Re: [syzbot] WARNING in follow_hugetlb_page

[David Hildenbrand <david@redhat.com>]

On 21.05.22 17:24, Minchan Kim wrote:
> On Fri, May 20, 2022 at 05:04:22PM -0700, Mike Kravetz wrote:
>> 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.

Hm. We primarily use CMA for gigantic pages only IIRC. Ordinary huge
pages come via the buddy.

Assume we allocated a (movable) 2MiB huge page ordinarily via the buddy
and it ended up on that CMA area by pure luck (as it's movable). If we'd
allow to pin it long-term, allocating a gigantic page from the
designated CMA area would fail.

So we'd want to allow long-term pinning a gigantic page but we'd not
want to allow long-term pinning an ordinary huge page. We'd want to
migrate the latter away.


The general rules are:

ZONE_MOVABLE: nobody is allowed to place unmovable allocations there; it
could prevent memory offlining/unplug.

CMA: nobody *but the designated owner* is allowed to place unmovable
memory there; it could prevent the actual owner to allocate contiguous
memory.

As explained above, it gets a bit weird if the owner (hugetlb) deals
with different allocation types (huge vs. gigantic pages).
>> Unless I do not understand, normal movable memory allocations can fall
>> back to CMA areas?

Yes, just like ZONE_MOVABLE IIRC.

> 
> In the case, Yes, it would be fallback if gfp_flag was __GFP_MOVABLE.
> 
> If HugeTLB support it(I think so), pin_user_pages with FOLL_LONGTERM
> will migrate the page out of movable/CMA before the longterm pinning
> so IMHO, we shouldn't have the problem.

As explained, the tricky bit would be hitting a gigantic page that's
valid to reside permanently on the designated CMA area. IIRC, some
gigantic pages are indeed movable, but we never place them on
ZONE_MOVABLE because migration is unlikely to work in practice.


-- 
Thanks,

David / dhildenb