linux-kernel - [PATCH v2] mm: fix the deadlock in finish

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Message-Id: <20210721074849.57004-1-zhengqi.arch@bytedance.com>
Date:   Wed, 21 Jul 2021 15:48:49 +0800
From:   Qi Zheng <zhengqi.arch@...edance.com>
To:     akpm@...ux-foundation.org, tglx@...utronix.de, hannes@...xchg.org,
        mhocko@...nel.org, vdavydov.dev@...il.com,
        kirill.shutemov@...ux.intel.com
Cc:     linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, songmuchun@...edance.com,
        Qi Zheng <zhengqi.arch@...edance.com>, stable@...r.kernel.org
Subject: [PATCH v2] mm: fix the deadlock in finish_fault()

The commit 63f3655f9501 ("mm, memcg: fix reclaim deadlock with
writeback") fix the following ABBA deadlock by pre-allocating
the pte page table without holding the page lock.

	                                lock_page(A)
                                        SetPageWriteback(A)
                                        unlock_page(A)
  lock_page(B)
                                        lock_page(B)
  pte_alloc_one
    shrink_page_list
      wait_on_page_writeback(A)
                                        SetPageWriteback(B)
                                        unlock_page(B)

                                        # flush A, B to clear the writeback

The commit f9ce0be71d1f ("mm: Cleanup faultaround and finish_fault()
codepaths") rework the relevant code but ignore this race. This will
cause the deadlock above to appear again, so fix it.

Fixes: f9ce0be71d1f ("mm: Cleanup faultaround and finish_fault() codepaths")
Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
CC: <stable@...r.kernel.org>
---
 Changelog in v1 -> v2:
  - Update commit log

 mm/memory.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index db86558791f1..94a17a9a48ab 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4026,8 +4026,17 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
 				return ret;
 		}

-		if (unlikely(pte_alloc(vma->vm_mm, vmf->pmd)))
+		if (vmf->prealloc_pte) {
+			vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
+			if (likely(pmd_none(*vmf->pmd))) {
+				mm_inc_nr_ptes(vma->vm_mm);
+				pmd_populate(vma->vm_mm, vmf->pmd, vmf->prealloc_pte);
+				vmf->prealloc_pte = NULL;
+			}
+			spin_unlock(vmf->ptl);
+		} else if (unlikely(pte_alloc(vma->vm_mm, vmf->pmd))) {
 			return VM_FAULT_OOM;
+		}
 	}

 	/* See comment in handle_pte_fault() */
--
2.11.0