On 2022/12/6 17:40, David Hildenbrand wrote:
> On 06.12.22 10:37, Miaohe Lin wrote:
>> 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.
>
> Want to send a patch? :)
Queued in my todo list. ;)
Thanks!
Miaohe Lin
