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

[Kefeng Wang]

On 2023/8/4 10:42, Zi Yan wrote:
> On 3 Aug 2023, at 21:45, Kefeng Wang wrote:
>
> > 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?
>
> Right. It will prevent the kernel from trying to isolate the same hugetlb page
> twice when two pages are in the same hugetlb folio. But looking at the
> code, if you try to isolate an already-isolated hugetlb folio, isolate_hugetlb()
> would return false, no error would show up. But it changes err value
> from -EACCES to -EBUSY and user will see a different page status than before.


When check man[1], the current -EACCES is not right, -EBUSY is not
precise but more suitable for this scenario,

 	-EACCES
              The page is mapped by multiple processes and can be moved
              only if MPOL_MF_MOVE_ALL is specified.

       -EBUSY The page is currently busy and cannot be moved.  Try again
              later.  This occurs if a page is undergoing I/O or another
              kernel subsystem is holding a reference to the page.
	-ENOENT
              The page is not present.

> I wonder why we do not have follow_folio() and returns -ENOENT error pointer
> when addr points to a non head page. It would make this patch more folio if
> follow_folio() can be used in place of follow_page(). One caveat is that
> user will see -ENOENT instead of -EACCES after this change.

-ENOENT is ok, but maybe the man need to be updated too.


	
[1] https://man7.org/linux/man-pages/man2/move_pages.2.html






> --
> Best Regards,
> Yan, Zi