khugepaged: collapse_pte_mapped_thp() flush the right range
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Message ID alpine.LSU.2.11.2008021204390.27773@eggly.anvils
State New
Headers show
Series
  • khugepaged: collapse_pte_mapped_thp() flush the right range
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Commit Message

Hugh Dickins Aug. 2, 2020, 7:12 p.m. UTC
pmdp_collapse_flush() should be given the start address at which the huge
page is mapped, haddr: it was given addr, which at that point has been
used as a local variable, incremented to the end address of the extent.

Found by source inspection while chasing a hugepage locking bug, which
I then could not explain by this. At first I thought this was very bad;
then saw that all of the page translations that were not flushed would
actually still point to the right pages afterwards, so harmless; then
realized that I know nothing of how different architectures and models
cache intermediate paging structures, so maybe it matters after all -
particularly since the page table concerned is immediately freed.

Much easier to fix than to think about.

Fixes: 27e1f8273113 ("khugepaged: enable collapse pmd for pte-mapped THP")
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: stable@vger.kernel.org # v5.4+
---

 mm/khugepaged.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Comments

Kirill A. Shutemov Aug. 2, 2020, 9:07 p.m. UTC | #1
On Sun, Aug 02, 2020 at 12:12:42PM -0700, Hugh Dickins wrote:
> pmdp_collapse_flush() should be given the start address at which the huge
> page is mapped, haddr: it was given addr, which at that point has been
> used as a local variable, incremented to the end address of the extent.
> 
> Found by source inspection while chasing a hugepage locking bug, which
> I then could not explain by this. At first I thought this was very bad;
> then saw that all of the page translations that were not flushed would
> actually still point to the right pages afterwards, so harmless; then
> realized that I know nothing of how different architectures and models
> cache intermediate paging structures, so maybe it matters after all -
> particularly since the page table concerned is immediately freed.
> 
> Much easier to fix than to think about.
> 
> Fixes: 27e1f8273113 ("khugepaged: enable collapse pmd for pte-mapped THP")
> Signed-off-by: Hugh Dickins <hughd@google.com>
> Cc: stable@vger.kernel.org # v5.4+

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

Patch
diff mbox series

--- 5.8-rc7/mm/khugepaged.c	2020-07-26 16:58:02.189038680 -0700
+++ linux/mm/khugepaged.c	2020-08-02 10:48:59.890925896 -0700
@@ -1502,7 +1502,7 @@  void collapse_pte_mapped_thp(struct mm_s
 
 	/* step 4: collapse pmd */
 	ptl = pmd_lock(vma->vm_mm, pmd);
-	_pmd = pmdp_collapse_flush(vma, addr, pmd);
+	_pmd = pmdp_collapse_flush(vma, haddr, pmd);
 	spin_unlock(ptl);
 	mm_dec_nr_ptes(mm);
 	pte_free(mm, pmd_pgtable(_pmd));