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Thu, 28 Jan 2021 19:49:59 +0000 Date: Thu, 28 Jan 2021 11:49:51 -0800 From: Saravanan D To: Matthew Wilcox CC: , , , , , , , , , Subject: Re: [PATCH V5] x86/mm: Tracking linear mapping split events Message-ID: <20210128194951.2sy3wt46ere4el3k@devvm1945.atn0.facebook.com> References: <20210128045153.GW308988@casper.infradead.org> <20210128104934.2916679-1-saravanand@fb.com> <20210128150444.GY308988@casper.infradead.org> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210128150444.GY308988@casper.infradead.org> X-Originating-IP: [2620:10d:c090:400::5:7429] X-ClientProxiedBy: MWHPR02CA0018.namprd02.prod.outlook.com (2603:10b6:300:4b::28) To SN6PR1501MB2126.namprd15.prod.outlook.com (2603:10b6:805:5::14) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from devvm1945.atn0.facebook.com (2620:10d:c090:400::5:7429) by MWHPR02CA0018.namprd02.prod.outlook.com (2603:10b6:300:4b::28) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3805.16 via Frontend Transport; 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Also, we have lots of kinds of tracing these days; are you > referring to kprobes? tracepoints? ftrace? Something else? It has to be kernel tracing (kprobes, tracepoints) as we are dealing with direct mapping splits. Kernel's direct mapping `` ffff888000000000 | -119.5 TB | ffffc87fffffffff | 64 TB | direct mapping of all physical memory (page_offset_base)`` The kernel text range ``ffffffff80000000 | -2 GB | ffffffff9fffffff | 512 MB | kernel text mapping, mapped to physical address 0`` Source : Documentation/x86/x86_64/mm.rst Kernel code segment points to the same physical addresses already mapped in the direct mapping range (0x20000000 = 512 MB) When we enable kernel tracing, we would have to modify attributes/permissions of the text segment pages that are direct mapped causing them to split. When we track the direct_pages_count[] in arch/x86/mm/pat/set_memory.c There are only splits from higher levels. They never coalesce back. Splits when we turn on dynamic tracing .... cat /proc/vmstat | grep -i direct_map_level direct_map_level2_splits 784 direct_map_level3_splits 12 bpftrace -e 'tracepoint:raw_syscalls:sys_enter { @ [pid, comm] = count(); }' cat /proc/vmstat | grep -i direct_map_level direct_map_level2_splits 789 direct_map_level3_splits 12 .... Thanks, Saravanan D