On 2022/12/6 16:43, David Hildenbrand wrote: >>> >> >> Hi David, sorry for the late respond and a possible inconsequential question. :) > > Better late than never! Thanks for the review, independently at which time it happens :) > >> >> <snip> >> >>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c >>> index 7a71ed679853..5add8bbd47cd 100644 >>> --- a/mm/hugetlb.c >>> +++ b/mm/hugetlb.c >>> @@ -4772,7 +4772,7 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src, >>> is_hugetlb_entry_hwpoisoned(entry))) { >>> swp_entry_t swp_entry = pte_to_swp_entry(entry); >>> - if (is_writable_migration_entry(swp_entry) && cow) { >>> + if (!is_readable_migration_entry(swp_entry) && cow) { >>> /* >>> * COW mappings require pages in both >>> * parent and child to be set to read. >>> @@ -5172,6 +5172,8 @@ static vm_fault_t hugetlb_cow(struct mm_struct *mm, struct vm_area_struct *vma, >>> set_huge_ptep_writable(vma, haddr, ptep); >>> return 0; >>> } >>> + VM_BUG_ON_PAGE(PageAnon(old_page) && PageAnonExclusive(old_page), >>> + old_page); >>> /* >>> * If the process that created a MAP_PRIVATE mapping is about to >>> @@ -6169,12 +6171,17 @@ unsigned long hugetlb_change_protection(struct vm_area_struct *vma, >>> } >>> if (unlikely(is_hugetlb_entry_migration(pte))) { >>> swp_entry_t entry = pte_to_swp_entry(pte); >>> + struct page *page = pfn_swap_entry_to_page(entry); >>> - if (is_writable_migration_entry(entry)) { >>> + if (!is_readable_migration_entry(entry)) { >> >> In hugetlb_change_protection(), is_writable_migration_entry() is changed to !is_readable_migration_entry(), >> but >> >>> pte_t newpte; >>> - entry = make_readable_migration_entry( >>> - swp_offset(entry)); >>> + if (PageAnon(page)) >>> + entry = make_readable_exclusive_migration_entry( >>> + swp_offset(entry)); >>> + else >>> + entry = make_readable_migration_entry( >>> + swp_offset(entry)); >>> newpte = swp_entry_to_pte(entry); >>> set_huge_swap_pte_at(mm, address, ptep, >>> newpte, huge_page_size(h)); >> >> <snip> >> >>> diff --git a/mm/mprotect.c b/mm/mprotect.c >>> index b69ce7a7b2b7..56060acdabd3 100644 >>> --- a/mm/mprotect.c >>> +++ b/mm/mprotect.c >>> @@ -152,6 +152,7 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd, >>> pages++; >>> } else if (is_swap_pte(oldpte)) { >>> swp_entry_t entry = pte_to_swp_entry(oldpte); >>> + struct page *page = pfn_swap_entry_to_page(entry); >>> pte_t newpte; >>> if (is_writable_migration_entry(entry)) { >> >> In change_pte_range(), is_writable_migration_entry() is not changed to !is_readable_migration_entry(). > > Yes, and also in change_huge_pmd(), is_writable_migration_entry() stays unchanged. > >> Is this done intentionally? Could you tell me why there's such a difference? I'm confused. It's very >> kind of you if you can answer my puzzle. > > For change protection, the only relevant part is to convert writable -> readable or writable -> readable_exclusive. > > If an entry is already readable or readable_exclusive, there is nothing to do. The only issues would be when turning a readable one into a readable_exclusive one or a readable_exclusive one into a readable one. > > > In hugetlb_change_protection(), the "!is_readable_migration_entry" could in fact be turned into a "is_writable_migration_entry()". Right now, it would convert writable -> readable or writable -> readable_exclusive AND readable -> readable AND readable_exclusive -> readable_exclusive, which isn't necessary but also shouldn't hurt either. Many thanks for your explanation. It's really helpful. :) > > > So yeah, it's not consistent but shouldn't be problematic. Do you see an issue with that? No, I don't see any issue with that. I just wonder whether we can change "!is_readable_migration_entry" to "is_writable_migration_entry()" to make code more consistent and avoid possible future puzzle. Also we can further remove this harmless unnecessary migration entry conversion. But this should be a separate cleanup patch anyway. > > It would be great to extend the "selftest/vm cow" test to also cover migration entries, however, that requires slightly more work and "luck" to fork() while migration is happening. > > Thanks! Many thanks for your work! Thanks, Miaohe Lin >