Re: [BUG] mm/hugetlb: possible temporary resv underflow

[Breno Leitao <leitao@debian.org>]

Hi Guillaume,

On Fri, Jul 10, 2026 at 05:02:43PM +0200, Guillaume Morin wrote:
> Hi,
> 
> We've encountered a temporary underflow in the reserved count after a fork when
> a parent unmaps a faulted hugetlb page before a child process. 
> 
> After a reservation is consumed (i.e the page is faulted), if there is a fork()
> and the parent munmaps the page before the child, the reserved count
> underflows. The count is restored when the child unmaps the page/exits. I have
> reproduced the issue in 6.12.95 and 6.18.38. I believe it's still present in
> 7.2-rc2 but didn't try to reproduce.

Thanks for the detailed report and the reproducer.

I reproduced the underflow on current linux-next (7.2.0-rc2-next-20260710)
with your program — it fires on every run. I also confirmed your
folio_mapcount() == 0 check fixes it: with it applied the underflow is gone
across many runs, and the existing hugetlb reservation selftests still pass.

> The patch that seems to have introduced the issue is df7a6d1f6405, "mm/hugetlb:
> restore the reservation if needed"
> 
> I am not sure what the best fix is but checking folio_mapcount() == 0 
> in __unmap_hugepage_range() seems to do the trick
> 
>                 adjust_reservation = false;
>  
>                 spin_lock_irq(&hugetlb_lock);
>                 if (!h->surplus_huge_pages && __vma_private_lock(vma) &&
> -                   folio_test_anon(folio)) {
> +                   folio_test_anon(folio) &&
> +                   folio_mapcount(folio) == 0) {
>                         folio_set_hugetlb_restore_reserve(page_folio(page));
>                         /* Reservation to be adjusted after the spin lock */
>                         adjust_reservation = true;

The fix looks correct to me.

> Reproducer (it only works if HugePages_Rsvd is 0 at the beginning of the test)

Would you be up for turning your reproducer into an expansion of the selftest
I've added a while ago for the fix I've added a while ago:

    tools/testing/selftests/mm/hugetlb_madv_vs_map.c

Your case is a nice complement to it, so it'd be great to have it there.