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
