Re: [PATCH v2 1/2] mm/migrate: Fix read-only page got writable when recover pte

David Hildenbrand
2022-11-15T09:13:31Z

>>
>> Any particular reason why not to simply glue this to pte_swp_uffd_wp(),
>> because only that needs special care:
>>
>> if (pte_swp_uffd_wp(*pvmw.pte)) {
>> pte = pte_wrprotect(pte);
>> pte = pte_mkuffd_wp(pte);
>> }
>>
>>
>> And that would match what actually should have been done in commit
>> f45ec5ff16a7 -- only special-case uffd-wp.
>>
>> Note that I think there are cases where we have a PTE that was !writable,
>> but after migration we can map it writable.
>
> The thing is recovering the pte into its original form is the safest
> approach to me, so I think we need justification on why it's always safe to
> set the write bit.
>
> Or do you perhaps have solid clue and think it's always safe
The problem I am having with this broader change, is that this changes
something independent of your original patch/problem.

If we identify this to be an actual problem, it should most probably be
separate fix + backport.

My understanding is that vma->vm_page_prot always tells you what the
default PTE protection in a mapping is.

If the mapping is private, it is never writable (due to COW). Similarly,
if the shared file mapping needs writenotify, it is never writable.

I consider UFFD-wp a special case: while the default VMA protection
might state that it is writable, you actually want individual PTEs to be
write-protected and have to manually remove the protection.

softdirty tracking is another special case: however, softdirty tracking
is enabled for the whole VMA. For remove_migration_pte() that should be
fine (I guess) because writenotify is active when the VMA needs to track
softdirty bits, and consequently vma->vm_page_prot has the proper
default permissions.

I wonder if the following (valid), for example is possible:

1) clear_refs() clears VM_SOFTDIRTY and pte_wrprotect() the pte.
-> writenotify is active and vma->vm_page_prot updated accordingly

VM_SOFTDIRTY is reset due to VMA merging and vma->vm_page_prot is
updated accordingly. See mmap_region() where we set VM_SOFTDIRTY.

If you now migrate the (still write-protected in the PTE) page, it was
not writable, but it can be writable on the destination.

>
>>
>> BTW, does unuse_pte() need similar care?
>>
>> new_pte = pte_mkold(mk_pte(page, vma->vm_page_prot));
>> if (pte_swp_uffd_wp(*pte))
>> new_pte = pte_mkuffd_wp(new_pte);
>> set_pte_at(vma->vm_mm, addr, pte, new_pte);
>
> I think unuse path is fine because unuse only applies to private mappings,
> so we should always have the W bit removed there within mk_pte().

You're right, however, shmem swapping confuses me. Maybe that does not
apply here.

--
Thanks,

David / dhildenb