On Thu, Aug 21, 2025 at 08:43:18PM +0530, Dev Jain wrote: > > 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@xxxxxxx/ > > The discussion about the creep problem started here: > https://lore.kernel.org/all/7098654a-776d-413b-8aca-28f811620df7@xxxxxxx/ > > and the discussion continuing here: > https://lore.kernel.org/all/37375ace-5601-4d6c-9dac-d1c8268698e9@xxxxxxxxxx/ > > ending with a summary I gave here: > https://lore.kernel.org/all/8114d47b-b383-4d6e-ab65-a0e88b99c873@xxxxxxx/ > > This should help you with the context. > > Thanks and I"ll have a look, but this series is unmergeable with a broken default in /sys/kernel/mm/transparent_hugepage/khugepaged/mthp_max_ptes_none_ratio sorry. We need to have a new tunable as far as I can tell. I also find the use of this PMD-specific value as an arbitrary way of expressing a ratio pretty gross. Thanks, Lorenzo
