On 15.11.22 18:22, David Hildenbrand wrote:
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:
- 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".
Forgot to reply to that part:
No it isn't natural. But sneaking such a change into your fix seems wrong. Touching !uffd-wp code should be separate, if we want to do this at all (as we discussed, maybe the better/cleaner approach is to eliminate writable migration entries if possible).