folio(h, folio);
		}


> to track the actual page with poison.  Note that we can not directly flag
> hugetlb 'subpages' because we may not have the struct pages due to vmemmap
> optimization.  Adding Naoya just to be sure.
>
> Do agree that this could be list_for_each_entry(folio ...

Will make this change in v3.

Thanks,
Sidhartha Kumar