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
