Re: [syzbot] WARNING in follow_hugetlb_page

[Minchan Kim <minchan@kernel.org>]

On Fri, May 20, 2022 at 03:56:31PM -0700, 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

Longterm pinned means the hugetlbfs page were mapped at userspace and
someone called FOLL_LONGTERM against on the page?

>             pages entirely (via unmounting hugetlbfs, I presume). In

Then, how can FS unmount successfully while something is accessing
on the page of the file in FS? (I expected FS should return -EBUSY).
Does hugetlbfs have something special?


>             this case, we now have CMA page that are long-term pinned,
>             and that's the state we want to avoid.