[PATCH] mm/hugetlb: Do not call vma_add_reservation upon ENOMEM

[Oscar Salvador]

sysbot reported a splat [1] on __unmap_hugepage_range().
This is because vma_needs_reservation() can return -ENOMEM if
allocate_file_region_entries() fails to allocate the file_region struct for
the reservation.
Check for that and do not call vma_add_reservation() if that is the case,
otherwise region_abort() and region_del() will see that we do not have any
file_regions.

If we detect that vma_needs_reservation() returned -ENOMEM, we clear the
hugetlb_restore_reserve flag as if this reservation was still consumed,
so free_huge_folio() will not increment the resv count.

[1] https://lore.kernel.org/linux-mm/0000000000004096100617c58d54@xxxxxxxxxx/T/#ma5983bc1ab18a54910da83416b3f89f3c7ee43aa

Fixes: df7a6d1f6405 ("mm/hugetlb: restore the reservation if needed")
Reported-and-tested-by: syzbot+d3fe2dc5ffe9380b714b@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://lore.kernel.org/linux-mm/0000000000004096100617c58d54@xxxxxxxxxx/
Signed-off-by: Oscar Salvador <osalvador@xxxxxxx>
---
 mm/hugetlb.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 6be78e7d4f6e..f35abff8be60 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5768,8 +5768,20 @@ void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct *vma,
 		 * do_exit() will not see it, and will keep the reservation
 		 * forever.
 		 */
-		if (adjust_reservation && vma_needs_reservation(h, vma, address))
-			vma_add_reservation(h, vma, address);
+		if (adjust_reservation) {
+			int rc = vma_needs_reservation(h, vma, address);
+
+			if (rc < 0)
+				/* Pressumably allocate_file_region_entries failed
+				 * to allocate a file_region struct. Clear
+				 * hugetlb_restore_reserve so that global reserve
+				 * count will not be incremented by free_huge_folio.
+				 * Act as if we consumed the reservation.
+				 */
+				folio_clear_hugetlb_restore_reserve(page_folio(page));
+			else if (rc)
+				vma_add_reservation(h, vma, address);
+		}
 
 		tlb_remove_page_size(tlb, page, huge_page_size(h));
 		/*
-- 
2.45.1