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(p200300cbc7054f0041f1185d4f9fd1c2.dip0.t-ipconnect.de. [2003:cb:c705:4f00:41f1:185d:4f9f:d1c2]) by smtp.gmail.com with ESMTPSA id z8-20020adfdf88000000b002258235bda3sm16231837wrl.61.2022.12.06.01.40.11 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 06 Dec 2022 01:40:12 -0800 (PST) Message-ID: Date: Tue, 6 Dec 2022 10:40:10 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.5.0 Subject: Re: [PATCH v4 12/17] mm: remember exclusively mapped anonymous pages with PG_anon_exclusive Content-Language: en-US To: Miaohe Lin , linux-kernel@vger.kernel.org Cc: Andrew Morton , Hugh Dickins , Linus Torvalds , David Rientjes , Shakeel Butt , John Hubbard , Jason Gunthorpe , Mike Kravetz , Mike Rapoport , Yang Shi , "Kirill A . Shutemov" , Matthew Wilcox , Vlastimil Babka , Jann Horn , Michal Hocko , Nadav Amit , Rik van Riel , Roman Gushchin , Andrea Arcangeli , Peter Xu , Donald Dutile , Christoph Hellwig , Oleg Nesterov , Jan Kara , Liang Zhang , Pedro Gomes , Oded Gabbay , linux-mm@kvack.org References: <20220428083441.37290-1-david@redhat.com> <20220428083441.37290-13-david@redhat.com> <90dd6a93-4500-e0de-2bf0-bf522c311b0c@huawei.com> <3c7fd5da-b3f8-5562-45a9-f83d7dbcdd7d@redhat.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 :) >> >>> >>> >>> >>>> 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)); >>> >>> >>> >>>> 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? :) Thanks! -- Thanks, David / dhildenb