Messages in this thread | | | Date | Fri, 2 Dec 2022 18:18:58 +0100 | From | David Hildenbrand <> | Subject | Re: [PATCH v3 1/2] mm/migrate: Fix read-only page got writable when recover pte |
| |
On 01.12.22 23:30, Andrew Morton wrote: > On Thu, 1 Dec 2022 16:42:52 +0100 David Hildenbrand <david@redhat.com> wrote: > >> On 01.12.22 16:28, Peter Xu wrote: >>> >>> I didn't reply here because I have already replied with the question in >>> previous version with a few attempts. Quotting myself: >>> >>> https://lore.kernel.org/all/Y3KgYeMTdTM0FN5W@x1n/ >>> >>> 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. >>> >>> I've also got another longer email trying to explain why I think it's the >>> other way round to be justfied, rather than justifying removal of the write >>> bit for a read migration entry, here: >>> >> >> And I disagree for this patch that is supposed to fix this hunk: >> >> >> @@ -243,11 +243,15 @@ static bool remove_migration_pte(struct page *page, struct vm_area_struct *vma, >> entry = pte_to_swp_entry(*pvmw.pte); >> if (is_write_migration_entry(entry)) >> pte = maybe_mkwrite(pte, vma); >> + else if (pte_swp_uffd_wp(*pvmw.pte)) >> + pte = pte_mkuffd_wp(pte); >> >> if (unlikely(is_zone_device_page(new))) { >> if (is_device_private_page(new)) { >> entry = make_device_private_entry(new, pte_write(pte)); >> pte = swp_entry_to_pte(entry); >> + if (pte_swp_uffd_wp(*pvmw.pte)) >> + pte = pte_mkuffd_wp(pte); >> } >> } > > David, I'm unclear on what you mean by the above. Can you please > expand? > >> >> There is really nothing to justify the other way around here. >> If it's broken fix it independently and properly backport it independenty. >> >> But we don't know about any such broken case. >> >> I have no energy to spare to argue further ;) > > This is a silent data loss bug, which is about as bad as it gets. > Under obscure conditions, fortunately. But please let's keep working > it. Let's aim for something minimal for backporting purposes. We can > revisit any cleanliness issues later. > > David, do you feel that the proposed fix will at least address the bug > without adverse side-effects?
Just to answer that question clearly: it will fix this bug, but it's likely that other similar bugs remain (suspecting NUMA hinting).
Adverse side effect will be that some PTEs that could we writable won't be writable. I assume it's not too bad in practice for this particular case.
I proposed an alternative fix and identified other possible broken cases. Again, I don't NAK this patch as is, it just logically doesn't make sense to me to handle this case differently to the other vma->vm_page_prot users. (more details in the other thread)
-- Thanks,
David / dhildenb
| |