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Then during the PMD scan, we use a bitmap to track chunks of > pages (defined by KHUGEPAGED_MTHP_MIN_ORDER) that are utilized. After the > PMD scan is done, we do binary recursion on the bitmap to find the optimal > mTHP sizes for the PMD range. The restriction on max_ptes_none is removed > during the scan, to make sure we account for the whole PMD range. When no > mTHP size is enabled, the legacy behavior of khugepaged is maintained. > max_ptes_none will be scaled by the attempted collapse order to determine > how full a mTHP must be to be eligible for the collapse to occur. If a > mTHP collapse is attempted, but contains swapped out, or shared pages, we > don't perform the collapse. It is now also possible to collapse to mTHPs > without requiring the PMD THP size to be enabled. > > With the default max_ptes_none=511, the code should keep its most of its > original behavior. When enabling multiple adjacent (m)THP sizes we need to > set max_ptes_none<=255. With max_ptes_none > HPAGE_PMD_NR/2 you will > experience collapse "creep" and constantly promote mTHPs to the next > available size. This is due the fact that a collapse will introduce at > least 2x the number of pages, and on a future scan will satisfy the > promotion condition once again. > > Patch 1: Refactor/rename hpage_collapse > Patch 2: Some refactoring to combine madvise_collapse and khugepaged > Patch 3-5: Generalize khugepaged functions for arbitrary orders > Patch 6-8: The mTHP patches > Patch 9-10: Allow khugepaged to operate without PMD enabled > Patch 11-12: Tracing/stats > Patch 13: Documentation > > For the next version, it will be really great if you can mention the lore links referencing important ideas guiding the evolution of the algorithm - say, a policy decision we make. (I frequently do that albeit I think I over-do it :)) I am asking because I am completely lost on the current discussion going on around the max_ptes_* scaling (been busy with other stuff).