Currently there is a bug that a huge page could be stolen, and when the original owner tries to fault in it, it causes a page fault. You can achieve that by: 1) Creating a single page echo 1 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages 2) mmap() the page above with MAP_HUGETLB into (void *ptr1). * This will mark the page as reserved 3) touch the page, which causes a page fault and allocates the page * This will move the page out of the free list. * It will also unreserved the page, since there is no more free page 4) madvise(MADV_DONTNEED) the page * This will free the page, but not mark it as reserved. 5) Allocate a secondary page with mmap(MAP_HUGETLB) into (void *ptr2). * it should fail, but, since there is no more available page. * But, since the page above is not reserved, this mmap() succeed. 6) Faulting at ptr1 will cause a SIGBUS * it will try to allocate a huge page, but there is none available A full reproducer is in selftest. See https://lore.kernel.org/all/20240105155419.1939484-1-leitao@xxxxxxxxxx/ Fix this by restoring the reserved page if necessary. If the page being unmapped has HPAGE_RESV_OWNER set, and needs a reservation, set the restore_reserve flag, which will move the page from free to reserved. Suggested-by: Rik van Riel <riel@xxxxxxxxxxx> Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx> --- mm/hugetlb.c | 10 ++++++++++ 1 file changed, 10 insertions(+) diff --git a/mm/hugetlb.c b/mm/hugetlb.c index ed1581b670d4..fa2c17767e44 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -5677,6 +5677,16 @@ void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct *vma, hugetlb_count_sub(pages_per_huge_page(h), mm); hugetlb_remove_rmap(page_folio(page)); + if (is_vma_resv_set(vma, HPAGE_RESV_OWNER) && + vma_needs_reservation(h, vma, start)) { + /* + * Restore the reservation if needed, otherwise the + * backing page could be stolen by someone. + */ + folio_set_hugetlb_restore_reserve(page_folio(page)); + vma_add_reservation(h, vma, address); + } + spin_unlock(ptl); tlb_remove_page_size(tlb, page, huge_page_size(h)); /* -- 2.34.1
