Re: A mapcount riddle

[Peter Xu <peterx@redhat.com>]

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