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From: Lance Yang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Rspamd-Server: rspam10 X-Rspam-User: X-Stat-Signature: 94rkxj9y6t14dtxcoab49141ckbg1aky X-Rspamd-Queue-Id: 79B24140009 X-HE-Tag: 1781057448-584850 X-HE-Meta: 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 multoS7V OtwaU5fW0kH4ZwzxJ8jsnp3wlUEYtxav4TZuxlbQJmsvpe+GbTxHUHhlPlPuifUqq8NVgRWAJe5q8KixtQ/tx63S8p2qIDYwjdpLzGpGprWJPti1xN09obYfGDm0fBpwZf1BqE75FR/X3/zEvwZ6D9Z/7OLOMrqy8IL5svl/yTbYGBl6XIAkSeU8M3jaxIJwgJ4PkuWhcyvkSEajVppr0qCdPn8xqc3UWmUyLNorT71C2xtnVLaw2QPSr86DWYukuQdXHbIjKv3etQ45NOLCOocGJ9qfKrwdjeMNy/flw/bqXhxj0e4/Vc4JSIQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2026/6/10 01:04, Nico Pache wrote: > On Tue, Jun 9, 2026 at 7:16 AM David Hildenbrand (Arm) wrote: >> >> On 6/9/26 14:04, Lance Yang wrote: >>> From: Lance Yang >>> >>> mthp_collapse() uses mthp_present_ptes to decide whether a range has >>> enough occupied PTEs to try collapse. Swap PTEs accepted by >>> collapse_scan_pmd() are counted in unmapped, but are not represented in >>> mthp_present_ptes. >>> >>> When lower orders are enabled, collapse_scan_pmd() relaxes max_ptes_none >>> so the scan can cover the whole PMD and build the bitmap. mthp_collapse() >>> then checks the PMD-order candidate using the bitmap. >>> >>> With max_ptes_none set to 0, a range with 511 present PTEs and one swap >>> PTE no longer reaches collapse_huge_page(), even though PMD collapse can >>> handle swap PTEs up to max_ptes_swap. >>> >>> Account unmapped PTEs only for PMD order. PMD collapse supports swap PTEs >>> through max_ptes_swap, while lower-order mTHP collapse does not currently >>> support non-present PTEs. Keep non-present PTEs out of the lower-order >>> eligibility check. >>> >>> Signed-off-by: Lance Yang >>> --- >>> Sent separately, as discussed in [1], to spell out the PMD-order swap PTE >>> case. Patch [2] is still only in mm-unstable, so no Fixes: tag. >>> >>> [1] https://lore.kernel.org/linux-mm/CAA1CXcD7WAiA1b9GTLAuNZ+kHaFx0SzZwpBkqAZ=s+RHsTUaow@mail.gmail.com/ >>> [2] https://lore.kernel.org/linux-mm/20260605161422.213817-12-npache@redhat.com/ >>> >>> mm/khugepaged.c | 8 ++++++++ >>> 1 file changed, 8 insertions(+) >>> >>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >>> index b12187709f6d..617bca76db49 100644 >>> --- a/mm/khugepaged.c >>> +++ b/mm/khugepaged.c >>> @@ -1508,6 +1508,14 @@ static enum scan_result mthp_collapse(struct mm_struct *mm, >>> nr_occupied_ptes = bitmap_weight_from(cc->mthp_present_ptes, offset, >>> offset + nr_ptes); >>> >>> + /* >>> + * Swap PTEs accepted during the scan are counted in @unmapped, >>> + * not in the present-PTE bitmap. Account them for the PMD-order >>> + * candidate. >>> + */ >>> + if (is_pmd_order(order)) >>> + nr_occupied_ptes += unmapped; >>> + >> >> LGTM, there is a bit of opportunity for cleanup in the future :) >> >> Acked-by: David Hildenbrand (Arm) >> >> >> For example, as we no longer have the VMA here, collapse_max_ptes_none is >> imprecise in uffd VMAs. We might try collapsing where there sure is nothing to >> collapse. >> >> We could likely handle the userfaultfd_armed() part easier: some indication that >> we must not have any pte_none() would be sufficient. >> >> Also, I don't see a good reason why uffd would not be allowed to collapse with >> zeropages ... it's really just about missing faults due to pte_none(). > > I have some patches exactly for this :) so far 2-3 patches for better > uffd handling Neat, I'll leave that to you then :) Looking forward to seeing those patches! Cheers, Lance