Patch "khugepaged: khugepaged_test_exit() check mmget_still_valid()" has been added to the 5.8-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    khugepaged: khugepaged_test_exit() check mmget_still_valid()

to the 5.8-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     khugepaged-khugepaged_test_exit-check-mmget_still_va.patch
and it can be found in the queue-5.8 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 458cad9ca976879fa071e4c0d892978125eb715f
Author: Hugh Dickins <hughd@xxxxxxxxxx>
Date:   Thu Aug 6 23:26:25 2020 -0700

    khugepaged: khugepaged_test_exit() check mmget_still_valid()
    
    [ Upstream commit bbe98f9cadff58cdd6a4acaeba0efa8565dabe65 ]
    
    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@xxxxxxxxxx>
    Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
    Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
    Cc: Song Liu <songliubraving@xxxxxx>
    Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
    Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
    Cc: <stable@xxxxxxxxxxxxxxx>    [4.8+]
    Link: http://lkml.kernel.org/r/alpine.LSU.2.11.2008021217020.27773@eggly.anvils
    Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 1d6a9b0b6a9fd..dd592ea9a4a06 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -431,7 +431,7 @@ static void insert_to_mm_slots_hash(struct mm_struct *mm,
 
 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_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;