On Wed, Jan 25, 2023 at 7:54 AM Peter Xu <peterx@xxxxxxxxxx> wrote:
>
> On Wed, Jan 25, 2023 at 07:26:49AM -0800, James Houghton wrote:
> > > At first thought this seems bad. However, I believe this has been the
> > > behavior since hugetlb PMD sharing was introduced in 2006 and I am
> > > unaware of any reported issues. I did a audit of code looking at
> > > mapcount. In addition to the above issue with smaps, there appears
> > > to be an issue with 'migrate_pages' where shared pages could be migrated
> > > without appropriate privilege.
> > >
> > > /* With MPOL_MF_MOVE, we migrate only unshared hugepage. */
> > > if (flags & (MPOL_MF_MOVE_ALL) ||
> > > (flags & MPOL_MF_MOVE && page_mapcount(page) == 1)) {
> > > if (isolate_hugetlb(page, qp->pagelist) &&
> > > (flags & MPOL_MF_STRICT))
> > > /*
> > > * Failed to isolate page but allow migrating pages
> > > * which have been queued.
> > > */
> > > ret = 1;
> > > }
> >
> > This isn't the exact same problem you're fixing Mike, but I want to
> > point out a related problem.
> >
> > This is the generic-mm-equivalent of the hugetlb code above:
> >
> > static int migrate_page_add(struct page *page, struct list_head
> > *pagelist, unsigned long flags)
> > {
> > struct page *head = compound_head(page);
> > /*
> > * Avoid migrating a page that is shared with others.
> > */
> > if ((flags & MPOL_MF_MOVE_ALL) || page_mapcount(head) == 1) {
> > if (!isolate_lru_page(head)) {
> > list_add_tail(&head->lru, pagelist);
> > mod_node_page_state(page_pgdat(head),
> > NR_ISOLATED_ANON + page_is_file_lru(head),
> > thp_nr_pages(head));
> > ...
> > }
> >
> > If you have a partially PTE-mapped THP, page_mapcount(head) will not
> > accurately determine if a page is mapped in multiple VMAs or not (it
> > only tells you how many times the head page is mapped).
> >
> > For example...
> > 1) You could have the THP PMD-mapped in one VMA, and then one tail
> > page of the THP can be mapped in another. page_mapcount(head) will be
> > 1.
> > 2) You could have two VMAs map two separate tail pages of the THP, in
> > which case page_mapcount(head) will be 0.
> >
> > I bring this up because we have the same problem with HugeTLB
> > high-granularity mapping.
>
> Maybe a better match here is total_mapcount() rather than page_mapcount()
> (despite the overheads on the sub-page loop)?
This would kind of fix the problem, but it would be too conservative now. :)
In both example 1 and 2 above, total_mapcount(head) for both would be
2, so that's ok. But now consider: you have one VMA that is
PTE-mapping two pieces of the same THP. total_mapcount(head) is still
2, even though only a single VMA is mapping the page.
- James
