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b=Pm6NVL8O6W3xFUJzWVW8VNRl36Qj66avYL9r9Ll0fdFORWcLjKdDxFrOqj4NP3kO8udevd3nIWj6WS5fuulZDqwt7Cblf4kSjmPNr+v7jpdr/6hREPhpBBjl37feQjW4qrFPDGdDRFNqGdOL958HGQ6neWhIsPA+WMdg7AryscI= Authentication-Results-Original: dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=arm.com; Received: from AM9PR08MB7120.eurprd08.prod.outlook.com (2603:10a6:20b:3dc::22) by GV1PR08MB8105.eurprd08.prod.outlook.com (2603:10a6:150:96::19) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.9094.22; Sun, 14 Sep 2025 17:03:30 +0000 Received: from AM9PR08MB7120.eurprd08.prod.outlook.com ([fe80::2933:29aa:2693:d12e]) by AM9PR08MB7120.eurprd08.prod.outlook.com ([fe80::2933:29aa:2693:d12e%5]) with mapi id 15.20.9094.021; Sun, 14 Sep 2025 17:03:30 +0000 Message-ID: <750a06dc-db3d-43c6-b234-95efb393a9df@arm.com> Date: Sun, 14 Sep 2025 22:33:22 +0530 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH mm-new 3/3] mm/khugepaged: abort collapse scan on guard PTEs To: Lance Yang , akpm@linux-foundation.org, david@redhat.com, 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, linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20250914143547.27687-1-lance.yang@linux.dev> <20250914143547.27687-4-lance.yang@linux.dev> Content-Language: en-US From: Dev Jain In-Reply-To: <20250914143547.27687-4-lance.yang@linux.dev> Content-Type: text/plain; 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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 14 Sep 2025 17:04:05.3011 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 355519c1-16ab-41c9-29ae-08ddf3b0b642 X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[4.158.2.129];Helo=[outbound-uk1.az.dlp.m.darktrace.com] X-MS-Exchange-CrossTenant-AuthSource: DU6PEPF0000B61B.eurprd02.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: DU0PR08MB8954 On 14/09/25 8:05 pm, 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 a new SCAN_PTE_GUARD > status, avoiding wasted work. > > Signed-off-by: Lance Yang > --- > mm/khugepaged.c | 12 ++++++++++++ > 1 file changed, 12 insertions(+) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index e54f99bb0b57..910a6f2ec8a9 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -59,6 +59,7 @@ enum scan_result { > SCAN_STORE_FAILED, > SCAN_COPY_MC, > SCAN_PAGE_FILLED, > + SCAN_PTE_GUARD, > }; > > #define CREATE_TRACE_POINTS > @@ -1317,6 +1318,16 @@ 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_GUARD; > + goto out_unmap; > + } > continue; This looks good, but see below. > } else { > result = SCAN_EXCEED_SWAP_PTE; > @@ -2860,6 +2871,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start, > case SCAN_PAGE_COMPOUND: > case SCAN_PAGE_LRU: > case SCAN_DEL_PAGE_LRU: > + case SCAN_PTE_GUARD: > last_fail = result; Should we not do this, and just send this case over to the default case. That would mean immediate exit with -EINVAL, instead of iterating over the complete range, potentially collapsing a non-guard range, and returning -EINVAL. I do not think we should spend a significant time in the kernel when the user is literally invoking madvise(MADV_GUARD_INSTALL) and madvise(MADV_COLLAPSE) on overlapping regions. > break; > default: