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Tue, 16 Dec 2025 19:31:55 -0800 (PST) Date: Wed, 17 Dec 2025 03:31:55 +0000 From: Wei Yang To: Vernon Yang Cc: akpm@linux-foundation.org, david@kernel.org, lorenzo.stoakes@oracle.com, ziy@nvidia.com, npache@redhat.com, baohua@kernel.org, lance.yang@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Vernon Yang Subject: Re: [PATCH 2/4] mm: khugepaged: remove mm when all memory has been collapsed Message-ID: <20251217033155.yhjerlthr36utnbr@master> Reply-To: Wei Yang References: <20251215090419.174418-1-yanglincheng@kylinos.cn> <20251215090419.174418-3-yanglincheng@kylinos.cn> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20251215090419.174418-3-yanglincheng@kylinos.cn> User-Agent: NeoMutt/20170113 (1.7.2) On Mon, Dec 15, 2025 at 05:04:17PM +0800, Vernon Yang wrote: >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_PMD_NONE are observed during a full scan by >khugepaged. > >@scan_pmd_status[1]: 1 ## SCAN_SUCCEED >@scan_pmd_status[4]: 158 ## SCAN_PMD_MAPPED >@scan_pmd_status[3]: 174 ## SCAN_PMD_NONE >total progress size: 701 MB >Total time : 440 seconds ## include khugepaged_scan_sleep_millisecs > >The khugepaged_scan list save all task that support collapse into hugepage, >as long as the take 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 all memory is either SCAN_PMD_MAPPED or >SCAN_PMD_NONE, the mm is automatically removed from khugepaged's scan >list. If the page fault or MADV_HUGEPAGE again, it is added back to >khugepaged. Two thing s come up my mind: * what happens if we split the huge page under memory pressure? * would this interfere with mTHP collapse? > >Signed-off-by: Vernon Yang >--- > mm/khugepaged.c | 35 +++++++++++++++++++++++++---------- > 1 file changed, 25 insertions(+), 10 deletions(-) > >diff --git a/mm/khugepaged.c b/mm/khugepaged.c >index 0598a19a98cc..1ec1af5be3c8 100644 >--- a/mm/khugepaged.c >+++ b/mm/khugepaged.c >@@ -115,6 +115,7 @@ struct khugepaged_scan { > struct list_head mm_head; > struct mm_slot *mm_slot; > unsigned long address; >+ bool maybe_collapse; > }; > > static struct khugepaged_scan khugepaged_scan = { >@@ -1420,22 +1421,19 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm, > return result; > } > >-static void collect_mm_slot(struct mm_slot *slot) >+static void collect_mm_slot(struct mm_slot *slot, bool maybe_collapse) > { > struct mm_struct *mm = slot->mm; > > lockdep_assert_held(&khugepaged_mm_lock); > >- if (hpage_collapse_test_exit(mm)) { >+ if (hpage_collapse_test_exit(mm) || !maybe_collapse) { > /* free mm_slot */ > hash_del(&slot->hash); > list_del(&slot->mm_node); > >- /* >- * Not strictly needed because the mm exited already. >- * >- * mm_flags_clear(MMF_VM_HUGEPAGE, mm); >- */ >+ if (!maybe_collapse) >+ mm_flags_clear(MMF_VM_HUGEPAGE, mm); > > /* khugepaged_mm_lock actually not necessary for the below */ > mm_slot_free(mm_slot_cache, slot); >@@ -2397,6 +2395,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result, > struct mm_slot, mm_node); > khugepaged_scan.address = 0; > khugepaged_scan.mm_slot = slot; >+ khugepaged_scan.maybe_collapse = false; > } > spin_unlock(&khugepaged_mm_lock); > >@@ -2470,8 +2469,18 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result, > khugepaged_scan.address, &mmap_locked, cc); > } > >- if (*result == SCAN_SUCCEED) >+ switch (*result) { >+ case SCAN_PMD_NULL: >+ case SCAN_PMD_NONE: >+ case SCAN_PMD_MAPPED: >+ case SCAN_PTE_MAPPED_HUGEPAGE: >+ break; >+ case SCAN_SUCCEED: > ++khugepaged_pages_collapsed; >+ fallthrough; If collapse successfully, we don't need to set maybe_collapse to true? >+ default: >+ khugepaged_scan.maybe_collapse = true; >+ } > > /* move to next address */ > khugepaged_scan.address += HPAGE_PMD_SIZE; >@@ -2500,6 +2509,11 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result, > * if we scanned all vmas of this mm. > */ > if (hpage_collapse_test_exit(mm) || !vma) { >+ bool maybe_collapse = khugepaged_scan.maybe_collapse; >+ >+ if (mm_flags_test(MMF_DISABLE_THP_COMPLETELY, mm)) >+ maybe_collapse = true; >+ > /* > * Make sure that if mm_users is reaching zero while > * khugepaged runs here, khugepaged_exit will find >@@ -2508,12 +2522,13 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result, > if (!list_is_last(&slot->mm_node, &khugepaged_scan.mm_head)) { > khugepaged_scan.mm_slot = list_next_entry(slot, mm_node); > khugepaged_scan.address = 0; >+ khugepaged_scan.maybe_collapse = false; > } else { > khugepaged_scan.mm_slot = NULL; > khugepaged_full_scans++; > } > >- collect_mm_slot(slot); >+ collect_mm_slot(slot, maybe_collapse); > } > > trace_mm_khugepaged_scan(mm, progress, khugepaged_scan.mm_slot == NULL); >@@ -2616,7 +2631,7 @@ static int khugepaged(void *none) > slot = khugepaged_scan.mm_slot; > khugepaged_scan.mm_slot = NULL; > if (slot) >- collect_mm_slot(slot); >+ collect_mm_slot(slot, true); > spin_unlock(&khugepaged_mm_lock); > return 0; > } >-- >2.51.0 > -- Wei Yang Help you, Help me