Re: [syzbot] WARNING in follow_hugetlb_page

John Hubbard
2022-05-14T02:55:16Z

On 5/13/22 17:56, John Hubbard wrote:
> On 5/13/22 17:26, Minchan Kim wrote:
>> Anything else further can we get insight from the warning?
>>
>> For example, pin_user_pages going on against a hugetlb page
>> which are concurrently running alloc_contig_range(it's
>> exported function so anyone can call randomly) so
>> alloc_contig_range changes pageblock type as MIGRATE_ISOLATE
>> under us so the hit at the warning?
>
> Well, yes. First of all, the comments above the warning that fired have
> gone a little bit stale: they claim that we can only hit the warning if
> the page refcount overflows. However, we almost certainly got here via:
>
> try_grab_folio()
>     /*
>      * Can't do FOLL_LONGTERM + FOLL_PIN gup fast path if not in a
>      * right zone, so fail and let the caller fall back to the slow
>      * path.
>      */
>     if (unlikely((flags & FOLL_LONGTERM) &&
>              !is_pinnable_page(page))) /* which we just changed */

Specifically, the recent patch effectively acted as an error injection
test, by forcing is_pinnable_page() to always return true (if CONFIG_CMA
is defined). Because: MIGRATE_CMA|MIGRATE_ISOLATE == 7, which will match
any of the MIGRATE_* enums when checked with bitwise AND.

I suspect this particular error path has not been exercised much, or if
it has, not reported here anyway. Until now.

thanks,
--
John Hubbard
NVIDIA