From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 7E6263033F2 for ; Fri, 23 Jan 2026 10:46:23 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769165186; cv=none; b=l39Jj+JOEvPCsRQ3nfXVPuesXbQgeP8LArz6pX3FMjLMhRi0JMmIcsG+a9oe4wS19Z6IGmDDyPm14K0wTKp8aN6EYieF7uBIgoqK9CeIYln98WWdmpi6Eamp4q8AM2FX/KjvG4HBRN74f3tqmBMeHawokSWC0F/JUM235Q+oT0o= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769165186; c=relaxed/simple; bh=tXCUB1/WFTSOTBv52rhHkV1rJ2/SLS9/q4CJM2Cb1AY=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Tkmm0BQkbI8HXzkTZsqDI8yH1EzGF0cf8jtV6G9IoIoj3Bh4t3wvmQmBcR70idcvxdMsDjmhhBUTq0EHn6PEqxOGruKoG92IL+wbk63EcruHDBHbRXdiMrark8J/HxiJ2Y63WS9VZOGhs+WrNj8NOead2BpPUfg8ex7ua/GnJmc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 7AE211476; Fri, 23 Jan 2026 02:46:16 -0800 (PST) Received: from [10.164.136.38] (unknown [10.164.136.38]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id B12DB3F740; Fri, 23 Jan 2026 02:46:19 -0800 (PST) Message-ID: <98bdfd16-cb50-47a8-999a-bbd9829c6350@arm.com> Date: Fri, 23 Jan 2026 16:16:16 +0530 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 v5 2/5] mm: khugepaged: refine scan progress number To: Vernon Yang , akpm@linux-foundation.org, david@kernel.org Cc: lorenzo.stoakes@oracle.com, ziy@nvidia.com, baohua@kernel.org, lance.yang@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Vernon Yang References: <20260123082232.16413-1-vernon2gm@gmail.com> <20260123082232.16413-3-vernon2gm@gmail.com> Content-Language: en-US From: Dev Jain In-Reply-To: <20260123082232.16413-3-vernon2gm@gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 23/01/26 1:52 pm, 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. > > @scan_pmd_status[1]: 1 ## SCAN_SUCCEED > @scan_pmd_status[6]: 2 ## SCAN_EXCEED_SHARED_PTE > @scan_pmd_status[3]: 142 ## SCAN_PMD_MAPPED > @scan_pmd_status[2]: 178 ## SCAN_NO_PTE_TABLE > 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_pmd_status[6]: 2 > @scan_pmd_status[3]: 147 > @scan_pmd_status[2]: 173 > total progress size: 45 MB > Total time : 20 seconds > > Signed-off-by: Vernon Yang > --- > include/linux/xarray.h | 9 ++++++++ > mm/khugepaged.c | 47 ++++++++++++++++++++++++++++++++++-------- > 2 files changed, 47 insertions(+), 9 deletions(-) > > diff --git a/include/linux/xarray.h b/include/linux/xarray.h > index be850174e802..f77d97d7b957 100644 > --- a/include/linux/xarray.h > +++ b/include/linux/xarray.h > @@ -1646,6 +1646,15 @@ static inline void xas_set(struct xa_state *xas, unsigned long index) > xas->xa_node = XAS_RESTART; > } > > +/** > + * xas_get_index() - Get XArray operation state for a different index. > + * @xas: XArray operation state. > + */ > +static inline unsigned long xas_get_index(struct xa_state *xas) > +{ > + return xas->xa_index; > +} > + Why is this needed? > /** > * xas_advance() - Skip over sibling entries. > * @xas: XArray operation state. > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 6f0f05148765..de95029e3763 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; > @@ -1255,6 +1259,9 @@ static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm, > > VM_BUG_ON(start_addr & ~HPAGE_PMD_MASK); > > + if (cur_progress) > + *cur_progress += 1; > + 1. Why do we need to do if (cur_progress). Isn't it guaranteed that the pointer will never be NULL. 2. Why do we increment this on function entry? > result = find_pmd_or_thp_or_none(mm, start_addr, &pmd); > if (result != SCAN_SUCCEED) > goto out; > @@ -1396,6 +1403,12 @@ static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm, > result = SCAN_SUCCEED; > } > out_unmap: > + if (cur_progress) { > + if (_pte >= pte + HPAGE_PMD_NR) > + *cur_progress += HPAGE_PMD_NR - 1; > + else > + *cur_progress += _pte - pte; > + } Why are two cases required here - shouldn't it just be _pte - pte? > pte_unmap_unlock(pte, ptl); > if (result == SCAN_SUCCEED) { > result = collapse_huge_page(mm, start_addr, referenced, > @@ -2286,8 +2299,9 @@ 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; > @@ -2376,6 +2390,18 @@ static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm, unsigned > cond_resched_rcu(); > } > } > + if (cur_progress) { > + unsigned long idx = xas_get_index(&xas) - start; > + > + if (folio == NULL) > + *cur_progress += HPAGE_PMD_NR; > + else if (xa_is_value(folio)) > + *cur_progress += idx + (1 << xas_get_order(&xas)); > + else if (folio_order(folio) == HPAGE_PMD_ORDER) > + *cur_progress += idx + 1; > + else > + *cur_progress += idx + folio_nr_pages(folio); > + } > rcu_read_unlock(); > > if (result == SCAN_SUCCEED) { > @@ -2456,6 +2482,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, enum scan_result > > while (khugepaged_scan.address < hend) { > bool mmap_locked = true; > + unsigned int cur_progress = 0; > > cond_resched(); > if (unlikely(hpage_collapse_test_exit_or_disable(mm))) > @@ -2472,7 +2499,8 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, enum scan_result > mmap_read_unlock(mm); > mmap_locked = false; > *result = hpage_collapse_scan_file(mm, > - khugepaged_scan.address, file, pgoff, cc); > + khugepaged_scan.address, file, pgoff, > + &cur_progress, cc); > fput(file); > if (*result == SCAN_PTE_MAPPED_HUGEPAGE) { > mmap_read_lock(mm); > @@ -2486,7 +2514,8 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, enum scan_result > } > } else { > *result = hpage_collapse_scan_pmd(mm, vma, > - khugepaged_scan.address, &mmap_locked, cc); > + khugepaged_scan.address, &mmap_locked, > + &cur_progress, cc); > } > > if (*result == SCAN_SUCCEED) > @@ -2494,7 +2523,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, enum scan_result > > /* move to next address */ > khugepaged_scan.address += HPAGE_PMD_SIZE; > - progress += HPAGE_PMD_NR; > + progress += cur_progress; > if (!mmap_locked) > /* > * We released mmap_lock so break loop. Note > @@ -2817,7 +2846,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start, > mmap_locked = false; > *lock_dropped = true; > result = hpage_collapse_scan_file(mm, addr, file, pgoff, > - cc); > + NULL, cc); > > if (result == SCAN_PAGE_DIRTY_OR_WRITEBACK && !triggered_wb && > mapping_can_writeback(file->f_mapping)) { > @@ -2832,7 +2861,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start, > fput(file); > } else { > result = hpage_collapse_scan_pmd(mm, vma, addr, > - &mmap_locked, cc); > + &mmap_locked, NULL, cc); > } > if (!mmap_locked) > *lock_dropped = true;