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Wed, 28 Jan 2026 06:34:59 -0800 (PST) Received: from localhost.localdomain ([221.227.246.159]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-82379c1fc6esm2829879b3a.47.2026.01.28.06.34.55 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 28 Jan 2026 06:34:58 -0800 (PST) Date: Wed, 28 Jan 2026 22:34:50 +0800 From: Vernon Yang To: Dev Jain Cc: akpm@linux-foundation.org, david@kernel.org, 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 Subject: Re: [PATCH mm-new v5 2/5] mm: khugepaged: refine scan progress number Message-ID: <4exllw7pf4yfhfzpyg3ictsnznf5lepv5tjd7zvycldjwmh6jm@j6rara3ogrux> References: <20260123082232.16413-1-vernon2gm@gmail.com> <20260123082232.16413-3-vernon2gm@gmail.com> <72dfd11f-dca4-447e-89c5-7a87cb675bda@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <72dfd11f-dca4-447e-89c5-7a87cb675bda@arm.com> X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 6D29B40005 X-Stat-Signature: ywskxwd1sw8kdsnb3n83qzphofqfmjor X-Rspam-User: X-HE-Tag: 1769610900-890847 X-HE-Meta: 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 Dy2n4sNy 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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 Wed, Jan 28, 2026 at 01:59:33PM +0530, Dev Jain wrote: > > 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 > > Could you elaborate what is [1], [6] etc and 1,2,142, etc? These 1,6 are value of "enum scan_result", you can directly refer to the notes on the right. These 1,2,142,178 are number of different "enum scan_result" from trace_mm_khugepaged_scan_pmd and trace_mm_khugepaged_scan_file. as example, SCAN_PMD_MAPPED has 142 times during a full scan by khugepaged. > > 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; > > +} > > + > > /** > > * 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; > > Why not be a little more explicit, and do this addition if find_pmd_or_thp_or_none fails, > or pte_offset_map_lock fails? The way you do it right now is not readable - it gives no > idea as to why on function entry we do a +1 right away. Please do add some comments too. If this way is not clear enough, we can directly add 1 in find_pmd_or_thp_or_none() etc, BUT it's a bit redundant. Please take a look at which one is better. case 1: as the V4 PATCH #2 [1] and #3 [2], only hpage_collapse_scan_pmd(). [1] https://lore.kernel.org/linux-mm/20260111121909.8410-3-yanglincheng@kylinos.cn [2] https://lore.kernel.org/linux-mm/20260111121909.8410-4-yanglincheng@kylinos.cn static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm, struct vm_area_struct *vma, unsigned long start_addr, bool *mmap_locked, unsigned int *cur_progress, struct collapse_control *cc) { ... result = find_pmd_or_thp_or_none(mm, start_addr, &pmd); if (result != SCAN_SUCCEED) { if (cur_progress) *cur_progress += 1; // here goto out; } ... pte = pte_offset_map_lock(mm, pmd, start_addr, &ptl); if (!pte) { if (cur_progress) *cur_progress += 1; // here 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; // here ... } } case 2: static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm, struct vm_area_struct *vma, unsigned long start_addr, bool *mmap_locked, unsigned int *cur_progress, struct collapse_control *cc) { ... result = find_pmd_or_thp_or_none(mm, start_addr, &pmd); if (result != SCAN_SUCCEED) { if (cur_progress) *cur_progress += 1; // here goto out; } ... pte = pte_offset_map_lock(mm, pmd, start_addr, &ptl); if (!pte) { if (cur_progress) *cur_progress += 1; // here result = SCAN_NO_PTE_TABLE; goto out; } for (addr = start_addr, _pte = pte; _pte < pte + HPAGE_PMD_NR; _pte++, addr += PAGE_SIZE) { ... } ... out_unmap: if (cur_progress) { if (_pte >= pte + HPAGE_PMD_NR) *cur_progress += HPAGE_PMD_NR; // here else *cur_progress += _pte - pte + 1; // here } } case 3: current patch, and add more comments to clearer. > > + > > 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; > > + } > > 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; > > I think this whole block needs some comments. Can you explain, why you > do a particular increment in each case? > > > + 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); > > + } The "idx" represent PTEs number already scanned when exiting xas_for_each(). However, the last valid folio size was not counted in "idx" (except folio == NULL, "idx" equal to HPAGE_PMD_NR), which can be further divided into three cases: 1. shmem swap entries (xa_is_value), add folio size. 2. the folio is HPAGE_PMD_ORDER, the memory has been collapsed to PMD, so add 1 only. 3. Normal folio, add folio size. Is the version below more readable? if (cur_progress) { *cur_progress += xas.xa_index - start; if (folio) { if (xa_is_value(folio)) *cur_progress += 1 << xas_get_order(&xas); else if (folio_order(folio) == HPAGE_PMD_ORDER) *cur_progress += 1; else *cur_progress += 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; > -- Thanks, Vernon