On Mon, 31 Aug 2020, Jann Horn wrote:

On Mon, Aug 31, 2020 at 8:07 AM Hugh Dickins [off-list ref] wrote:

quoted

On Thu, 27 Aug 2020, Jann Horn wrote:

quoted

The preceding patches have ensured that core dumping properly takes the
mmap_lock. Thanks to that, we can now remove mmget_still_valid() and all
its users.

Hi Jann, while the only tears to be shed over losing mmget_still_valid()
will be tears of joy, I think you need to explain why you believe it's
safe to remove the instance in mm/khugepaged.c: which you'll have found
I moved just recently, to cover an extra case (sorry for not Cc'ing you).

quoted

--- 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 || !mmget_still_valid(mm);
+     return atomic_read(&mm->mm_users) == 0;
 }

 static bool hugepage_vma_check(struct vm_area_struct *vma,

The movement (which you have correctly followed) was in
bbe98f9cadff ("khugepaged: khugepaged_test_exit() check mmget_still_valid()")
but the "pmd .. physical page 0" issue is explained better in its parent
18e77600f7a1 ("khugepaged: retract_page_tables() remember to test exit")

I think your core dumping is still reading the page tables without
holding mmap_lock

Where? get_dump_page() takes mmap_lock now:
<https://lore.kernel.org/lkml/20200827114932.3572699-7-jannh@google.com/ (local)>

Right, sorry for the noise, that's precisely what 6/7 is all about,
and properly declares itself there in its Subject - I plead that I
got distracted by the vma snapshot part of the series, and paid too
little attention before bleating.

Looks good to me - thanks.

I don't think there should be any paths into __get_user_pages() left
that don't hold the mmap_lock. Actually, we should probably try
sticking mmap_assert_locked() in there now as a follow-up?

Maybe: I haven't given it thought, to be honest.

Hugh