Re: [syzbot] WARNING in follow_hugetlb_page

Minchan Kim
2022-05-20T23:43:25Z

On Fri, May 20, 2022 at 04:31:31PM -0700, Mike Kravetz wrote:
> On 5/20/22 15:56, John Hubbard wrote:
> > On 5/20/22 15:19, Minchan Kim wrote:
> >> The memory offline would be an issue so we shouldn't allow pinning of any
> >> pages in *movable zone*.
> >>
> >> Isn't alloc_contig_range just best effort? Then, it wouldn't be a big
> >> problem to allow pinning on those area. The matter is what target range
> >> on alloc_contig_range is backed by CMA or movable zone and usecases.
> >>
> >> IOW, movable zone should be never allowed. But CMA case, if pages
> >> are used by normal process memory instead of hugeTLB, we shouldn't
> >> allow longterm pinning since someone can claim those memory suddenly.
> >> However, we are fine to allow longterm pinning if the CMA memory
> >> already claimed and mapped at userspace(hugeTLB case IIUC).
> >>
> >
> > From Mike's comments and yours, plus a rather quick reading of some
> > CMA-related code in mm/hugetlb.c (free_gigantic_page(), alloc_gigantic_pages()), the following seems true:
> >
> > a) hugetlbfs can allocate pages *from* CMA, via cma_alloc()
> >
> > b) while hugetlbfs is using those CMA-allocated pages, it is debatable
> > whether those pages should be allowed to be long term pinned. That's
> > because there are two cases:
> >
> >     Case 1: pages are longterm pinned, then released, all while
> >             owned by hugetlbfs. No problem.
> >
> >     Case 2: pages are longterm pinned, but then hugetlbfs releases the
> >             pages entirely (via unmounting hugetlbfs, I presume). In
> >             this case, we now have CMA page that are long-term pinned,
> >             and that's the state we want to avoid.
>
> I do not think case 2 can happen. A hugetlb page can only be changed back
> to 'normal' (buddy) pages when ref count goes to zero.
>
> It should also be noted that hugetlb code sets up the CMA area from which
> hugetlb pages can be allocated. This area is never unreserved/freed.
>
> I do not think there is a reason to disallow long term pinning of hugetlb
> pages allocated from THE hugetlb CMA area.
>
> But, I wonder if it is possible for hugetlb pages to be allocated from
> another (non-hugetlb) area. For example if someone sets up a huge CMA area
> and hugetlb allocations spill over into that area. If this is possible
> (still need to research), then we would not want to long term pin such
> hugetlb pages. We can check this in the hugetlb code to determine if
> long term pinning is allowed.

I don't think it's possible because cma_alloc needs "struct cma" just
like handle and VM doesn't maintain any fallback list of cma chains
so unless someone could steal the handle somehow, there is no way to
claim memory others reserved for the CMA purpose.