mm, thp: fix infinite loop on memcg OOM

Masayoshi Mizuma reported a bug with the hang of an application under
the memcg limit.  It happens on write-protection fault to huge zero page

If we successfully allocate a huge page to replace zero page but hit the
memcg limit we need to split the zero page with split_huge_page_pmd()
and fallback to small pages.

The other part of the problem is that VM_FAULT_OOM has special meaning
in do_huge_pmd_wp_page() context.  __handle_mm_fault() expects the page
to be split if it sees VM_FAULT_OOM and it will will retry page fault
handling.  This causes an infinite loop if the page was not split.

do_huge_pmd_wp_zero_page_fallback() can return VM_FAULT_OOM if it failed
to allocate one small page, so fallback to small pages will not help.

The solution for this part is to replace VM_FAULT_OOM with
VM_FAULT_FALLBACK is fallback required.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reported-by: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 9 insertions, 14 deletions

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index da23eb96779f..4df39b1bde91 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1166,8 +1166,10 @@ alloc:
                 } else {
                         ret = do_huge_pmd_wp_page_fallback(mm, vma, address,
                                         pmd, orig_pmd, page, haddr);
-                        if (ret & VM_FAULT_OOM)
+                        if (ret & VM_FAULT_OOM) {
                                 split_huge_page(page);
+                                ret |= VM_FAULT_FALLBACK;
+                        }
                         put_page(page);
                 }
                 count_vm_event(THP_FAULT_FALLBACK);
@@ -1179,9 +1181,10 @@ alloc:
                 if (page) {
                         split_huge_page(page);
                         put_page(page);
-                }
+                } else
+                        split_huge_page_pmd(vma, address, pmd);
+                ret |= VM_FAULT_FALLBACK;
                 count_vm_event(THP_FAULT_FALLBACK);
-                ret |= VM_FAULT_OOM;
                 goto out;
         }
 
diff --git a/mm/memory.c b/mm/memory.c
index 5f2001a7ab31..22dfa617bddb 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3704,7 +3704,6 @@ static 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)
@@ -3742,20 +3741,13 @@ retry:
                         if (dirty && !pmd_write(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;
+                                if (!(ret & VM_FAULT_FALLBACK))
+                                        return ret;
                         } else {
                                 huge_pmd_set_accessed(mm, vma, address, pmd,
                                                       orig_pmd, dirty);
+                                return 0;
                         }
-
-                        return 0;
                 }
         }