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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 28 Aug 2025 10:49:34.2669 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: edb5cf43-3ce7-4da9-d577-08dde6209371 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: DB1PEPF000509E9.eurprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: PAVPR08MB8800 X-Stat-Signature: 46bio59oc8oaybskane9htrn1ac5zpn9 X-Rspam-User: X-Rspamd-Queue-Id: BB26BA0012 X-Rspamd-Server: rspam05 X-HE-Tag: 1756378178-517720 X-HE-Meta: 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 K23vK8Db 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 28/08/25 3:16 pm, Baolin Wang wrote: > (Sorry for chiming in late) > > On 2025/8/22 22:10, 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. >> >>> >>> 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. >> >> 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". > > If we only allow setting 0 or 511, as Nico mentioned before, "At 511, > no mTHP collapses would ever occur anyway, unless you have 2MB > disabled and other mTHP sizes enabled. Technically, at 511, only the > highest enabled order would ever be collapsed." I didn't understand this statement. At 511, mTHP collapses will occur if khugepaged cannot get a PMD folio. Our goal is to collapse to the highest order folio. > > In other words, for the scenario you described, although there are > only 2 PTEs present in a PMD, it would still get collapsed into a > PMD-sized THP. In reality, what we probably need is just an order-2 > mTHP collapse. > > If 'khugepaged_max_ptes_none' is set to 255, I think this would > achieve the desired result: when there are only 2 PTEs present in a > PMD, an order-2 mTHP collapse would be successed, but it wouldn’t > creep up to an order-3 mTHP collapse. That’s because: > When attempting an order-3 mTHP collapse, 'threshold_bits' = 1, while > 'bits_set' = 1 (means only 1 chunk is present), so 'bits_set > > threshold_bits' is false, then an order-3 mTHP collapse wouldn’t be > attempted. No? > > So I have some concerns that if we only allow setting 0 or 511, it may > not meet the goal we have for mTHP collapsing. > >>>> 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. >> >> 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. >> >