On 5/14/26 05:10, Wei Yang wrote: > On Tue, May 12, 2026 at 03:42:02PM +0800, Lance Yang wrote: >> >> On Mon, May 11, 2026 at 12:58:04PM -0600, Nico Pache wrote: >>> generalize the order of the __collapse_huge_page_* and collapse_max_* >>> functions to support future mTHP collapse. >>> >>> The current mechanism for determining collapse with the >>> khugepaged_max_ptes_none value is not designed with mTHP in mind. This >>> raises a key design issue: if we support user defined max_pte_none values >>> (even those scaled by order), a collapse of a lower order can introduces >>> an feedback loop, or "creep", when max_ptes_none is set to a value greater >>> than HPAGE_PMD_NR / 2. [1] >>> >>> With this configuration, a successful collapse to order N will populate >>> enough pages to satisfy the collapse condition on order N+1 on the next >>> scan. This leads to unnecessary work and memory churn. >>> >>> To fix this issue introduce a helper function that will limit mTHP >>> collapse support to two max_ptes_none values, 0 and HPAGE_PMD_NR - 1. >>> This effectively supports two modes: [2] >>> >>> - max_ptes_none=0: never collapses if it encounters an empty PTE or a PTE >>> that maps the shared zeropage. Consequently, no memory bloat. >>> - max_ptes_none=511 (on 4k pagesz): Always collapse to the highest >>> available mTHP order. >>> >>> This removes the possiblilty of "creep", while not modifying any uAPI >>> expectations. A warning will be emitted if any non-supported >>> max_ptes_none value is configured with mTHP enabled. >>> >>> mTHP collapse will not honor the khugepaged_max_ptes_shared or >>> khugepaged_max_ptes_swap parameters, and will fail if it encounters a >>> shared or swapped entry. >>> >>> No functional changes in this patch; however it defines future behavior >>> for mTHP collapse. >>> >>> [1] - >>> https://lore.kernel.org/all/[email protected] >>> [2] - >>> https://lore.kernel.org/all/[email protected] >>> >>> Co-developed-by: Dev Jain <[email protected]> >>> Signed-off-by: Dev Jain <[email protected]> >>> Signed-off-by: Nico Pache <[email protected]> >>> --- >>> include/trace/events/huge_memory.h | 3 +- >>> mm/khugepaged.c | 117 ++++++++++++++++++++--------- >>> 2 files changed, 85 insertions(+), 35 deletions(-) >>> >>> diff --git a/include/trace/events/huge_memory.h >>> b/include/trace/events/huge_memory.h >>> index bcdc57eea270..443e0bd13fdb 100644 >>> --- a/include/trace/events/huge_memory.h >>> +++ b/include/trace/events/huge_memory.h >>> @@ -39,7 +39,8 @@ >>> EM( SCAN_STORE_FAILED, "store_failed") \ >>> EM( SCAN_COPY_MC, "copy_poisoned_page") \ >>> EM( SCAN_PAGE_FILLED, "page_filled") \ >>> - EMe(SCAN_PAGE_DIRTY_OR_WRITEBACK, "page_dirty_or_writeback") >>> + EM(SCAN_PAGE_DIRTY_OR_WRITEBACK, "page_dirty_or_writeback") \ >>> + EMe(SCAN_INVALID_PTES_NONE, "invalid_ptes_none") >>> >>> #undef EM >>> #undef EMe >>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >>> index f68853b3caa7..27465161fa6d 100644 >>> --- a/mm/khugepaged.c >>> +++ b/mm/khugepaged.c >>> @@ -61,6 +61,7 @@ enum scan_result { >>> SCAN_COPY_MC, >>> SCAN_PAGE_FILLED, >>> SCAN_PAGE_DIRTY_OR_WRITEBACK, >>> + SCAN_INVALID_PTES_NONE, >>> }; >>> >>> #define CREATE_TRACE_POINTS >>> @@ -353,37 +354,60 @@ static bool pte_none_or_zero(pte_t pte) >>> * PTEs for the given collapse operation. >>> * @cc: The collapse control struct >>> * @vma: The vma to check for userfaultfd >>> + * @order: The folio order being collapsed to >>> * >>> * Return: Maximum number of none-page or zero-page PTEs allowed for the >>> * collapse operation. >>> */ >>> -static unsigned int collapse_max_ptes_none(struct collapse_control *cc, >>> - struct vm_area_struct *vma) >>> +static int collapse_max_ptes_none(struct collapse_control *cc, >>> + struct vm_area_struct *vma, unsigned int order) >>> { >>> + unsigned int max_ptes_none = khugepaged_max_ptes_none; >>> // If the vma is userfaultfd-armed, allow no none-page or zero-page >>> PTEs. >> >> One thing I still want to call out: kernel code usually uses C-style >> comments :) >> >>> if (vma && userfaultfd_armed(vma)) >>> return 0; >>> // for MADV_COLLAPSE, allow any none-page or zero-page PTEs. >>> if (!cc->is_khugepaged) >>> return HPAGE_PMD_NR; >>> - // For all other cases repect the user defined maximum. >>> - return khugepaged_max_ptes_none; >>> + // for PMD collapse, respect the user defined maximum. >>> + if (is_pmd_order(order)) >>> + return max_ptes_none; >>> + /* Zero/non-present collapse disabled. */ >>> + if (!max_ptes_none) >>> + return 0; >>> + // for mTHP collapse with the sysctl value set to >>> KHUGEPAGED_MAX_PTES_LIMIT, >>> + // scale the maximum number of PTEs to the order of the collapse. >>> + if (max_ptes_none == KHUGEPAGED_MAX_PTES_LIMIT) >>> + return (1 << order) - 1; >>> + >>> + // We currently only support max_ptes_none values of 0 or >>> KHUGEPAGED_MAX_PTES_LIMIT. >>> + // Emit a warning and return -EINVAL. >>> + pr_warn_once("mTHP collapse only supports max_ptes_none values of 0 or >>> %u\n", >>> + KHUGEPAGED_MAX_PTES_LIMIT); >> >> Maybe fallback to 0 instead, as David suggested earlier? >> > > It looks reasonable to fallback to 0. > > But as the updated Document says in patch 14: > > For mTHP collapse, only 0 or (HPAGE_PMD_NR - 1) are supported. Any other > value will emit a warning and no mTHP collapse will be attempted. > > This is why it does like this now. > > mthp_collapse() > max_ptes_none = collapse_max_ptes_none(); > if (max_ptes_none < 0) > return collapsed; > >> max_ptes_none is mostly legacy PMD THP behavior. mTHP is new, and any >> intermediate value in (0, KHUGEPAGED_MAX_PTES_LIMIT) would implicitly >> disable it :( >> > > So it depends on what we want to do here :-) > > For me, I would vote for fallback to 0.
At this point I'll prefer to not return errors from collapse_max_ptes_none(). It's just rather awkward to return an error deep down in collapse code for a configuration problem. For mthp collapse, we only support max_ptes_none==0 and max_ptes_none=="HPAGE_PMD_NR - 1" (default). If another value is specified while collapsing mTHP, print a warning and treat it as 0 (save value, no creep, no memory waste). In a sense, this is similar to how we handle max_ptes_shared + max_ptes_swap: for mTHP: we always treat them as being 0 for mTHP collapse (and don't issue a warning, because we would issue a warning with the default settings). @Lorenzo, fine with you? -- Cheers, David
