On Wed, May 28, 2025 at 6:39 AM Baolin Wang <baolin.w...@linux.alibaba.com> wrote: > > > > On 2025/5/15 11:22, Nico Pache wrote: > > The following series provides khugepaged and madvise collapse with the > > capability to collapse anonymous memory regions to mTHPs. > > > > To achieve this we generalize the khugepaged functions to no longer depend > > on PMD_ORDER. Then during the PMD scan, we keep track of chunks of pages > > (defined by KHUGEPAGED_MTHP_MIN_ORDER) that are utilized. This info is > > tracked using a bitmap. 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 THP must be to be > > eligible. If a mTHP collapse is attempted, but contains swapped out, or > > shared pages, we dont perform the collapse. > > > > With the default max_ptes_none=511, the code should keep its most of its > > original behavior. To exercise mTHP collapse 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 it will introduce at least 2x > > the number of pages, and on a future scan will satisfy that 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 6-9: The mTHP patches > > Patch 10-11: Tracing/stats > > Patch 12: Documentation > > When I tested 64K mTHP collapse and disabled PMD-sized THP, I found that > khugepaged couldn't scan and collapse 64K mTHP. I send out two fix > patches[1], and with these patches applied, 64K mTHP collapse works > well. I hope my two patches can be folded into your next version series > if you think there are no issues. Thanks.
Thank you for looking into that and fixing it, I had originally decided to only allow khugepaged to collapse to mTHP if the PMD size was enabled as well. It was on my todo list :) I'll work on adding your patches to my set, and do some proper testing again! > > [1] > https://lore.kernel.org/all/ac9ed6d71b439611f9c94b3506a8ce975d4636e9.1748435162.git.baolin.w...@linux.alibaba.com/ >
