Re: [syzbot] WARNING in follow_hugetlb_page

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

On 5/20/22 16:43, Minchan Kim wrote:
> 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.

I was thinking about the case where a hugetlb page is allocated via
__alloc_pages().  Not sure if that can fall back to a CMA area that
someone else might have created/reserved.

Unless I do not understand, normal movable memory allocations can fall
back to CMA areas?
-- 
Mike Kravetz