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From: Lance Yang In-Reply-To: <7d081256-5b30-4e3c-b948-85ba76ad0e1d@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT X-Rspamd-Queue-Id: 217FE1A0005 X-Stat-Signature: 6fdnmxu7zuijffni358kedztgfkymx5f X-Rspam-User: X-Rspamd-Server: rspam12 X-HE-Tag: 1781022512-224125 X-HE-Meta: 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 I7qJHIUe jAHtEYOaRgi/NagjLsSxE/eRrwwPrsJHO5RypKfQ50Np3qXcRIjWIFTQjajwr3BdBlx4If/dU442IC/7y0CilLoUCr9XoysQd5O2hVq00wUg+ROGnrKxLQREJJ02JGt+Jmjyom7s/8+TzZEyQbakYNa7jzKlOPSwHekhgAE+wjIG9XKHtodVi6aIS433yubVGarINhLyh41XcIQGopDEDjPQKv/4G6l3b2ba4lvLECxzMeDtK8tndeRUo79hSZxP43FO+sqZZo0T0VtZlRFtHx3VVc0ZKarKQvEtN Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2026/6/9 21:16, 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 :) Yes, follow-up cleanup material :) > Acked-by: David Hildenbrand (Arm) Thanks! > 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. Oh, good catch. We may end up trying a collapse that cannot really go anywhere ... One for a follow-up. > We could likely handle the userfaultfd_armed() part easier: some indication that > we must not have any pte_none() would be sufficient. Right. By the time we get to mthp_collapse(), we probably only need to carry that as a small "no pte_none" constraint for the candidate range. > 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(). Makes sense to me. I'll take a look when I get a chance. And yeah, as Lorenzo said, better to clean up the khugepaged mess first before piling more on top :) Cheers, Lance