From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 95C73255E34 for ; Wed, 18 Feb 2026 08:05:28 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771401928; cv=none; b=ZBelX+ef+pIxTErNvgeVq8FzjAFWkpBBpQOCCDpKMjzd7skvQ2br2mdK3cSRNyqaGK+m5oOg2Vxxe4tX9DB8lyu1xU+uOJu5QC4xtNFlF2/lYt0g0VkM5OIwJPYr1fSfIw2v1OVSftd1xpb1yJObTOJvzNlY+2wGKAwTLWHBG3I= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771401928; c=relaxed/simple; bh=qw9gzxcNy19n9/pZuKUBlr0jWF8fKT2TdJbUSdrsj5I=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=PvmY78g4y0XJwSQbQ8KvRVyAIw2xdCuf2UEp0EevH0GUZLDytBhoNLH1Yz5uRj3E5QUtn+/mrl0n0C4nZaVq8Vjakio0pNrM1TNLqbI5LtNVcY954nDJN4uxKr42+oKy7BQqUQ4TJlhZCrFZ3SeCrp4f/lGWsKQk4n+QzAtqweI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ifcTjY9H; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="ifcTjY9H" Received: by smtp.kernel.org (Postfix) with ESMTPSA id CCFF2C19421; Wed, 18 Feb 2026 08:05:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1771401928; bh=qw9gzxcNy19n9/pZuKUBlr0jWF8fKT2TdJbUSdrsj5I=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=ifcTjY9HX2ZaJSz7QsVRODHUNOV5bYWzzj/OLHRzEPynMGd8/dS72kINYOXhHKrDd wiGJrXNVyd8i1siWYDCDLWSwUGWZZtQvmhdRIZQWYnQ72YiUYdeSS7Jx5EFGfjrMw0 jtKsVL3AUZ9xbEzIHVD6dUWgDLqvA3ZU4yYjPUEcjqWJytnFDJEMcRzUeiFH3eK4tO Tv+1NJIXuE07xMqPu+BLELA7pbuMHx5ShdZ2zSFIhXacczreKk+qFo84QDyZ9rdgrk vZxwBa+KEecNI70kqKMW8ZAOlwT2WxYqml/5ofnRBihY+zi9cOCMIV5iceZOxBeEQ6 r1zM8vGRr6saQ== Message-ID: <392f45e0-0942-47c9-8833-d54833842e9e@kernel.org> Date: Wed, 18 Feb 2026 09:05:22 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH mm-new v7 2/5] mm: khugepaged: refine scan progress number To: Vernon Yang , akpm@linux-foundation.org Cc: lorenzo.stoakes@oracle.com, ziy@nvidia.com, dev.jain@arm.com, baohua@kernel.org, lance.yang@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Vernon Yang References: <20260207081613.588598-1-vernon2gm@gmail.com> <20260207081613.588598-3-vernon2gm@gmail.com> <4qdu7owpmxfh3ugsue775fxarw5g2gcggbxdf5psj75nnu7z2u@cv2uu2yocaxq> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 2/18/26 04:55, Vernon Yang wrote: > On Sat, Feb 07, 2026 at 04:16:10PM +0800, Vernon Yang wrote: >> From: Vernon Yang >> >> Currently, each scan always increases "progress" by HPAGE_PMD_NR, >> even if only scanning a single PTE/PMD entry. >> >> - When only scanning a sigle PTE entry, let me provide a detailed >> example: >> >> static int hpage_collapse_scan_pmd() >> { >> for (addr = start_addr, _pte = pte; _pte < pte + HPAGE_PMD_NR; >> _pte++, addr += PAGE_SIZE) { >> pte_t pteval = ptep_get(_pte); >> ... >> if (pte_uffd_wp(pteval)) { <-- first scan hit >> result = SCAN_PTE_UFFD_WP; >> goto out_unmap; >> } >> } >> } >> >> During the first scan, if pte_uffd_wp(pteval) is true, the loop exits >> directly. In practice, only one PTE is scanned before termination. >> Here, "progress += 1" reflects the actual number of PTEs scanned, but >> previously "progress += HPAGE_PMD_NR" always. >> >> - When the memory has been collapsed to PMD, let me provide a detailed >> example: >> >> The following data is traced by bpftrace on a desktop system. After >> the system has been left idle for 10 minutes upon booting, a lot of >> SCAN_PMD_MAPPED or SCAN_NO_PTE_TABLE are observed during a full scan >> by khugepaged. >> >> From trace_mm_khugepaged_scan_pmd and trace_mm_khugepaged_scan_file, the >> following statuses were observed, with frequency mentioned next to them: >> >> SCAN_SUCCEED : 1 >> SCAN_EXCEED_SHARED_PTE: 2 >> SCAN_PMD_MAPPED : 142 >> SCAN_NO_PTE_TABLE : 178 >> total progress size : 674 MB >> Total time : 419 seconds, include khugepaged_scan_sleep_millisecs >> >> The khugepaged_scan list save all task that support collapse into hugepage, >> as long as the task is not destroyed, khugepaged will not remove it from >> the khugepaged_scan list. This exist a phenomenon where task has already >> collapsed all memory regions into hugepage, but khugepaged continues to >> scan it, which wastes CPU time and invalid, and due to >> khugepaged_scan_sleep_millisecs (default 10s) causes a long