From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 9185525FA0E for ; Fri, 19 Sep 2025 04:11:49 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758255111; cv=none; b=jjSTIpbx2A61QN5NBxGyE4ZHF7beE82km6eEf8543Mv+k+I3omZ4oYr8nFSJRpHVG3c1nzYARzfH0rf1mdRbIKOJA0JUX2liVT52AnaqQGhn3ndy/OeSjxYQW9cMgwy7wo1uacXOVu0aRqGve1974j4PNeEMN6Dz61dqPAQ5O3Y= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758255111; c=relaxed/simple; bh=CmDuLxpMDxf5AchlYUIvjaFeP+19N3HTntvzKsfXsMU=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Tv/HK4DwUct7GvWZuqxoTL1H+xHyhChcYRCzUdleq09I/7fNDfu+T/eui3NLfOs5rkKtfk42i2k1vTLoPMDZ1RCuvhQ/okqM0CmJSE3GisqTLVw2QCpKkelzff9xolCakFx67sCngTnT6v+Tt9K3mRdbs6sZZ6nw7++EJKdDU6M= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 9E5AF1758; Thu, 18 Sep 2025 21:11:40 -0700 (PDT) Received: from [10.164.18.52] (MacBook-Pro.blr.arm.com [10.164.18.52]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 5D1413F673; Thu, 18 Sep 2025 21:11:38 -0700 (PDT) Message-ID: <6a6f03df-8dc4-44aa-ad25-40f51ad98266@arm.com> Date: Fri, 19 Sep 2025 09:41:35 +0530 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 v2 2/2] mm/khugepaged: abort collapse scan on guard PTEs To: David Hildenbrand , Lance Yang , akpm@linux-foundation.org, lorenzo.stoakes@oracle.com Cc: ziy@nvidia.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, baohua@kernel.org, ioworker0@gmail.com, kirill@shutemov.name, hughd@google.com, mpenttil@redhat.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20250918050431.36855-1-lance.yang@linux.dev> <20250918050431.36855-3-lance.yang@linux.dev> Content-Language: en-US From: Dev Jain In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 19/09/25 12:17 am, David Hildenbrand wrote: > On 18.09.25 07:04, Lance Yang wrote: >> From: Lance Yang >> >> Guard PTE markers are installed via MADV_GUARD_INSTALL to create >> lightweight guard regions. >> >> Currently, any collapse path (khugepaged or MADV_COLLAPSE) will fail >> when >> encountering such a range. >> >> MADV_COLLAPSE fails deep inside the collapse logic when trying to >> swap-in >> the special marker in __collapse_huge_page_swapin(). >> >> hpage_collapse_scan_pmd() >>   `- collapse_huge_page() >>       `- __collapse_huge_page_swapin() -> fails! >> >> khugepaged's behavior is slightly different due to its max_ptes_swap >> limit >> (default 64). It won't fail as deep, but it will still needlessly >> scan up >> to 64 swap entries before bailing out. >> >> IMHO, we can and should detect this much earlier. >> >> This patch adds a check directly inside the PTE scan loop. If a guard >> marker is found, the scan is aborted immediately with >> SCAN_PTE_NON_PRESENT, >> avoiding wasted work. >> >> Suggested-by: Lorenzo Stoakes >> Signed-off-by: Lance Yang >> --- >>   mm/khugepaged.c | 10 ++++++++++ >>   1 file changed, 10 insertions(+) >> >> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >> index 9ed1af2b5c38..70ebfc7c1f3e 100644 >> --- a/mm/khugepaged.c >> +++ b/mm/khugepaged.c >> @@ -1306,6 +1306,16 @@ static int hpage_collapse_scan_pmd(struct >> mm_struct *mm, >>                       result = SCAN_PTE_UFFD_WP; >>                       goto out_unmap; >>                   } >> +                /* >> +                 * Guard PTE markers are installed by >> +                 * MADV_GUARD_INSTALL. Any collapse path must >> +                 * not touch them, so abort the scan immediately >> +                 * if one is found. >> +                 */ >> +                if (is_guard_pte_marker(pteval)) { >> +                    result = SCAN_PTE_NON_PRESENT; >> +                    goto out_unmap; >> +                } > > Thinking about it, this is interesting. > > Essentially we track any non-swap swap entries towards > khugepaged_max_ptes_swap, which is rather weird. > > I think we might also run into migration entries here and hwpoison > entries? > > So what about just generalizing this: > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index af5f5c80fe4ed..28f1f4bf0e0a8 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -1293,7 +1293,24 @@ static int hpage_collapse_scan_pmd(struct > mm_struct *mm, >         for (_address = address, _pte = pte; _pte < pte + HPAGE_PMD_NR; >              _pte++, _address += 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))) { > +                               result = SCAN_PTE_NON_PRESENT; > +                               goto out_unmap; > +                       } > + >                         ++unmapped; >                         if (!cc->is_khugepaged || >                             unmapped <= khugepaged_max_ptes_swap) { > @@ -1313,18 +1330,7 @@ static int hpage_collapse_scan_pmd(struct > mm_struct *mm, >                                 goto out_unmap; >                         } >                 } > -               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; > -                       } > -               } > + >                 if (pte_uffd_wp(pteval)) { >                         /* >                          * Don't collapse the page if any of the small > > > With that, the function flow looks more similar to > __collapse_huge_page_isolate(), > except that we handle swap entries in there now. This looks good! > > > And with that in place, couldn't we factor out a huge chunk of both > scanning > functions into some helper (passing whether swap entries are allowed > or not?). > > Yes, I know, refactoring khugepaged, crazy idea. >