On 17.02.25 20:08, Matthew Wilcox (Oracle) wrote:
The intent is to add folio_mk_pte() to remove the conversion from folio to page necessary to call mk_pte(). Eventually we might end up removing mk_pte(), but that's not what's being proposed today. I didn't want to add folio_mk_pte() to each architecture, and I didn't want to lose any optimisations that architectures have from their own implementation of mk_pte(). Fortunately, most architectures have by now turned their mk_pte() into a fairly bland variant of pfn_pte(), but s390 is different. So patch 1 hoists the optimisation of calling pte_mkdirty() from s390 to generic code. I'd appreciate some eyes on this from mm people who understand this better than I do. I originally had - if (write) + if (write || folio_test_dirty(folio)) entry = maybe_mkwrite(pte_mkdirty(entry), vma); and I think that broke COW under some circumstances that 01.org could reproduce and I couldn't.
If it's an anon folio that logic would be broken, yes (anon CoW). We do have can_change_pte_writable() that tells you when it is safe to upgrade write permissions for a PTE.
Looking at can_change_pte_writable(), I don't know if filesystems with writenotify might have a problem when setting the PTE dirty and allowing for write access, just because the folio is dirty.
So I assume that it would break fs-level CoW indeed. -- Cheers, David / dhildenb