wait for >> scanning a large number of invalid task, so scanning really valid task >> is later. >> >> After applying this patch, when the memory is either SCAN_PMD_MAPPED or >> SCAN_NO_PTE_TABLE, just skip it, as follow: >> >> SCAN_EXCEED_SHARED_PTE: 2 >> SCAN_PMD_MAPPED : 147 >> SCAN_NO_PTE_TABLE : 173 >> total progress size : 45 MB >> Total time : 20 seconds >> >> Signed-off-by: Vernon Yang >> --- >> mm/khugepaged.c | 38 ++++++++++++++++++++++++++++---------- >> 1 file changed, 28 insertions(+), 10 deletions(-) >> >> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >> index 4049234e1c8b..8b68ae3bc2c5 100644 >> --- a/mm/khugepaged.c >> +++ b/mm/khugepaged.c >> @@ -68,7 +68,10 @@ enum scan_result { >> static struct task_struct *khugepaged_thread __read_mostly; >> static DEFINE_MUTEX(khugepaged_mutex); >> >> -/* default scan 8*HPAGE_PMD_NR ptes (or vmas) every 10 second */ >> +/* >> + * default scan 8*HPAGE_PMD_NR ptes, pmd_mapped, no_pte_table or vmas >> + * every 10 second. >> + */ >> static unsigned int khugepaged_pages_to_scan __read_mostly; >> static unsigned int khugepaged_pages_collapsed; >> static unsigned int khugepaged_full_scans; >> @@ -1240,7 +1243,8 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a >> } >> >> static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm, >> - struct vm_area_struct *vma, unsigned long start_addr, bool *mmap_locked, >> + struct vm_area_struct *vma, unsigned long start_addr, >> + bool *mmap_locked, unsigned int *cur_progress, >> struct collapse_control *cc) >> { >> pmd_t *pmd; >> @@ -1256,19 +1260,27 @@ static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm, >> VM_BUG_ON(start_addr & ~HPAGE_PMD_MASK); >> >> result = find_pmd_or_thp_or_none(mm, start_addr, &pmd); >> - if (result != SCAN_SUCCEED) >> + if (result != SCAN_SUCCEED) { >> + if (cur_progress) >> + *cur_progress = 1; >> goto out; >> + } >> >> memset(cc->node_load, 0, sizeof(cc->node_load)); >> nodes_clear(cc->alloc_nmask); >> pte = pte_offset_map_lock(mm, pmd, start_addr, &ptl); >> if (!pte) { >> + if (cur_progress) >> + *cur_progress = 1; >> result = SCAN_NO_PTE_TABLE; >> goto out; >> } >> >> for (addr = start_addr, _pte = pte; _pte < pte + HPAGE_PMD_NR; >> _pte++, addr += PAGE_SIZE) { >> + if (cur_progress) >> + *cur_progress += 1; >> + >> pte_t pteval = ptep_get(_pte); >> if (pte_none_or_zero(pteval)) { >> ++none_or_zero; >> @@ -2288,8 +2300,9 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr, >> return result; >> } >> >> -static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr, >> - struct file *file, pgoff_t start, struct collapse_control *cc) >> +static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm, >> + unsigned long addr, struct file *file, pgoff_t start, >> + unsigned int *cur_progress, struct collapse_control *cc) >> { >> struct folio *folio = NULL; >> struct address_space *mapping = file->f_mapping; >> @@ -2378,6 +2391,8 @@ static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm, unsigned >> cond_resched_rcu(); >> } >> } >> + if (cur_progress) >> + *cur_progress = HPAGE_PMD_NR; > > Hi David, > > When using Fedora Server, I found a lot of SCAN_PTE_MAPPED_HUGEPAGE. > > The following data is traced by bpftrace[1] on Fedora Server. After > the system has been left idle for 10 minutes upon booting, a lot of > SCAN_PTE_MAPPED_HUGEPAGE are observed during a full scan by khugepaged, > as shown below: > > SCAN_SUCCEED : 1 > SCAN_PMD_MAPPED : 22 > SCAN_EXCEED_NONE_PTE : 67 > SCAN_PTE_MAPPED_HUGEPAGE: 919 > > I simply handled SCAN_PTE_MAPPED_HUGEPAGE by "cur_progress" equal to 1, > as follows: > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 437783cf2873..7f301bebfb11 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -2405,8 +2405,12 @@ static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm, > } > } > rcu_read_unlock(); > - if (cur_progress) > - *cur_progress = HPAGE_PMD_NR; > + if (cur_progress) { > + if (result == SCAN_PTE_MAPPED_HUGEPAGE) > + *cur_progress = 1; > + else > + *cur_progress = HPAGE_PMD_NR; > + } That makes sense to me. -- Cheers, David