Re: RFC for new feature to move pages from one vma to another without split

[Peter Xu]

On Wed, Apr 12, 2023 at 10:47:52AM +0200, David Hildenbrand wrote:
> > Personally it was always a mistery to me on how vm_pgoff works with
> > anonymous vmas and why it needs to be setup with vm_start >> PAGE_SHIFT.
> > 
> > Just now I tried to apply below oneliner change:
> > 
> > @@ -1369,7 +1369,7 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
> >                          /*
> >                           * Set pgoff according to addr for anon_vma.
> >                           */
> > -                       pgoff = addr >> PAGE_SHIFT;
> > +                       pgoff = 0;
> >                          break;
> >                  default:
> >                          return -EINVAL;
> > 
> > The kernel even boots without a major problem so far..
> 
> I think it's for RMAP purposes.
> 
> Take a look at linear_page_index() and how it's, for example, used in
> ksm_might_need_to_copy() alongside page->index.

>From what I read, the vma's vm_pgoff is set before setup any page->index
within the vma, while the latter will be calculated out of the vma pgoff
with linear_page_index() (in __page_set_anon_rmap()).

	folio->index = linear_page_index(vma, address);

I think I missed something, but it seems to me any comparisions between
page->index and linear_page_index() will just keep working for anonymous
even if we change vma pgoff to 0 when vma is mapped.

Do you perhaps mean this is needed for ksm only?  I really am not familiar
enough with ksm, especially when it's swapped out.  I do see that
ksm_might_need_to_copy() wants to avoid reusing a page if anon_vma is setup
not for current vma, but I don't know when it'll happen.

	if (PageKsm(page)) {
		if (page_stable_node(page) &&
		    !(ksm_run & KSM_RUN_UNMERGE))
			return page;	/* no need to copy it */
	} else if (!anon_vma) {
		return page;		/* no need to copy it */
	} else if (page->index == linear_page_index(vma, address) &&
			anon_vma->root == vma->anon_vma->root) {
		return page;		/* still no need to copy it */
	}

I think when all these paths don't trigger (aka, we need to copy) it means
there's anon_vma assigned to the page but not the right one (even though I
don't know how that could happen..).  Meanwhile I don't see either on how
vma pg_off affects this (and I assume a real KSM page ignores page->index
completely).

Thanks,

-- 
Peter Xu