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([2a09:80c0:192:0:5dac:bf3d:c41:c3e7]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-426ce583424sm23814996f8f.21.2025.10.14.08.12.19 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 14 Oct 2025 08:12:19 -0700 (PDT) Message-ID: Date: Tue, 14 Oct 2025 17:12:18 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH mm-new v3 1/1] mm/khugepaged: abort collapse scan on non-swap entries To: Lance Yang , Lorenzo Stoakes Cc: akpm@linux-foundation.org, Liam.Howlett@oracle.com, baohua@kernel.org, baolin.wang@linux.alibaba.com, dev.jain@arm.com, hughd@google.com, ioworker0@gmail.com, kirill@shutemov.name, linux-kernel@vger.kernel.org, linux-mm@kvack.org, mpenttil@redhat.com, npache@redhat.com, ryan.roberts@arm.com, ziy@nvidia.com, richard.weiyang@gmail.com References: <20251008032657.72406-1-lance.yang@linux.dev> <0bfdbccd-9d4a-409f-ae43-b44bb7347d70@linux.dev> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 14.10.25 17:01, Lance Yang wrote: > > > On 2025/10/14 22:39, Lorenzo Stoakes wrote: >> On Tue, Oct 14, 2025 at 10:26:20PM +0800, Lance Yang wrote: >>> >>> >>> On 2025/10/14 19:08, Lorenzo Stoakes wrote: >>>> On Wed, Oct 08, 2025 at 11:26:57AM +0800, Lance Yang wrote: >>>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >>>>> index abe54f0043c7..bec3e268dc76 100644 >>>>> --- a/mm/khugepaged.c >>>>> +++ b/mm/khugepaged.c >>>>> @@ -1020,6 +1020,11 @@ static int __collapse_huge_page_swapin(struct mm_struct *mm, >>>>> if (!is_swap_pte(vmf.orig_pte)) >>>>> continue; >>>>> >>>>> + if (non_swap_entry(pte_to_swp_entry(vmf.orig_pte))) { >>>>> + result = SCAN_PTE_NON_PRESENT; >>>>> + goto out; >>>>> + } >>>> >>>> OK seems in line with what we were discussing before... >>> >>> Yep. That's the idea :) >>> >>>> >>>>> + >>>>> vmf.pte = pte; >>>>> vmf.ptl = ptl; >>>>> ret = do_swap_page(&vmf); >>>>> @@ -1281,7 +1286,23 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm, >>>>> for (addr = start_addr, _pte = pte; _pte < pte + HPAGE_PMD_NR; >>>>> _pte++, addr += PAGE_SIZE) { >>>>> pte_t pteval = ptep_get(_pte); >>>>> - if (is_swap_pte(pteval)) { >>>>> + if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) { >>>>> + ++none_or_zero; >>>>> + if (!userfaultfd_armed(vma) && >>>>> + (!cc->is_khugepaged || >>>>> + none_or_zero <= khugepaged_max_ptes_none)) { >>>>> + continue; >>>>> + } else { >>>>> + result = SCAN_EXCEED_NONE_PTE; >>>>> + count_vm_event(THP_SCAN_EXCEED_NONE_PTE); >>>>> + goto out_unmap; >>>>> + } >>>>> + } else if (!pte_present(pteval)) { >>>>> + if (non_swap_entry(pte_to_swp_entry(pteval))) { >>>> >>> >>> Thanks for pointing that out! >> >> You've deleted what I've said here and also not indicated whether you'll do what >> I asked :) >> >> Please be clearer... > > Oh, I didn't delete your comment at all ... It's just below ... > >> >>>>>> Hm but can't this be pte_protnone() at this stage (or something > else)? And then <-- Here! >>> >>> Yeah. The funny thing is, a protnone pte cannot actually get here, IIUC. >>> >>> ``` >>> static inline int pte_protnone(pte_t pte) >>> { >>> return (pte_flags(pte) & (_PAGE_PROTNONE | _PAGE_PRESENT)) >>> == _PAGE_PROTNONE; >>> } >>> >>> static inline int pte_present(pte_t a) >>> { >>> return pte_flags(a) & (_PAGE_PRESENT | _PAGE_PROTNONE); >>> } >>> ``` >>> >>> On x86, pte_present() returns true for a protnone pte. And I'd assume >>> other archs behave similarly ... >> >> This was one example, we may make changes in the future that result in entries >> that are non-present but also non-swap. >> >> I don't see the point in eliminating this check based on an implicit, open-coded >> assumption that this can never be the case, this is just asking for trouble. >> >>> >>>> we're just assuming pte_to_swp_entry() is operating on a swap entry when it in >>>> fact might not be? >>>> >>>> Couldn't we end up with false positives here? >>> >>> Emm, I think we're good here and the code is doing the right thing. >> >> I mean sorry but just - NO - to doing swap operations based on open-coded checks >> that you implicitly feel must imply a swap entry. >> >> This makes the code a lot more confusing, it opens us up to accidentally >> breaking things in future and has little to no benefit, I don't see why we're >> doing it. >> >> I don't think every little 'aha X must imply Y so just eliminate Z' idea need be >> implemented, this feels like a sort of 'mathematical reduction of code ignoring >> all other factors'. > > Understood. Changing !pte_present() to is_swap_pte() will resolve all your > concerns, right? > > ``` > if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) { > [...] > } else if (is_swap_pte(pteval)) { <-- Here > if (non_swap_entry(pte_to_swp_entry(pteval))) { > [...] > } > [...]} Can we please take a step back and make sure we are not starting to do stuff differently than elswehere in the kernel, please? -- Cheers David / dhildenb