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([2a09:80c0:192:0:5dac:bf3d:c41:c3e7]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-46fb32a84edsm132343605e9.4.2025.10.14.08.33.18 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 14 Oct 2025 08:33:19 -0700 (PDT) Message-ID: Date: Tue, 14 Oct 2025 17:33:18 +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 From: David Hildenbrand 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> 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 17:12, David Hildenbrand wrote: > 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? > For the sake of progress, I assume the compiler will optimize out the additional pte_none() stuff. I absolutely, absolutely hate is_swap_pte(). To me, it makes the code more confusing that talking about something that is !present but also !none: there is something that is not an ordinary page table mapping. The underlying problem is how we hacked in non-swap into swap (and that's exactly where it gets confusing). Well, which this series is all about. So, I don't care in the end here. -- Cheers David / dhildenb