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

[Naoya Horiguchi <naoya.horiguchi@linux.dev>]

On Fri, Jan 13, 2023 at 09:16:23AM -0600, Sidhartha Kumar wrote:
> 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.

Yes, the above code and the reason totally make sense to me.
Thanks for the patch and the update.

- Naoya Horiguchif