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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 22 Aug 2025 15:34:27.1936 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: d136ca62-044b-4c9d-3992-08dde1916124 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: DB5PEPF00014B9F.eurprd02.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: PA4PR08MB6239 On 22/08/25 8:19 pm, Lorenzo Stoakes wrote: > On Fri, Aug 22, 2025 at 04:10:35PM +0200, David Hildenbrand wrote: >>>> Once could also easily support the value 255 (HPAGE_PMD_NR / 2- 1), but not sure >>>> if we have to add that for now. >>> Yeah not so sure about this, this is a 'just have to know' too, and yes you >>> might add it to the docs, but people are going to be mightily confused, esp if >>> it's a calculated value. >>> >>> I don't see any other way around having a separate tunable if we don't just have >>> something VERY simple like on/off. >> Yeah, not advocating that we add support for other values than 0/511, >> really. > Yeah I'm fine with 0/511. > >>> Also the mentioned issue sounds like something that needs to be fixed elsewhere >>> honestly in the algorithm used to figure out mTHP ranges (I may be wrong - and >>> happy to stand corrected if this is somehow inherent, but reallly feels that >>> way). >> I think the creep is unavoidable for certain values. >> >> If you have the first two pages of a PMD area populated, and you allow for >> at least half of the #PTEs to be non/zero, you'd collapse first a >> order-2 folio, then and order-3 ... until you reached PMD order. > Feels like we should be looking at this in reverse? What's the largest, then > next largest, then etc.? > > Surely this is the sensible way of doing it? What David means to say is, for example, suppose all orders are enabled, and we fail to collapse for order-9, then order-8, then order-7, and so on, *only* because the distribution of ptes did not obey the scaled max_ptes_none. Let order-4 collapse succeed. Next time, khugepaged comes and tries for order-9, fails, then order-8, fails and so on. Then it checks for order-5, and it comes under the scaled max_ptes_none constraint only because the previous cycle's order-4 collapse changed the ptes' distribution. > >> So for now we really should just support 0 / 511 to say "don't collapse if >> there are holes" vs. "always collapse if there is at least one pte used". > Yes. > >>>> Because, as raised in the past, I'm afraid nobody on this earth has a clue how >>>> to set this parameter to values different to 0 (don't waste memory with khugepaged) >>>> and 511 (page fault behavior). >>> Yup >>> >>>> >>>> If any other value is set, essentially >>>> pr_warn("Unsupported 'max_ptes_none' value for mTHP collapse"); >>>> >>>> for now and just disable it. >>> Hmm but under what circumstances? I would just say unsupported value not mention >>> mTHP or people who don't use mTHP might find that confusing. >> Well, we can check whether any mTHP size is enabled while the value is set >> to something unexpected. We can then even print the problematic sizes if we >> have to. > Ack > >> We could also just just say that if the value is set to something else than >> 511 (which is the default), it will be treated as being "0" when collapsing >> mthp, instead of doing any scaling. > Or we could make it an error to set anything but 0, 511, but on the other hand > that's likely to break userspace so yeah probably not. > > Maybe have a warning saying 'this is no longer supported and will be ignored' > then set the value to 0 for anything but 511 or 0. > > Then can remove the warning later. > > By having 0/511 we can really simplify the 'scaling' logic too which would be > fantastic! :) FWIW here was my implementation of this thing, for ease of everyone: https://lore.kernel.org/all/20250211111326.14295-17-dev.jain@arm.com/ > > Cheers, Lorenzo