On Wed, 28 Apr 2021 18:50:26 -0400 Peter Xu <peterx@xxxxxxxxxx> wrote:
> We tried to do something similar in b569a1760782 ("userfaultfd: wp: drop
> _PAGE_UFFD_WP properly when fork") previously, but it's not doing it all
> right.. A few fixes around the code path:
>
> 1. We were referencing VM_UFFD_WP vm_flags on the _old_ vma rather than the
> new vma. That's overlooked in b569a1760782, so it won't work as expected.
> Thanks to the recent rework on fork code (7a4830c380f3a8b3), we can easily
> get the new vma now, so switch the checks to that.
>
> 2. Dropping the uffd-wp bit in copy_huge_pmd() could be wrong if the huge pmd
> is a migration huge pmd. When it happens, instead of using pmd_uffd_wp(),
> we should use pmd_swp_uffd_wp(). The fix is simply to handle them separately.
>
> 3. Forget to carry over uffd-wp bit for a write migration huge pmd entry.
> This also happens in copy_huge_pmd(), where we converted a write huge
> migration entry into a read one.
>
> 4. In copy_nonpresent_pte(), drop uffd-wp if necessary for swap ptes.
>
> 5. In copy_present_page() when COW is enforced when fork(), we also need to
> pass over the uffd-wp bit if VM_UFFD_WP is armed on the new vma, and when
> the pte to be copied has uffd-wp bit set.
>
> Remove the comment in copy_present_pte() about this. It won't help a huge lot
> to only comment there, but comment everywhere would be an overkill. Let's
> assume the commit messages would help.
>
This run afoul of Alistair's "mm: Device exclusive memory access",
https://lkml.kernel.org/r/20210524132725.12697-8-apopple@xxxxxxxxxx
`vma' is now undeclared. I think this?
--- a/mm/memory.c~mm-userfaultfd-fix-uffd-wp-special-cases-for-fork-fix
+++ a/mm/memory.c
@@ -850,8 +850,8 @@ copy_nonpresent_pte(struct mm_struct *ds
* exclusive entries currently only support private writable
* (ie. COW) mappings.
*/
- VM_BUG_ON(!is_cow_mapping(vma->vm_flags));
- if (try_restore_exclusive_pte(src_mm, src_pte, vma, addr))
+ VM_BUG_ON(!is_cow_mapping(dst_vma->vm_flags));
+ if (try_restore_exclusive_pte(src_mm, src_pte, dst_vma, addr))
return -EBUSY;
return -ENOENT;
}
_
