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Date: Thu, 15 Jan 2026 11:48:47 +0100 From: Roger Pau =?utf-8?B?TW9ubsOp?= To: Jan Beulich Cc: Andrew Cooper , Anthony PERARD , Michal Orzel , Julien Grall , Stefano Stabellini , xen-devel@lists.xenproject.org Subject: Re: [PATCH 2/2] xen/mm: limit non-scrubbed allocations to a specific order Message-ID: References: <20260108175536.82153-1-roger.pau@citrix.com> <20260108175536.82153-3-roger.pau@citrix.com> Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-ClientProxiedBy: PAZP264CA0208.FRAP264.PROD.OUTLOOK.COM (2603:10a6:102:237::21) To CH7PR03MB7860.namprd03.prod.outlook.com (2603:10b6:610:24e::14) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: CH7PR03MB7860:EE_|SA3PR03MB8253:EE_ X-MS-Office365-Filtering-Correlation-Id: 01f6bdd9-a4ff-4295-d83b-08de5423abc6 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|366016|1800799024; X-Microsoft-Antispam-Message-Info: =?utf-8?B?YUdBaFVGUS84S3dBL3RhZnF3M1FSL3hta2U4Q09Qd2VMVTlTSFhRTDBFeWk0?= =?utf-8?B?a2ZnMm5LMmRBbWpZOE1Zc1dHZ1RBS2pESVBVdXpsbm9FelB2UngrdTNSSW53?= =?utf-8?B?c054RmNEcXVKN0pIdXRvYklaTDFsZFJua0xRU2l2RHRGMjRzUTBtRHV5bUti?= =?utf-8?B?OWs1TzNmTnNQT3loa0szWllYUlFEbnQ3emV1WUVUb0ZZNkN1dVBESmVLTFNY?= =?utf-8?B?WS9vRVJjckJERVNWTFBJRStJVVJKYk9yUGIyYjdGVElWaTRWcmdHVGxBc0M5?= =?utf-8?B?b1V1eWFNMFU1czI5ZjNZclcrTE1CcmpGd1E0dGFIdm92aGx4V1IzaTF2N29x?= =?utf-8?B?QnAvOUxUSHlpdEdpSjhuTEJ2UmpqeTNKV0dLWGtsejZEM2NraXIzSnNWWkJF?= =?utf-8?B?cDhiRzhYTXBEM1M5cDFFWjAwbzd3ZjV5VCsxcGRiOXRlVXZOaW5ucjhodXVU?= =?utf-8?B?UkpmL0o0cTVxTmZxMk9kK0U4SExWSEhaUlYrdFllKzY3RlJoWkNzTTN0dHl0?= =?utf-8?B?bTBwVUlpa2g0TjBSZ2xqZVcwZklORzZOK29lMEVTcGFrUm5NT0FhcXR3MU11?= =?utf-8?B?QmJnSnpSSmZuVW5DT053WTAvVVB4a05FVExvOFpiQVdWL0lDOFJ1Y3MvYWFD?= =?utf-8?B?OHBnN01qQTEvd2g3NkUvWElxaTU1M252a1o2djBBZkNYL2ZrSjFVSmpqZ1px?= =?utf-8?B?SkxCQ21RZ1FjYlBLWVhnZ2xJczliSisrQVlLNnEwOEdJUFA5eUVuL2d0czRP?= =?utf-8?B?Y3VVR3p0YlFxUFlVbFBUR0U1Mmk1SFcvaDk4NkMxeFhRVit5VXg1WGhTMWhI?= =?utf-8?B?MW5Wd2g3cXRhK1dnaWsvMnNrakZNbU5CWHovdFNmN05DOHRndmFnOENzRHVF?= =?utf-8?B?bkNFcFB1VVZiRnBicEYvOUs5cHhQYmNjL09CcFlkR250aHdPZ211UjR2Y2tP?= =?utf-8?B?eVRZMTNQY3pSbk0vU21rY0pTWUhXSTBFTE5pbm16TkpqSVdyNm9mTkFPNTQ0?= =?utf-8?B?YlphWUoxRVFiclFXMnhNd0VlUEd0VDhEL01CcnhIcjRpd3IvYkgzZGtTRDFn?= =?utf-8?B?VXBSa2REZE85ZFRNb0ttaWZlQ2ZlSG9XM1J1aW1YdlNTcldOMzZsRXAyMFQ5?= =?utf-8?B?L1lmdHA0RS9MaW1kZk8yWHgvLzVsZ251d1FBbHJtbHY0cU5qUFY0MUhiQm02?= =?utf-8?B?bXhNemR6dEVBY2MyRG94QUVZMGN3Wm43akVEUGszdTJTK2NmQTNnYTRVNnBl?= =?utf-8?B?NFJvY3NGcHl6QlhVM0ZHSUQxSnhXQVZtMmt0SmZ5cjkxNXNxUk1SWFR6aWtN?