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From: Hugh Dickins <hughd@google.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Song Liu <songliubraving@fb.com>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: [PATCH] khugepaged: khugepaged_test_exit() check mmget_still_valid()
Date: Sun, 2 Aug 2020 12:18:35 -0700 (PDT)
Message-ID: <alpine.LSU.2.11.2008021217020.27773@eggly.anvils> (raw)
In-Reply-To: <alpine.LSU.2.11.2008021204390.27773@eggly.anvils>

Move collapse_huge_page()'s mmget_still_valid() check into
khugepaged_test_exit() itself.  collapse_huge_page() is used for anon
THP only, and earned its mmget_still_valid() check because it inserts
a huge pmd entry in place of the page table's pmd entry; whereas
collapse_file()'s retract_page_tables() or collapse_pte_mapped_thp()
merely clears the page table's pmd entry.  But core dumping without
mmap lock must have been as open to mistaking a racily cleared pmd
entry for a page table at physical page 0, as exit_mmap() was.  And
we certainly have no interest in mapping as a THP once dumping core.

Fixes: 59ea6d06cfa9 ("coredump: fix race condition between collapse_huge_page() and core dumping")
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: stable@vger.kernel.org # v4.8+
---
Note on stable backporting: the coredump fix was backported as far as
v3.16, but this extension only needed where shmem or file THP supported.

 mm/khugepaged.c |    5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

--- 5.8-rc7/mm/khugepaged.c	2020-07-26 16:58:02.189038680 -0700
+++ linux/mm/khugepaged.c	2020-08-02 10:56:11.105617241 -0700
@@ -431,7 +431,7 @@ static void insert_to_mm_slots_hash(stru
 
 static inline int khugepaged_test_exit(struct mm_struct *mm)
 {
-	return atomic_read(&mm->mm_users) == 0;
+	return atomic_read(&mm->mm_users) == 0 || !mmget_still_valid(mm);
 }
 
 static bool hugepage_vma_check(struct vm_area_struct *vma,
@@ -1100,9 +1100,6 @@ static void collapse_huge_page(struct mm
 	 * handled by the anon_vma lock + PG_lock.
 	 */
 	mmap_write_lock(mm);
-	result = SCAN_ANY_PROCESS;
-	if (!mmget_still_valid(mm))
-		goto out;
 	result = hugepage_vma_revalidate(mm, address, &vma);
 	if (result)
 		goto out;

  parent reply index

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-02 19:12 [PATCH] khugepaged: collapse_pte_mapped_thp() flush the right range Hugh Dickins
2020-08-02 19:15 ` [PATCH] khugepaged: collapse_pte_mapped_thp() protect the pmd lock Hugh Dickins
2020-08-02 21:23   ` Kirill A. Shutemov
2020-08-02 19:16 ` [PATCH] khugepaged: retract_page_tables() remember to test exit Hugh Dickins
2020-08-02 21:44   ` Kirill A. Shutemov
2020-08-03  0:35     ` Hugh Dickins
2020-08-03  8:59       ` Kirill A. Shutemov
2020-08-02 19:18 ` Hugh Dickins [this message]
2020-08-02 21:07 ` [PATCH] khugepaged: collapse_pte_mapped_thp() flush the right range Kirill A. Shutemov

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