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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 21 Aug 2025 15:14:02.2178 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: e80b4314-7618-4522-0742-08dde0c55c94 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: DU6PEPF0000A7E0.eurprd02.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: DU5PR08MB10552 X-Rspamd-Queue-Id: 48AC440010 X-Rspamd-Server: rspam04 X-Rspam-User: X-Stat-Signature: bxzwd6kspxm9ipgkq1f6mk66cb4eesrn X-HE-Tag: 1755789248-510503 X-HE-Meta: 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 +C/ruzM3 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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 21/08/25 8:31 pm, Lorenzo Stoakes wrote: > OK so I noticed in patch 13/13 (!) where you change the documentation that you > essentially state that the whole method used to determine the ratio of PTEs to > collapse to mTHP is broken: > > khugepaged uses max_ptes_none scaled to the order of the enabled > mTHP size to determine collapses. When using mTHPs it's recommended > to set max_ptes_none low-- ideally less than HPAGE_PMD_NR / 2 (255 > on 4k page size). This will prevent undesired "creep" behavior that > leads to continuously collapsing to the largest mTHP size; when we > collapse, we are bringing in new non-zero pages that will, on a > subsequent scan, cause the max_ptes_none check of the +1 order to > always be satisfied. By limiting this to less than half the current > order, we make sure we don't cause this feedback > loop. max_ptes_shared and max_ptes_swap have no effect when > collapsing to a mTHP, and mTHP collapse will fail on shared or > swapped out pages. > > This seems to me to suggest that using > /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none as some means > of establishing a 'ratio' to do this calculation is fundamentally flawed. > > So surely we ought to introduce a new sysfs tunable for this? Perhaps > > /sys/kernel/mm/transparent_hugepage/khugepaged/mthp_max_ptes_none_ratio > > Or something like this? > > It's already questionable that we are taking a value that is expressed > essentially in terms of PTE entries per PMD and then use it implicitly to > determine the ratio for mTHP, but to then say 'oh but the default value is > known-broken' is just a blocker for the series in my opinion. > > This really has to be done a different way I think. > > Cheers, Lorenzo FWIW this was my version of the documentation patch: https://lore.kernel.org/all/20250211111326.14295-18-dev.jain@arm.com/ The discussion about the creep problem started here: https://lore.kernel.org/all/7098654a-776d-413b-8aca-28f811620df7@arm.com/ and the discussion continuing here: https://lore.kernel.org/all/37375ace-5601-4d6c-9dac-d1c8268698e9@redhat.com/ ending with a summary I gave here: https://lore.kernel.org/all/8114d47b-b383-4d6e-ab65-a0e88b99c873@arm.com/ This should help you with the context.