[PATCH 1/2] mm: hugetlb: Return -ENOSPC on memcg charge failure

[Ackerley Tng via B4 Relay]

From: Ackerley Tng <ackerleytng@xxxxxxxxxx>

When mem_cgroup_charge_hugetlb() fails with -ENOMEM, alloc_hugetlb_folio()
currently propagates this error. This results in the page fault handler
returning VM_FAULT_OOM.

Because HugeTLB allocations are high-order and use __GFP_RETRY_MAYFAIL,
they bypass the OOM killer. Returning VM_FAULT_OOM to the #PF handler
without triggering the OOM killer (or having it make progress) leads to
an infinite loop of retrying the fault.

Avoid this loop by returning -ENOSPC when charging fails, which maps to
VM_FAULT_SIGBUS, terminating the process cleanly.

Make mem_cgroup_charge_hugetlb() fault handling use a common error handling
path, the same handling used for hugetlb_cgroup_uncharge_cgroup{,_rsvd}(),
which also don't trigger the OOM killer and hence opt to terminate the
process with a SIGBUS.

Fixes: 991135774c0e0 ("memcg/hugetlb: introduce mem_cgroup_charge_hugetlb")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
---
 mm/hugetlb.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 7857728457952..01b57f6d3b804 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2823,7 +2823,6 @@ void wait_for_freed_hugetlb_folios(void)
  *
  * Return: A pointer to the allocated folio, or an ERR_PTR on failure.
  *         -ENOSPC if cgroup charging fails or no folio is available.
- *         -ENOMEM if mem cgroup charging fails.
  */
 struct folio *hugetlb_alloc_folio(struct hstate *h,
 		struct mempolicy_interpreted *mpoli, u8 alloc_flags)
@@ -2896,7 +2895,18 @@ struct folio *hugetlb_alloc_folio(struct hstate *h,
 		 * were committed to the folio and freeing the folio
 		 * would have cleared those up.
 		 */
-		return ERR_PTR(ret);
+		/*
+		 * Return -ENOSPC when this function fails to allocate
+		 * or charge a huge page. If a standard (PAGE_SIZE)
+		 * page allocation fails, the OOM killer is given a
+		 * chance to run, which may resolve the failure on
+		 * retry. However, for HugeTLB allocations, the OOM
+		 * killer is not triggered.  Returning -ENOMEM (or
+		 * anything resulting in VM_FAULT_OOM) would leak to
+		 * the #PF handler, causing it to loop indefinitely
+		 * retrying the fault.
+		 */
+		return ERR_PTR(-ENOSPC);
 	}
 
 	return folio;

-- 
2.55.0.970.g62bdec98f9-goog