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Tue, 17 Feb 2026 19:55:48 -0800 (PST) Received: from localhost.localdomain ([114.231.118.96]) by smtp.gmail.com with ESMTPSA id 41be03b00d2f7-c6e531e6dccsm10739572a12.19.2026.02.17.19.55.43 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 17 Feb 2026 19:55:47 -0800 (PST) Date: Wed, 18 Feb 2026 11:55:39 +0800 From: Vernon Yang To: akpm@linux-foundation.org, david@kernel.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 Subject: Re: [PATCH mm-new v7 2/5] mm: khugepaged: refine scan progress number Message-ID: <4qdu7owpmxfh3ugsue775fxarw5g2gcggbxdf5psj75nnu7z2u@cv2uu2yocaxq> References: <20260207081613.588598-1-vernon2gm@gmail.com> <20260207081613.588598-3-vernon2gm@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260207081613.588598-3-vernon2gm@gmail.com> X-Stat-Signature: tikjj84fbkyfudihb6u53rzeyxe4jfr4 X-Rspam-User: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 56361A0008 X-HE-Tag: 1771386949-304694 X-HE-Meta: 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 Btu0+BGP 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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; + } The below is some performance test results. kernbench results (testing on x86_64 machine): baseline w/o patches test w/ patches Amean user-32 18633.85 ( 0.00%) 18346.30 * 1.54%* Amean syst-32 1138.82 ( 0.00%) 1109.68 * 2.56%* Amean elsp-32 669.60 ( 0.00%) 659.79 * 1.47%* BAmean-95 user-32 18631.33 ( 0.00%) 18340.10 ( 1.56%) BAmean-95 syst-32 1138.36 ( 0.00%) 1108.05 ( 2.66%) BAmean-95 elsp-32 669.55 ( 0.00%) 659.61 ( 1.48%) BAmean-99 user-32 18631.33 ( 0.00%) 18340.10 ( 1.56%) BAmean-99 syst-32 1138.36 ( 0.00%) 1108.05 ( 2.66%) BAmean-99 elsp-32 669.55 ( 0.00%) 659.61 ( 1.48%) Kernbench performance improved by 2.56%, so we truly need to address this issue. I will fix it in the next version. If I missed something, please let me know, Thanks! [1] https://github.com/vernon2gh/app_and_module/blob/main/khugepaged/khugepaged_mm.bt -- Cheers, Vernon