Messages in this thread | | | Date | Fri, 13 May 2022 18:16:46 -0700 | Subject | Re: [syzbot] WARNING in follow_hugetlb_page | From | John Hubbard <> |
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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
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