On Wed, Jan 8, 2025 at 11:27 PM Dev Jain <dev.jain@xxxxxxx> wrote: > > > On 09/01/25 5:01 am, Nico Pache wrote: > > The following series provides khugepaged and madvise collapse with the > > capability to collapse 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 MTHP_MIN_ORDER) that are fully 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. max_ptes_none is mapped to a 0-100 range to > > determine how full a mTHP order needs to be before collapsing it. > > > > Some design choices to note: > > - bitmap structures are allocated dynamically because on some arch's > > (like PowerPC) the value of MTHP_BITMAP_SIZE cannot be computed at > > compile time leading to warnings. > > - The recursion is masked through a stack structure. > > - A MTHP_MIN_ORDER was added to compress the bitmap, and ensure it was > > 64bit on x86. This provides some optimization on the bitmap operations. > > if other arches/configs that have larger than 512 PTEs per PMD want to > > compress their bitmap further we can change this value per arch. > > > > Patch 1-2: Some refactoring to combine madvise_collapse and khugepaged > > Patch 3: A minor "fix"/optimization > > Patch 4: Refactor/rename hpage_collapse > > Patch 5-7: Generalize khugepaged functions for arbitrary orders > > Patch 8-11: The mTHP patches > > > > This series acts as an alternative to Dev Jain's approach [1]. The two > > series differ in a few ways: > > - My approach uses a bitmap to store the state of the linear scan_pmd to > > then determine potential mTHP batches. Devs incorporates his directly > > into the scan, and will try each available order. > > - Dev is attempting to optimize the locking, while my approach keeps the > > locking changes to a minimum. I believe his changes are not safe for > > uffd. > > - Dev's changes only work for khugepaged not madvise_collapse (although > > i think that was by choice and it could easily support madvise) > > - Dev scales all khugepaged sysfs tunables by order, while im removing > > the restriction of max_ptes_none and converting it to a scale to > > determine a (m)THP threshold. > > - Dev turns on khugepaged if any order is available while mine still > > only runs if PMDs are enabled. I like Dev's approach and will most > > likely do the same in my PATCH posting. > > - mTHPs need their ref count updated to 1<<order, which Dev is missing. > > Well, I did not miss it :) Sorry! I missed that in my initial review of your code. Seeing that would have saved me a few hours of debugging xD > > int nr_pages = folio_nr_pages(folio); > folio_ref_add(folio, nr_pages - 1); Once I found the fix I forgot to cross reference with your series. Missing this ref update was causing the issue I alluded to in your RFC thread. When you said you ran into some issues on the debug configs I figured it was the same one. > > > > > Patch 11 was inspired by one of Dev's changes. > > > > [1] https://lore.kernel.org/lkml/20241216165105.56185-1-dev.jain@xxxxxxx/ > > > > Nico Pache (11): > > introduce khugepaged_collapse_single_pmd to collapse a single pmd > > khugepaged: refactor madvise_collapse and khugepaged_scan_mm_slot > > khugepaged: Don't allocate khugepaged mm_slot early > > khugepaged: rename hpage_collapse_* to khugepaged_* > > khugepaged: generalize hugepage_vma_revalidate for mTHP support > > khugepaged: generalize alloc_charge_folio for mTHP support > > khugepaged: generalize __collapse_huge_page_* for mTHP support > > khugepaged: introduce khugepaged_scan_bitmap for mTHP support > > khugepaged: add mTHP support > > khugepaged: remove max_ptes_none restriction on the pmd scan > > khugepaged: skip collapsing mTHP to smaller orders > > > > include/linux/khugepaged.h | 4 +- > > mm/huge_memory.c | 3 +- > > mm/khugepaged.c | 436 +++++++++++++++++++++++++------------ > > 3 files changed, 306 insertions(+), 137 deletions(-) > > >
