[PATCH 3/4] 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")
Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
---
 mm/hugetlb.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 46e8dfffcfa5d..e42049e2c038b 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2997,7 +2997,7 @@ struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma,
 
 	if (ret == -ENOMEM) {
 		folio_put(folio);
-		return ERR_PTR(-ENOMEM);
+		goto err;
 	}
 
 	return folio;
@@ -3020,6 +3020,17 @@ struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma,
 out_end_reservation:
 	if (map_chg != MAP_CHG_ENFORCED)
 		vma_end_reservation(h, vma, addr);
+err:
+	/*
+	 * 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);
 }
 

-- 
2.55.0.795.g602f6c329a-goog