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

[David Hildenbrand <david@redhat.com>]

>> 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.
> 
> I didn't even notice merging could work with soft-dirty enabled, that's
> interesting to know.
> 
> Yes I think it's possible and I agree it's safe, as VM_SOFTDIRTY is set for
> the merged vma so afaiu the write bit is safe to set.  We get a bunch of
> false positives but that's how soft-dirty works.
> 
> I think the whole problem is easier if we see this at a higher level.
> You're discussing this from vma pov and it's fair to do so, at least I
> agree with what you mentioned so far and I can't see anything outside
> uffd-wp that can be affected.  However, it is also true when you noticed we
> already have quite a few paragraphs trying to discuss the safety for this
> and that, that's the part where I think we need justification and it's not
> that "natural".
> 
> For "natural", I meant fundamentally we're talking about page migrations
> here.  The natural way (at least to me) for page migration to happen as a
> fundamental rule is that, we leverag the migration pte to make sure the pte
> being stable so we can do the migration work, then we "recover" the pte to
> present either by a full recovery or just (hopefully) only replace the pfn,
> keeping all the rest untouched.
> 
> One thing to prove that is we have two migration entries not one (I'm
> temporarily put the exclusive read one aside since that's solving different
> problems): MIGRATION_READ and MIGRATION_WRITE.  If we only rely on vma
> flags logically we don't need MIGRATION_READ and MIGRATION_WRITE, we only
> need MIGRATION generic swap pte then we recover the write bit from vma
> flags and other things (like uffd-wp, currently we have the bit set in swap
> pte besides the swap entry type).
> 
> So maybe one day we can use two migration types rather than three
> (MIGRATION and MIGRATION_EXCLUSIVE)?  I can't tell, but hopefully that
> shows what I meant, that we need further justification to grant write bit
> only base on vma, rather than recovering write bit based on migration entry
> type.

That's precisely what I had in mind recently, and I am happy to hear 
that you have similar idea:

https://lkml.kernel.org/r/20221108174652.198904-6-david@redhat.com

"
Note that we don't optimize for the actual migration case:
(1) When migration succeeds the new PTE will not be writable because the
     source PTE was not writable (protnone); in the future we
     might just optimize that case similarly by reusing
     can_change_pte_writable()/can_change_pmd_writable() when removing
     migration PTEs.
"

Currently, "readable_migration_entry" is even wrong: it might be 
PROT_NONE and not even readable.

-- 
Thanks,

David / dhildenb