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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 3D9E3C001A X-Rspamd-Server: rspam11 X-Rspam-User: X-Stat-Signature: qpi4zy48yhmqppi6444qdqcd91usp1bs X-HE-Tag: 1760454680-348763 X-HE-Meta: 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 Tbawr6I4 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 14.10.25 16:39, 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... > >> >>> Hm but can't this be pte_protnone() at this stage (or something else)? And then >> >> 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'. Not sure I follow. If something is !none && !present it's what we call a swap PTE (that includes actual swap and non-swap PTEs). We have the exact same code flow for example in copy_pte_range()->copy_nonpresent_pte() and I don't see a problem with it. If we were to ever change what we call a "swap PTE" (I don't think so?) we'd have to fix stuff all over the place. Maybe I get the concern here wrong? -- Cheers David / dhildenb