[patch v2] thp, memcg: split hugepage for memcg oom on cow

[David Rientjes <rientjes@google.com>]

On COW, a new hugepage is allocated and charged to the memcg.  If the
system is oom or the charge to the memcg fails, however, the fault
handler will return VM_FAULT_OOM which results in an oom kill.

Instead, it's possible to fallback to splitting the hugepage so that the
COW results only in an order-0 page being allocated and charged to the
memcg which has a higher liklihood to succeed.  This is expensive because
the hugepage must be split in the page fault handler, but it is much
better than unnecessarily oom killing a process.

Signed-off-by: David Rientjes <rientjes@google.com>
---
 mm/huge_memory.c |    3 +++
 mm/memory.c      |   18 +++++++++++++++---
 2 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -950,6 +950,8 @@ int do_huge_pmd_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
 		count_vm_event(THP_FAULT_FALLBACK);
 		ret = do_huge_pmd_wp_page_fallback(mm, vma, address,
 						   pmd, orig_pmd, page, haddr);
+		if (ret & VM_FAULT_OOM)
+			split_huge_page(page);
 		put_page(page);
 		goto out;
 	}
@@ -957,6 +959,7 @@ int do_huge_pmd_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
 
 	if (unlikely(mem_cgroup_newpage_charge(new_page, mm, GFP_KERNEL))) {
 		put_page(new_page);
+		split_huge_page(page);
 		put_page(page);
 		ret |= VM_FAULT_OOM;
 		goto out;
diff --git a/mm/memory.c b/mm/memory.c
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3489,6 +3489,7 @@ int handle_mm_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 	if (unlikely(is_vm_hugetlb_page(vma)))
 		return hugetlb_fault(mm, vma, address, flags);
 
+retry:
 	pgd = pgd_offset(mm, address);
 	pud = pud_alloc(mm, pgd, address);
 	if (!pud)
@@ -3502,13 +3503,24 @@ int handle_mm_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 							  pmd, flags);
 	} else {
 		pmd_t orig_pmd = *pmd;
+		int ret;
+
 		barrier();
 		if (pmd_trans_huge(orig_pmd)) {
 			if (flags & FAULT_FLAG_WRITE &&
 			    !pmd_write(orig_pmd) &&
-			    !pmd_trans_splitting(orig_pmd))
-				return do_huge_pmd_wp_page(mm, vma, address,
-							   pmd, orig_pmd);
+			    !pmd_trans_splitting(orig_pmd)) {
+				ret = do_huge_pmd_wp_page(mm, vma, address, pmd,
+							  orig_pmd);
+				/*
+				 * If COW results in an oom, the huge pmd will
+				 * have been split, so retry the fault on the
+				 * pte for a smaller charge.
+				 */
+				if (unlikely(ret & VM_FAULT_OOM))
+					goto retry;
+				return ret;
+			}
 			return 0;
 		}
 	}

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