Commit 9845cbbd authored Feb 25, 2014 by Kirill A. Shutemov Committed by Linus Torvalds Feb 25, 2014

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>

parent 01412886

mm/huge_memory.c

+6 −3

Original line number	Diff line number	Diff line
		@@ -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;
		}

mm/memory.c

+3 −11

Original line number	Diff line number	Diff line
		@@ -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)
		@@ -3742,22 +3741,15 @@ 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;
		if (!(ret & VM_FAULT_FALLBACK))
		return ret;
		} else {
		huge_pmd_set_accessed(mm, vma, address, pmd,
		orig_pmd, dirty);
		}

		return 0;
		}
		}
		}

		/* THP should already have been handled */
		BUG_ON(pmd_numa(*pmd));