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
