Re: A mapcount riddle

[Mike Kravetz <mike.kravetz@oracle.com>]

On 01/25/23 09:24, Michal Hocko wrote:
> On Tue 24-01-23 12:56:24, Mike Kravetz 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;
> > 	}
> 
> Could you elaborate what is problematic about that? The whole pmd
> sharing is a cooperative thing. So if some of the processes decides to
> migrate the page then why that should be a problem for others sharing
> that page via page table? Am I missing something obvious?

Nothing obvious.  It is just that the semantics seem to be that you can
only move shared pages if you have CAP_SYS_NICE.  Certainly cooperation
is implied for shared PMDs, but I would guess that most applications are
not even aware they are sharing PMDs.

Consider a group of processes sharing a hugetlb mapping.  If the mapping
is PUD_SIZE - huge_page_size, there is no sharing of PMDs and a process
without CAP_SYS_NICE can not migrate the shared pages.  However, if nothing
else changes and the mapping size is PUD_SIZE (and appropriately aligned)
the PMDs are shared.  Should we allow a process to migrate shared pages
without CAP_SYS_NICE in this case?
-- 
Mike Kravetz