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Message-ID: <d51a7163-b082-d43a-1d0d-13ebf8cb538d@huawei.com>
Date: Tue, 6 Dec 2022 17:37:31 +0800
From: Miaohe Lin <linmiaohe@...wei.com>
To: David Hildenbrand <david@...hat.com>,
<linux-kernel@...r.kernel.org>
CC: Andrew Morton <akpm@...ux-foundation.org>,
Hugh Dickins <hughd@...gle.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
David Rientjes <rientjes@...gle.com>,
Shakeel Butt <shakeelb@...gle.com>,
John Hubbard <jhubbard@...dia.com>,
Jason Gunthorpe <jgg@...dia.com>,
Mike Kravetz <mike.kravetz@...cle.com>,
Mike Rapoport <rppt@...ux.ibm.com>,
Yang Shi <shy828301@...il.com>,
"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
Matthew Wilcox <willy@...radead.org>,
Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
Michal Hocko <mhocko@...nel.org>,
Nadav Amit <namit@...are.com>, Rik van Riel <riel@...riel.com>,
Roman Gushchin <guro@...com>,
Andrea Arcangeli <aarcange@...hat.com>,
Peter Xu <peterx@...hat.com>,
Donald Dutile <ddutile@...hat.com>,
Christoph Hellwig <hch@....de>,
Oleg Nesterov <oleg@...hat.com>, Jan Kara <jack@...e.cz>,
Liang Zhang <zhangliang5@...wei.com>,
Pedro Gomes <pedrodemargomes@...il.com>,
Oded Gabbay <oded.gabbay@...il.com>, <linux-mm@...ck.org>
Subject: Re: [PATCH v4 12/17] mm: remember exclusively mapped anonymous pages
with PG_anon_exclusive
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
>
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