Re: [PATCH 1/4] mm: migrate: use a folio in add_page_for_migration()

[Kefeng Wang]

On 2023/8/3 20:30, Matthew Wilcox wrote:
> On Thu, Aug 03, 2023 at 03:13:21PM +0800, Kefeng Wang wrote:
> > On 2023/8/2 20:21, Matthew Wilcox wrote:
> > > On Wed, Aug 02, 2023 at 05:53:43PM +0800, Kefeng Wang wrote:
> > > >    	err = -EACCES;
> > > > -	if (page_mapcount(page) > 1 && !migrate_all)
> > > > -		goto out_putpage;
> > > > +	if (folio_estimated_sharers(folio) > 1 && !migrate_all)
> > > > +		goto out_putfolio;
> > >
> > > I do not think this is the correct change.  Maybe leave this line
> > > alone.
> >
> > Ok, I am aware of the discussion about this in other mail, will not
> > change it(also the next two patch about this function), or wait the
> > new work of David.
> > > > -	if (PageHuge(page)) {
> > > > -		if (PageHead(page)) {
> > > > -			isolated = isolate_hugetlb(page_folio(page), pagelist);
> > > > +	if (folio_test_hugetlb(folio)) {
> > > > +		if (folio_test_large(folio)) {
> > >
> > > This makes no sense when you read it.  All hugetlb folios are large,
> > > by definition.  Think about what this code used to do, and what it
> > > should be changed to.
> >
> > hugetlb folio is self large folio, will drop redundant check
>
> No, that's not the difference.  Keep thinking about it.  This is not
> a mechanical translation!


  if (PageHuge(page))  // page must be a hugetlb page
	if (PageHead(page)) // page must be a head page, not tail
             isolate_hugetlb() // isolate the hugetlb page if head

After using folio,

  if (folio_test_hugetlb(folio)) // only check folio is hugetlb or not

I don't check the page is head or not, since the follow_page could
return a sub-page, so the check PageHead need be retained, right?