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Subject: Re: [PATCH v4 12/17] mm: remember exclusively mapped anonymous pages
 with PG_anon_exclusive
To:     David Hildenbrand <david@redhat.com>,
        <linux-kernel@vger.kernel.org>
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 <3c7fd5da-b3f8-5562-45a9-f83d7dbcdd7d@redhat.com>
From:   Miaohe Lin <linmiaohe@huawei.com>
Message-ID: <d51a7163-b082-d43a-1d0d-13ebf8cb538d@huawei.com>
Date:   Tue, 6 Dec 2022 17:37:31 +0800
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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

>