From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-182.mta0.migadu.com (out-182.mta0.migadu.com [91.218.175.182]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id C194E25BEF2 for ; Tue, 14 Oct 2025 15:01:33 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.182 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1760454096; cv=none; b=tOFNqWj9bAYCWuTSrmspW3lB78meUv44Q0XbOdLzxe7OBvcY1wf7XIhoIwg4nGdzUemFIWAIfoN70O9FuxkHhJKw4PFWRtiw9QbCH5tnsYVShCOXIs6CqKtJV9pZ5fwhA0qxL6idHhubN7RvwsMyTgw8E/j3o9UqkgRf6/h8Ywk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1760454096; c=relaxed/simple; bh=mhe+ISGY1+JYt+BLY6+x29jcecp3fFER7VeOt+v8I3E=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=CZ9tHmli32EPFIQ+JMBqdJPkmEIthCf3hT+5waSv/ziCoinxruSb1C07nHglzdbvsgwXOyi/RKuLVPbtwWv9A0cyawYj263lGdmg80PBeumJVT5O9XX0bozbmvLx7qSSwH75vCz7feiPFWWmF1t7QJfDDtFU018JTYKdxSTNW68= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=mc3BGk1l; arc=none smtp.client-ip=91.218.175.182 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="mc3BGk1l" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1760454091; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=AB9j6O3TPji8YNEzc+bgFaMn1t+j1zQSsTJC9AP4fx4=; b=mc3BGk1laLOWzAF5w/uZV0flGEZx0baY7aeoTRTSkf+Blbng98lexfcRFo5vWyJFzc98eP 0pkpMSXcdutVbpE9GhGemQE1cLazHEkR0wheL8UQ2RfZeOpZEQSyeMDgmlpQEX2+LUQaC+ WdXJX3Hr31s7/2r7jIDxnIUFzo2+4A4= Date: Tue, 14 Oct 2025 23:01:13 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH mm-new v3 1/1] mm/khugepaged: abort collapse scan on non-swap entries Content-Language: en-US To: Lorenzo Stoakes Cc: akpm@linux-foundation.org, david@redhat.com, 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> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Lance Yang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT 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))) { [...] } [...]} ``` > >> >>> >>>> + result = SCAN_PTE_NON_PRESENT; >>>> + goto out_unmap; >>>> + } >>>> + >>>> ++unmapped; >>>> if (!cc->is_khugepaged || >>>> unmapped <= khugepaged_max_ptes_swap) { >>>> @@ -1290,7 +1311,7 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm, >>>> * enabled swap entries. Please see >>>> * comment below for pte_uffd_wp(). >>>> */ >>>> - if (pte_swp_uffd_wp_any(pteval)) { >>>> + if (pte_swp_uffd_wp(pteval)) { >>> >>> Again you're assuming it's a swap entry but you're not asserting this is a swap >>> entry in this branch? >> >> As we discussed above, the non_swap_entry() check has already kicked out >> anything that isn't a true swap entry, right? > > This is a different function? > > Actually I'm mistaken here I think - you check in the code above: > > if (is_swap_pte(pteval)) { > ... > } > > So this is fine, please ignore sorry :) No worries at all, thanks for double-checking and clarifying! > >> >>> >>> Also an aside - I hate, hate, hate how this uffd wp stuff has infiltrated all >>> kinds of open-coded stuff. It's so gross (not your fault, just a general >>> comment...) >> >> Haha, tell me about it. No argument from me there ;) > > :) > >> >> Thanks, >> Lance > > Cheers, Lorenzo