On Mon, May 18, 2026 at 1:33 PM Lorenzo Stoakes <[email protected]> wrote: > > On Mon, May 18, 2026 at 03:16:11PM +0200, David Hildenbrand (Arm) wrote: > > 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? > > Yes 100%, this sounds sensible both in terms of the error and the default. > Let's > keep our lives simple(-ish) please :)
Ok thank you im glad we finally came to consensus on this! phew! > > > > > -- > > Cheers, > > > > David > > Cheers, Lorenzo >
