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Wed, 17 Dec 2025 19:48:02 -0800 (PST) Date: Thu, 18 Dec 2025 03:48:01 +0000 From: Wei Yang To: Vernon Yang Cc: Wei Yang , 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 2/4] mm: khugepaged: remove mm when all memory has been collapsed Message-ID: <20251218034801.jyuu437dbtvcnpzw@master> Reply-To: Wei Yang References: <20251215090419.174418-1-yanglincheng@kylinos.cn> <20251215090419.174418-3-yanglincheng@kylinos.cn> <20251217033155.yhjerlthr36utnbr@master> <77paexkefc7qkfjgv6reuf7jxlysgkinuswsck5tthqkpcjkpr@aelvplwvafnt> 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: <77paexkefc7qkfjgv6reuf7jxlysgkinuswsck5tthqkpcjkpr@aelvplwvafnt> User-Agent: NeoMutt/20170113 (1.7.2) On Thu, Dec 18, 2025 at 11:27:24AM +0800, Vernon Yang wrote: >On Wed, Dec 17, 2025 at 03:31:55AM +0000, Wei Yang wrote: >> 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? > >static unsigned int shrink_folio_list(struct list_head *folio_list, > struct pglist_data *pgdat, struct scan_control *sc, > struct reclaim_stat *stat, bool ignore_references, > struct mem_cgroup *memcg) >{ > ... > > folio = lru_to_folio(folio_list); > > ... > > references = folio_check_references(folio, sc); > switch (references) { > case FOLIOREF_ACTIVATE: > goto activate_locked; > case FOLIOREF_KEEP: > stat->nr_ref_keep += nr_pages; > goto keep_locked; > case FOLIOREF_RECLAIM: > case FOLIOREF_RECLAIM_CLEAN: > ; /* try to reclaim the folio below */ > } > > ... > > split_folio_to_list(folio, folio_list); >} > >During memory reclaim above, only inactive folios are split. This also >implies that the folio is cold, meaning it hasn't been used recently, so >we do not expect to put the mm back onto the khugepaged scan list to >continue scan/collapse. khugeapged needs to scan hot folios as much as >possible priorityly and collapse hot folios to avoid wasting CPU. > So we will never pout this process back onto the scan list, right? >> * would this interfere with mTHP collapse? > >It has no impact on mTHP collapse, only when all memory is either >SCAN_PMD_MAPPED or SCAN_PMD_NONE, the mm will be removed automatically. >other cases will not be removed. > >Let me know if I missed something please, thanks! > >> >> > >> >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? > >Above "fallthrough" explicitly tells the compiler that when the collapse is >successful, run below "khugepaged_scan.maybe_collapse = true" :) > Got it, thanks. >> >+ 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 > >-- >Thanks, >Vernon -- Wei Yang Help you, Help me