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Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Pedro Demarchi Gomes Subject: [PATCH mm-stable] mm/khugepaged: avoid unnecessary checking for swap entries when collapsing a mTHP Date: Tue, 25 Aug 2026 16:24:33 -0300 Message-ID: <20260825192433.3185880-1-pedrodemargomes@gmail.com> X-Mailer: git-send-email 2.55.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit mthp_collapse() tries to swap in PTEs when collapsing a mTHP if there are any swap PTEs in the PMD range, even if none of those swap PTEs are actually part of the mTHP's range. Track swap PTEs in cc->mthp_unmapped_ptes so that mthp_collapse() can tell whether the mTHP's own range contains a swap PTE, and skip the swapin check when it doesn't. Signed-off-by: Pedro Demarchi Gomes --- mm/khugepaged.c | 29 +++++++++++++++++++---------- 1 file changed, 19 insertions(+), 10 deletions(-) diff --git a/mm/khugepaged.c b/mm/khugepaged.c index 11ff98d55c76..38596172f7dc 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -117,6 +117,9 @@ struct collapse_control { /* Each bit represents a single occupied (!none/zero) page. */ DECLARE_BITMAP(mthp_present_ptes, MAX_PTRS_PER_PTE); + + /* Each bit represents a single not present and not none/zero pte. */ + DECLARE_BITMAP(mthp_unmapped_ptes, MAX_PTRS_PER_PTE); }; /** @@ -634,6 +637,7 @@ static void collapse_control_init_scan(struct collapse_control *cc) memset(cc->node_load, 0, sizeof(cc->node_load)); nodes_clear(cc->alloc_nmask); bitmap_zero(cc->mthp_present_ptes, MAX_PTRS_PER_PTE); + bitmap_zero(cc->mthp_unmapped_ptes, MAX_PTRS_PER_PTE); } static void release_pte_folio(struct folio *folio) @@ -1291,7 +1295,7 @@ static enum scan_result alloc_charge_folio(struct folio **foliop, struct mm_stru * Note that the VMA must be rechecked after grabbing the mmap_lock again. */ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long start_addr, - int referenced, int unmapped, struct collapse_control *cc, + int referenced, bool swapin, struct collapse_control *cc, unsigned int order) { const unsigned long pmd_addr = start_addr & HPAGE_PMD_MASK; @@ -1330,7 +1334,7 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long s goto out_nolock; } - if (unmapped) { + if (swapin) { /* * __collapse_huge_page_swapin() will return with mmap_lock * released when it fails. So we jump out_nolock directly in @@ -1502,10 +1506,10 @@ static unsigned int max_order_from_offset(unsigned int offset) * mTHP. */ static enum scan_result mthp_collapse(struct mm_struct *mm, - unsigned long address, int referenced, int unmapped, + unsigned long address, int referenced, struct collapse_control *cc, unsigned long enabled_orders) { - unsigned int nr_occupied_ptes, nr_ptes, max_ptes_none; + unsigned int nr_occupied_ptes, nr_unmapped_ptes, nr_ptes, max_ptes_none; enum scan_result last_result = SCAN_FAIL; int collapsed = 0; bool alloc_failed = false; @@ -1522,21 +1526,25 @@ static enum scan_result mthp_collapse(struct mm_struct *mm, max_ptes_none = collapse_max_ptes_none(cc, NULL, order); nr_occupied_ptes = bitmap_weight_from(cc->mthp_present_ptes, offset, offset + nr_ptes); + nr_unmapped_ptes = bitmap_weight_from(cc->mthp_unmapped_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. + * Swap PTEs accepted during the scan are counted in + * nr_unmapped_ptes, not in the present-PTE bitmap. Account + * them for the PMD-order candidate. */ if (is_pmd_order(order)) - nr_occupied_ptes += unmapped; + nr_occupied_ptes += nr_unmapped_ptes; if (nr_occupied_ptes >= nr_ptes - max_ptes_none) { enum scan_result ret; + bool swapin = nr_unmapped_ptes > 0; collapse_address = address + offset * PAGE_SIZE; ret = collapse_huge_page(mm, collapse_address, referenced, - unmapped, cc, order); + swapin, cc, order); switch (ret) { /* Cases where we continue to next collapse candidate */ @@ -1667,6 +1675,7 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm, continue; } if (!pte_present(pteval)) { + __set_bit(i, cc->mthp_unmapped_ptes); if (++unmapped > max_ptes_swap) { result = SCAN_EXCEED_SWAP_PTE; count_collapse_event(HPAGE_PMD_ORDER, THP_SCAN_EXCEED_SWAP_PTE, @@ -1785,7 +1794,7 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm, /* collapse_huge_page() expects the lock to be dropped before calling */ mmap_read_unlock(mm); result = mthp_collapse(mm, start_addr, referenced, - unmapped, cc, enabled_orders); + cc, enabled_orders); /* mmap_lock was released above, set lock_dropped */ *lock_dropped = true; } -- 2.55.0