= =?utf-8?B?a0I2VXRHOHV6aXNGV3gwL2V2VFFXbC8rd1hWOHdmbDU0b2UyVWV5SGF6cENq?= =?utf-8?B?MDhWanRUSFRYNU4vMTBXSmlyZ3hFSXB1cVhPUlJ2c3NLYXptMHJydDg4dTJp?= =?utf-8?B?SkJFOUVQN0JOWWhaSlhrUHUzWmxNQmRtQmljNThwTzdKUkgxQ1VwdlljNU5u?= =?utf-8?B?NnpqVnFnNmxNQ2NDWm5DWlhpQ29FT2RtSHV6MzVWQUtvMGxFOWZLYkI3VjU4?= =?utf-8?B?ZXhjQ05xLzZjNmdQYk5YYm9ISVVYK3RlSGFRTWFBbUV2Q0dLSVVIV0RETFA4?= =?utf-8?B?MGRINW9CN0d5VU9MZ2ZPaCtHdG9IRzhTUU1DUFQ5d3BvTTEyV1VtOW5VRVdF?= =?utf-8?B?VU9UZTdmVkdmRmtnbW1zaUo1d1BiTDRWeDFOSjRDS3pPcENXQVY0NEtiWEcv?= =?utf-8?B?Y3Q3WUtTc1E3a1F0OUtLSm5vVkNzejZmU2hNUTdqQlo4YnBRM1JCU1AydE9Z?= =?utf-8?B?Z1djY1FYbUlnR1QrWlluNnptc0hpeUkvUDdqcTNjYXA1RWxVZWV3K2RKZ29T?= =?utf-8?B?TVh5MTRXeU5Td0RLcm02UW9SSkM5Ui9CRFgxanJCZ3JkZDJSNEtKbU4xeTJW?= =?utf-8?B?WjF6QWhtcDF3QncxZk9iSUxYdy9HQTRDRGNjTWg1NEVDb0hhZGhVWWttUkdy?= =?utf-8?B?Z2xIT3JnMldNS0RwckhtSk9sbWFZenYvbU9HS2NiWERsdVlVQUd2S3pVMXM1?= =?utf-8?B?U3ZMaFFqN09INnFtSjhSbE1IYitpZlhuUEVpL050dFlwSXh1YzRjclZNNlFB?= =?utf-8?Q?zIqc=3D?= X-Forefront-Antispam-Report: CIP:255.255.255.255;CTRY:;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:CH7PR03MB7860.namprd03.prod.outlook.com;PTR:;CAT:NONE;SFS:(13230040)(376014)(366016)(1800799024);DIR:OUT;SFP:1101; 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> >>> + > >>> + if ( memory_scrub_pending(node) || > >>> + (node != NUMA_NO_NODE && > >>> + !(a->memflags & MEMF_exact_node) && > >>> + memory_scrub_pending(node = NUMA_NO_NODE)) ) > >>> + { > >>> + scrub_free_pages(node); > >>> + a->preempted = 1; > >>> + goto out; > >>> + } > >> > >> At least for order 0 requests there's no point in trying this. With the > >> current logic, actually for orders up to MAX_DIRTY_ORDER. > > > > Yes, otherwise we might force the CPU to do some scrubbing work when > > it won't satisfy it's allocation request anyway. > > > >> Further, from a general interface perspective, wouldn't we need to do the > >> same for at least XENMEM_increase_reservation? > > > > Possibly yes. TBH I would also be fine with strictly limiting > > XENMEM_increase_reservation to 2M order extents, even for the control > > domain. The physmap population is the only that actually requires > > bigger extents. > > Hmm, that's an option, yes, but an ABI-changing one. I don't think it changes the ABI: Xen has always reserved the right to block high order allocations. See for example how max_order() has different limits depending on the domain permissions, and I would not consider those limits part of the ABI, they can be changed from the command line. Thanks, Roger.