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)?
--
Peter Xu
