From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 7E108C61DC6 for ; Thu, 27 Aug 2026 12:12:01 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 67D086B008C; Thu, 27 Aug 2026 08:12:00 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 653596B0092; Thu, 27 Aug 2026 08:12:00 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 5933D6B0095; Thu, 27 Aug 2026 08:12:00 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0016.hostedemail.com [216.40.44.16]) by kanga.kvack.org (Postfix) with ESMTP id 373EB6B008C for ; Thu, 27 Aug 2026 08:12:00 -0400 (EDT) Received: from smtpin09.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay09.hostedemail.com (Postfix) with ESMTP id 9346F80404 for ; Thu, 27 Aug 2026 12:11:59 +0000 (UTC) X-FDA: 85146935958.09.DC1D3AF Received: from mta0.migadu.com (out-74.mta0.migadu.com [91.218.175.74]) by imf05.hostedemail.com (Postfix) with ESMTP id 7C44E10000E for ; Thu, 27 Aug 2026 12:11:57 +0000 (UTC) Authentication-Results: imf05.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=sghyhz76; spf=pass (imf05.hostedemail.com: domain of lance.yang@linux.dev designates 91.218.175.74 as permitted sender) smtp.mailfrom=lance.yang@linux.dev; dmarc=pass (policy=none) header.from=linux.dev ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1787832717; b=ufdIEv/+lSxZjWRouzMTztI8LMDasjDDXxM7NImaSekeeS9d6/v8bjifTFoeQwlqghdRuN 0PH7QttFJhtaXGqY6lDt5ZPiqKLWyEA/fxp3qK8Mr9Fi/9mMliA7zdRi+IJyNKV2TnWdoN 7mQpjgORnU+MjI7WJVjmiJIN/dA5x7w= ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1787832717; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=ajudW1cVrHI5Aype0yKSDVlP8hrDj5Jx8rF/kHBErJE=; b=hiGKvgFpi0DxuqT7jJnVK5pmUPEax7IU0noVoNSfDysMCg0aeTxgjW18Xo3qj2CFgrQU4z S2yd64i9kZ4s9MRO40wBDRVS0S+Cv6Oc3eW7sOCFaDc6WJS2DimrKAvHkqejgRlHCoLCkL ZHHvqObx3TXsYrQL3+i2y8xWkGN63G4= ARC-Authentication-Results: i=1; imf05.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=sghyhz76; spf=pass (imf05.hostedemail.com: domain of lance.yang@linux.dev designates 91.218.175.74 as permitted sender) smtp.mailfrom=lance.yang@linux.dev; dmarc=pass (policy=none) header.from=linux.dev X-Envelope-To: linux-mm@kvack.org DKIM-Signature: a=rsa-sha256; bh=OmZ4HMNJEKn5n8PIiCPbESGHuRzCDj7TC5nRy66T5lU=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1787832716; v=1; x=1788437516; b=sghyhz76ZGQ1HyGNO4mpw7Wuj33xXQly/O+AM/JrD3r07379EZYipF+0lEbXN9QV1uK02byQ ahUMrm/rxCXIm+hlkLYX2lrCvs3Qf/qiq/ZFNm5XSXTYlSla9raOOh49GujPhLGOwfP93ABdo5m v5YsGQwDhgDnRBgxu8sKBNeQ= X-Envelope-To: linux-mm@kvack.org Received: by smtp.migadu.com with ESMTPS id a5aa79f41822b67a; Thu, 27 Aug 2026 12:11:55 +0000 X-Mizu-Trace-ID: a5aa79f41822b67a X-Migadu-Flow: FLOW_OUT From: Lance Yang To: david@kernel.org, baolin.wang@linux.alibaba.com, pedrodemargomes@gmail.com Cc: akpm@linux-foundation.org, ljs@kernel.org, ziy@nvidia.com, liam@infradead.org, nico.pache@linux.dev, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, lance.yang@linux.dev, usama.arif@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH mm-stable] mm/khugepaged: avoid unnecessary checking for swap entries when collapsing a mTHP Date: Thu, 27 Aug 2026 20:11:51 +0800 Message-Id: <20260827121151.7837-1-lance.yang@linux.dev> X-Mailer: git-send-email 2.39.3 (Apple Git-146) In-Reply-To: References: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 7C44E10000E X-Stat-Signature: m6st64a9hask5dpkp9csh3buun9fqodm X-HE-Tag: 1787832717-520146 X-HE-Meta: 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 ilVJcgMw zGQDicEt8JtuOAsRqctIAwASY/QZyylkE8QM/L0ggFgWmN3iNefYhcA/+Di5cHKtm3i4oNVI8DUKtSOE3PH/0pSBsGtwww1W+841kK38rzg62UFw4AXP9HdM7neUD3YPE3xVFJ/SC8r807RMVp9BicXEvNpcd5AAij0ElBRAXaxLmPHMNANn0kOAQPkuLmDrjHb5DdUMIoGiTU88IUTTpOOwr+NaMeA8Eu+seGzw0Ck52gY7sI0vDKXXoA9OxMnkGHsF/aXsC5PvB5fw0eksJzhaEG4/Azaqr7jC3 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Aug 27, 2026 at 01:02:14PM +0200, David Hildenbrand (Arm) wrote: >On 8/27/26 10:06, Baolin Wang wrote: >> >> >> On 8/26/26 3:24 AM, Pedro Demarchi Gomes wrote: >>> 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. >> >> Are you sure? I wonder how you tested your patch? Because we never swapin PTEs >> for mTHP collapse, see the code in __collapse_huge_page_swapin(): > >I'm confused as well, this doesn't really make sense. Well ... the change does remove a redundant PTE walk, IIUC ... I think Pedro's wording is causing the confusion :) Yeah, Baolin is right that __collapse_huge_page_swapin() never reaches do_swap_page() for a mTHP. For an otherwise eligible lower-order candidate, current code still calls it and walks the candidate's PTE range when an unrelated swap PTE exists elsewhere in the same PMD :) Assume an otherwise eligible lower-order candidate has no swap PTE, while another subrange in the PMD has one. collapse_scan_pmd() counts unmapped over the full PMD and passes that PMD-wide value into mthp_collapse(): static enum scan_result collapse_scan_pmd(struct mm_struct *mm, struct vm_area_struct *vma, unsigned long start_addr, bool *lock_dropped, struct collapse_control *cc) { ... int node = NUMA_NO_NODE, unmapped = 0; ... for (i = 0; i < HPAGE_PMD_NR; i++) { _pte = pte + i; addr = start_addr + i * PAGE_SIZE; pteval = ptep_get(_pte); ... if (pte_none_or_zero(pteval)) { ... continue; } if (!pte_present(pteval)) { if (++unmapped > max_ptes_swap) { ... } ... if (pte_swp_uffd_any(pteval)) { result = SCAN_PTE_UFFD; goto out_unmap; } continue; } ... } if (cc->is_khugepaged && (!referenced || (unmapped && referenced < HPAGE_PMD_NR / 2))) { result = SCAN_LACK_REFERENCED_PAGE; } else { result = SCAN_SUCCEED; } ... if (result == SCAN_SUCCEED) { ... result = mthp_collapse(mm, start_addr, referenced, unmapped, cc, enabled_orders); ... } ... return result; } unmapped is PMD-wide above. mthp_collapse() then passes the same value to every attempted candidate: static enum scan_result mthp_collapse(struct mm_struct *mm, unsigned long address, int referenced, int unmapped, struct collapse_control *cc, unsigned long enabled_orders) { unsigned int nr_occupied_ptes, nr_ptes, max_ptes_none; ... unsigned int order = HPAGE_PMD_ORDER; while (offset < HPAGE_PMD_NR) { nr_ptes = 1UL << order; if (!test_bit(order, &enabled_orders)) goto next_order; max_ptes_none = collapse_max_ptes_none(cc, NULL, order); 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; if (nr_occupied_ptes >= nr_ptes - max_ptes_none) { enum scan_result ret; collapse_address = address + offset * PAGE_SIZE; ret = collapse_huge_page(mm, collapse_address, referenced, unmapped, cc, order); ... } next_order: ... if (order > KHUGEPAGED_MIN_MTHP_ORDER && (enabled_orders & GENMASK(order - 1, 0))) { order--; continue; } next_offset: ... offset += nr_ptes; order = max_order_from_offset(offset); } ... } Once it reaches the lower-order candidate with no swap PTE, unmapped is still nonzero and collapse_huge_page() calls the swapin helper: static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long start_addr, int referenced, int unmapped, struct collapse_control *cc, unsigned int order) { ... if (unmapped) { ... result = __collapse_huge_page_swapin(mm, vma, start_addr, pmd, referenced, order); ... } ... } __collapse_huge_page_swapin() only calls do_swap_page() after finding a non-present, non-none PTE, and lower orders return before that call: static enum scan_result __collapse_huge_page_swapin(struct mm_struct *mm, struct vm_area_struct *vma, unsigned long start_addr, pmd_t *pmd, int referenced, unsigned int order) { ... unsigned long addr, end = start_addr + (PAGE_SIZE << order); enum scan_result result; pte_t *pte = NULL; spinlock_t *ptl; for (addr = start_addr; addr < end; addr += PAGE_SIZE) { ... vmf.orig_pte = ptep_get_lockless(pte); if (pte_none(vmf.orig_pte) || pte_present(vmf.orig_pte)) continue; ... if (!is_pmd_order(order)) { ... result = SCAN_EXCEED_SWAP_PTE; goto out; } vmf.pte = pte; vmf.ptl = ptl; ret = do_swap_page(&vmf); ... } ... result = SCAN_SUCCEED; out: ... return result; } For the candidate above, the loop only reads its PTEs and returns SCAN_SUCCEED. Pedro's bitmap lets mthp_collapse() determine that upfront and skip the walk. Emm ... as I asked before[1], any numbers showing how much the extra scan costs? [1] https://lore.kernel.org/lkml/20260825192433.3185880-1-pedrodemargomes@gmail.com/ Cheers, Lance