Re: [PATCH v3 1/3] mm/migrate_device.c: Flush TLB while holding PTL

Alistair Popple [off-list ref] writes:

"Huang, Ying" [off-list ref] writes:

quoted

Alistair Popple [off-list ref] writes:

quoted

When clearing a PTE the TLB should be flushed whilst still holding the
PTL to avoid a potential race with madvise/munmap/etc. For example
consider the following sequence:

  CPU0                          CPU1
  ----                          ----

  migrate_vma_collect_pmd()
  pte_unmap_unlock()
                                madvise(MADV_DONTNEED)
                                -> zap_pte_range()
                                pte_offset_map_lock()
                                [ PTE not present, TLB not flushed ]
                                pte_unmap_unlock()
                                [ page is still accessible via stale TLB ]
  flush_tlb_range()

In this case the page may still be accessed via the stale TLB entry
after madvise returns. Fix this by flushing the TLB while holding the
PTL.

Signed-off-by: Alistair Popple <apopple@nvidia.com>
Reported-by: Nadav Amit <redacted>
Fixes: 8c3328f1f36a ("mm/migrate: migrate_vma() unmap page from vma while collecting pages")
Cc: <redacted>

---

Changes for v3:

 - New for v3
---
 mm/migrate_device.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index 27fb37d..6a5ef9f 100644
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c

@@ -254,13 +254,14 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
 		migrate->dst[migrate->npages] = 0;
 		migrate->src[migrate->npages++] = mpfn;
 	}
-	arch_leave_lazy_mmu_mode();
-	pte_unmap_unlock(ptep - 1, ptl);

 	/* Only flush the TLB if we actually modified any entries */
 	if (unmapped)
 		flush_tlb_range(walk->vma, start, end);

It appears that we can increase "unmapped" only if ptep_get_and_clear()
is used?

In other words you mean we only need to increase unmapped if pte_present
&& !anon_exclusive?

Agree, that's a good optimisation to make. However I'm just trying to
solve a data corruption issue (not dirtying the page) here, so will post
that as a separate optimisation patch. Thanks.

OK.  Then the patch looks good to me.  Feel free to add my,

Reviewed-by: "Huang, Ying" <redacted>

Best Regards,
Huang, Ying

quoted

quoted

+	arch_leave_lazy_mmu_mode();
+	pte_unmap_unlock(ptep - 1, ptl);
+
 	return 0;
 }


base-commit: ffcf9c5700e49c0aee42dcba9a12ba21338e8136